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LT4356MP-1/LT4356MP-2 Surge Stopper FEATURES
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DESCRIPTION
The LT(R)4356 surge stopper protects loads from high voltage transients. It regulates the output during an overvoltage event, such as load dump in automobiles, by controlling the gate of an external N-channel MOSFET. The output is limited to a safe value thereby allowing the loads to continue functioning. The LT4356MP also monitors the voltage drop between the VCC and SNS pins to protect against overcurrent faults. An internal amplifier limits the current sense voltage to 50mV. In either fault condition, a timer is started inversely proportional to MOSFET stress. If the timer expires, the FLT pin pulls low to warn of an impending power-down. If the condition persists, the MOSFET is turned off. After a cooldown period, the GATE pin pulls up turning on the MOSFET again. The spare amplifier may be used as a voltage detection comparator or as a linear regulator controller driving an external PNP pass transistor. Back-to-back FETs can be used in lieu of a Schottky diode for reverse input protection, reducing voltage drop and power loss. A shutdown pin reduces the quiescent current to less than 7A during shutdown for the LT4356-1 and 60A for the LT4356-2 while keeping the reference and auxiliary amplifier functioning.
Stops High Voltage Surges Adjustable Output Clamp Voltage Overcurrent Protection Wide Operation Range: 4V to 80V Reverse Input Protection to -60V Low 7A Shutdown Current, LT4356-1 Adjustable Fault Timer Controls N-channel MOSFET Shutdown Pin Withstands -60V to 100V Fault Output Indication Guaranteed Operation -55C to 125C Spare Amplifier for Level Detection Comparator or Linear Regulator Controller Available in 10-Pin MSOP or 16-Pin SO Packages
APPLICATIONS
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Automotive/Avionic Surge Protection Hot Swap/Live Insertion High Side Switch for Battery Powered Systems Intrinsic Safety Applications
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
4A, 12V Overvoltage Output Regulator
VIN 12V 10m IRLR2908 VOUT 80V INPUT SURGE 10 383k VCC SHDN IN+ 100k EN UNDERVOLTAGE AOUT GND TMR FLT
4356mp12 TA01
Overvoltage Protector Regulates Output at 27V During Transient
CTMR = 6.8F ILOAD = 500mA
102k OUT FB 4.99k VCC DC-DC CONVERTER SHDN GND FAULT VOUT 20V/DIV 12V
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SNS
GATE
VIN 20V/DIV
12V 27V ADJUSTABLE CLAMP
LT4356S
100ms/DIV
0.1F
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LT4356MP-1/LT4356MP-2 ABSOLUTE MAXIMUM RATINGS (Notes 1 and 2)
VCC, SHDN ................................................ -60V to 100V SNS............................. VCC - 30V or -60V to VCC + 0.3V OUT, AOUT, FLT, EN ..................................... -0.3V to 80V GATE (Note 3) .................................-0.3V to VOUT + 10V FB, TMR, IN+ ................................................ -0.3V to 6V AOUT, EN, FLT, IN+ ..................................................-3mA Operating Temperature Range LT4356M ............................................ -55C to 125C Storage Temperature Range MS, SO .............................................. -65C to 150C Lead Temperature (Soldering, 10 sec) MS, SO ............................................................. 300C
PIN CONFIGURATION
TOP VIEW TMR 1 TOP VIEW FB OUT GATE SNS VCC 1 2 3 4 5 10 9 8 7 6 TMR GND EN FLT SHDN FB 2 NC 3 OUT 4 GATE 5 NC 6 SNS 7 VCC 8 16 IN+ 15 NC 14 AOUT 13 NC 12 GND 11 EN 10 FLT 9 SHDN
MS PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125C, JA = 160C/W
S PACKAGE 16-LEAD PLASTIC SO TJMAX = 150C, JA = 100C/W
ORDER INFORMATION
LEAD FREE FINISH LT4356MPMS-1#PBF LT4356MPS-1#PBF LT4356MPS-2#PBF LEAD BASED FINISH LT4356MPMS-1 LT4356MPS-1 LT4356MPS-2 TAPE AND REEL LT4356MPMS-1#TRPBF LT4356MPS-1#TRPBF LT4356MPS-2#TRPBF TAPE AND REEL LT4356MPMS-1#TR LT4356MPS-1#TR LT4356MPS-2#TR PART MARKING LTFGD LT4356MPS-1 LT4356MPS-2 PART MARKING LTFGD LT4356MPS-1 LT4356MPS-2 PACKAGE DESCRIPTION 10-Lead Plastic MSOP 16-Lead Plastic SO 16-Lead Plastic SO PACKAGE DESCRIPTION 10-Lead Plastic MSOP 16-Lead Plastic SO 16-Lead Plastic SO TEMPERATURE RANGE -55C to 125C -55C to 125C -55C to 125C TEMPERATURE RANGE -55C to 125C -55C to 125C -55C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LT4356MP-1/LT4356MP-2 ELECTRICAL CHARACTERISTICS
SYMBOL VCC ICC PARAMETER Operating Voltage Range VCC Supply Current CONDITIONS
l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 12V unless otherwise noted.
MIN 4 1 7 7 60 60 0.3 0.8 4.5 10 -4 -4.5 75 5 1.5 1.215 42.5 43 5
l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l
TYP
MAX 80 1.5 25 40 70 250 1 2 8 16 -36 -50
UNITS V mA A A A A mA mA V V A A mA mA mA V A mV mV A A A A A A A A A V V mV V A V mV A mA V V V V A s s
IR VGATE IGATE(UP) IGATE(DN)
Reverse Input Current GATE Pin Output High Voltage GATE Pin Pull-Up Current GATE Pin Pull-Down Current
VFB IFB VSNS ISNS ILEAK ITMR
FB Pin Servo Voltage FB Pin Input Current Overcurrent Fault Threshold SNS Pin Input Current FLT, EN Pins Leakage Current AOUT Pin Leakage Current TMR Pin Pull-Up Current
VSHDN = Float VSHDN = 0V, IN+ = 1.3V, LT4356MP-1 LT4356MP-1 VSHDN = 0V, IN+ = 1.3V, LT4356MP-2 LT4356MP-2 VSNS = VCC = -30V, SHDN Open VSNS = VCC = VSHDN = -30V VCC = 4V; (VGATE - VOUT) 80V VCC 8V; (VGATE - VOUT) VGATE = 12V; VCC = 12V VGATE = 48V; VCC = 48V Overvoltage, VFB = 1.4V, VGATE = 12V Overcurrent, VCC - VSNS = 120mV, VGATE = 12V Shutdown Mode, VSHDN = 0V, VGATE = 12V VGATE = 12V; VOUT = 12V VFB = 1.25V VSNS = (VCC - VSNS), VCC = 12V VSNS = (VCC - VSNS), VCC = 48V VSNS = VCC = 12V to 48V FLT, EN = 80V AOUT = 80V VTMR = 1V, VFB = 1.5V, (VCC - VOUT) = 0.5V VTMR = 1V, VFB = 1.5V, (VCC - VOUT) = 75V VTMR = 1.3V, VFB = 1.5V VTMR = 1V, VSNS = 60mV, (VCC - VOUT) = 0.5V VTMR = 1V, VSNS = 60mV, (VCC - VOUT) = 80V VTMR = 1V, VFB = 1V, VSNS = 0V FLT From High to Low, VCC = 5V to 80V VGATE From Low to High, VCC = 5V to 80V From FLT Going Low to GATE Going Low, VCC = 5V to 80V VIN+ = 1.25V ISINK = 2mA ISINK = 0.1mA VOUT = VCC = 12V VOUT = VCC = 12V, VSHDN = 0V VOUT = VCC - VOUT; EN from Low to High VCC = 12V to 48V VCC = 12V to 48V VSHDN = 0V GATE from High to Low, VSNS = 0 120mV GATE from High to Low, VFB = 0 1.5V
-23 -30 150 10 5 1.25 0.3 50 51 10
1.275 1 55 56 22 2.5 4.5 -4 -56 -8.5 -6.5 -315 2.7 1.28 0.52 120 1.28 1 8 800 300 14 0.7 1.7 2.1 2 -8 4 1
TMR Pin Pull-Down Current VTMR VTMR VIN+ IIN+ VOL IOUT VOUT VSHDN TMR Pin Thresholds Early Warning Period IN+ Pin Threshold IN+ Pin Input Current FLT, EN, AOUT Pins Output Low OUT Pin Input Current OUT Pin High Threshold SHDN Pin Threshold
-1.5 -44 -3.5 -2.5 -195 1.5 1.22 0.48 80 1.22
-2.5 -50 -5.5 -4.5 -260 2.2 1.25 0.5 100 1.25 0.3 2 300 200 6 0.5 1.4 1.2 -4 2 0.25
0.25 0.6 0.4 0.6 -1
VSHDN(FLT) SHDN Pin Float Voltage SHDN Pin Current ISHDN Overcurrent Turn-Off Delay Time tOFF(OC) tOFF(OV) Overvoltage Turn-Off Delay Time
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.
Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to GND unless otherwise specified. Note 3: An internal clamp limits the GATE pin to a minimum of 10V above the OUT pin. Driving this pin to voltages beyond the clamp may damage the device.
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LT4356MP-1/LT4356MP-2 TYPICAL PERFORMANCE CHARACTERISTICS
Specifications are at VCC = 12V, TA = 25C unless otherwise noted. ICC (Shutdown) vs VCC
60 50 40 ICC (A) ICC (A) 30 20 10 0 0 10 20 30 40 50 VCC (V) 60 70 80 LT4356-1 120 100 80 60 40 20 0 0 10 20 30 40 50 VCC (V) 60 70 80 200 ICC (A) 600
ICC (Shutdown) vs VCC
LT4356-2 IN+ = 1.3V 1000
ICC vs VCC
800
400
0 0 10 20 30 40 50 VCC (V) 60 70 80
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ICC (Shutdown) vs Temperature
35 30 25 ICC (A) ICC (A) 20 15 10 5 0 -50 LT4356-1 300 250 200 150 100 50
ICC (Shutdown) vs Temperature
LT4356-2 6 5 4 ISHDN (A) -25 25 75 0 50 TEMPERATURE (C) 100 125 3 2 1
SHDN Current vs Temperature
VSHDN = 0V
-25
25 75 0 50 TEMPERATURE (C)
100
125
0 -50
0 -50
-25
25 75 0 50 TEMPERATURE (C)
100
125
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GATE Pull-Up Current vs VCC
40 35 30 IGATE (A) IGATE (A) 25 20 15 10 5 0 0 10 20 30 40 50 VCC (V) 60 70 80 35 30
GATE Pull-Up Current vs Temperature
VGATE = VOUT = 12V 220 200 IGATE(DOWN) (mA) 180 160 140 120
GATE Pull-Down Current vs Temperature
OVERVOLTAGE CONDITION VFB = 1.5V
25 20 15 10 5 0 -50
-25
25 75 0 50 TEMPERATURE (C)
100
125
100 -50
-25
25 75 0 50 TEMPERATURE (C)
100
125
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4356mp12 G08
4356mp12 G09
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LT4356MP-1/LT4356MP-2 TYPICAL PERFORMANCE CHARACTERISTICS
Specifications are at VCC = 12V, TA = 25C unless otherwise noted. GATE Pull-Down Current vs Temperature
12 10 IGATE(DOWN) (mA) 8 VGATE (V) 6 4 2 0 -50 OVERCURRENT CONDITION VSNS = 120mV 14 12 10 VGATE (V) 8 6 4 2 0 0 2 4 6 10 IGATE (A) 8 12 14 16
VGATE vs IGATE
VOUT = 12V 14 12 10 8 6 4 2
VGATE vs Temperature
IGATE = -1A VCC = 8V
VCC = 4V
-25
25 75 0 50 TEMPERATURE (C)
100
125
0 -50
-25
0 50 25 75 TEMPERATURE (C)
100
125
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VGATE vs VCC
16 14 12 VGATE (V) 10 8 6 4 2I GATE = -1A VOUT = VCC 0 0 10 20 8 0 30 40 50 VCC (V) 60 70 80 TA = 25C TA = 130C 48
Overvoltage TMR Current vs (VCC - VOUT)
OVERVOLTAGE CONDITION VOUT = 5V 40 VTMR = 1V 32 ITMR (A) ITMR (A) 0 10 20 30 40 50 VCC - VOUT (V) 60 70 80 24 16 280
Overcurrent TMR Current vs (VCC - VOUT)
OVERCURRENT CONDITION VOUT = 0V 240 VTMR = 1V 200 160 120 80 40 0 0 10 20 30 40 50 VCC - VOUT (V) 60 70 80
TA = -45C
4356mp12 G13
4356mp12 G14
4356mp12 G15
Warning Period TMR Current vs VCC
14 OVERVOLTAGE, EARLY WARNING PERIOD 12 VFB = 1.5V VTMR = 1.3V 10 ITMR (A) ITMR (A) 8 6 4 2 0 0 10 20 30 40 50 VCC (V) 60 70 80 3.0 2.5 2.0
TMR Pull-Down Current vs Temperature
VTMR = 1V 4.0 3.5 3.0 2.5 VOL (V) 2.0 1.5 1.0 0.5 0 -50 0.5 0 -25 0 25 50 75 TEMPERATURE (C) 100 125
Output Low Voltage vs Current
AOUT
FLT EN
1.5 1.0
0
0.5
1.0 1.5 2.0 CURRENT (mA)
2.5
3.0
4356mp12 G16
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4356mp12 G18
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LT4356MP-1/LT4356MP-2 TYPICAL PERFORMANCE CHARACTERISTICS
Specifications are at VCC = 12V, TA = 25C unless otherwise noted. Overvoltage Turn-Off Time vs Temperature
500 OVERVOLTAGE CONDITION VFB = 1.5V 4.0 3.5 -15 3.0 tOFF (ns) tOFF (s) 2.5 2.0 -5 100 1.5 1.0 -50 0 ICC (mA) 300 -10
Overcurrent Turn-Off Time vs Temperature
OVERCURRENT CONDITION VSNS = 120mV -20
Reverse Current vs Reverse Voltage
VCC = SNS
400
200
0 -50
-25
0 25 50 75 TEMPERATURE (C)
100
125
-25
0 25 50 75 TEMPERATURE (C)
100
125
0
-20
-40 VCC (V)
-60
-80
4356mp12 G21
4356mp12 G19
4356mp12 G20
PIN FUNCTIONS
AOUT : Amplifier Output. Open collector output of the auxiliary amplifier. It is capable of sinking up to 2mA from 80V. The negative input of the amplifier is internally connected to a 1.25V reference. EN: Open-Collector Enable Output. The EN pin goes high impedance when the voltage at the OUT pin is above (VCC - 0.7V), indicating the external MOSFET is fully on. The state of the pin is latched until the OUT pin voltage resets at below 0.5V and goes back up above 2V. The internal NPN is capable of sinking up to 3mA of current from 80V to drive an LED or opto-coupler. Exposed Pad: Exposed pad may be left open or connected to device ground (GND). FB: Voltage Regulator Feedback Input. Connect this pin to the center tap of the output resistive divider connected between the OUT pin and ground. During an overvoltage condition, the GATE pin is servoed to maintain a 1.25V threshold at the FB pin. This pin is clamped internally to 7V. Tie to GND to disable the OV clamp. FLT: Open-Collector Fault Output. This pin pulls low after the voltage at the TMR pin has reached the fault threshold of 1.25V. It indicates the pass transistor is about to turn off because either the supply voltage has stayed at an elevated level for an extended period of time (voltage fault) or the device is in an overcurrent condition (current fault). The internal NPN is capable of sinking up to 3mA of current from 80V to drive an LED or opto-coupler. GATE: N-channel MOSFET Gate Drive Output. The GATE pin is pulled up by an internal charge pump current source and clamped to 14V above the OUT pin. Both voltage and current amplifiers control the GATE pin to regulate the output voltage and limit the current through the MOSFET. GND: Device Ground. IN+: Positive Input of the Auxiliary Amplifier. This amplifier can be used as a level detection comparator with external hysteresis or linear regulator controlling an external PNP transistor. This pin is clamped internally to 7V. Connect to ground if unused. OUT: Output Voltage Sense Input. This pin senses the voltage at the source of the N-channel MOSFET and sets the fault timer current. When the OUT pin voltage reaches 0.7V away from VCC, the EN pin goes high impedance. SHDN: Shutdown Control Input. The LT4356 can be shut down to a low current mode by pulling the SHDN pin below the shutdown threshold of 0.6V. Pull this pin above 1.7V or disconnect it and allow the internal current source to turn the part back on. The leakage current to ground at the pin
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LT4356MP-1/LT4356MP-2 PIN FUNCTIONS
should be limited to no more than 1A if no pull-up device is used to turn the part on. The SHDN pin can be pulled up to 100V or below GND by 60V without damage. SNS: Current Sense Input. Connect this pin to the output of the current sense resistor. The current limit circuit controls the GATE pin to limit the sense voltage between VCC and SNS pins to 50mV. At the same time the sense amplifier also starts a current source to charge up the TMR pin. This pin can be pulled below GND by up to 60V, though the voltage difference with the VCC pin must be limited to less than 30V. Connect to VCC if unused. TMR: Fault Timer Input. Connect a capacitor between this pin and ground to set the times for early warning, fault and cooldown periods. The current charging up this pin during fault conditions depends on the voltage difference between the VCC and OUT pins. When VTMR reaches 1.25V, the FLT pin pulls low to indicate the detection of a fault condition. If the condition persists, the pass transistor turns off when VTMR reaches the threshold of 1.35V. As soon as the fault condition disappears, the pull-up current stops and a 2A current starts to pull the TMR pin down. When VTMR reaches the retry threshold of 0.5V, the GATE pin pulls high turning back on the pass transistor. VCC: Positive Supply Voltage Input. The positive supply input ranges from 4V to 80V for normal operation. It can also be pulled below ground potential by up to 60V during a reverse battery condition, without damaging the part. The supply current is reduced to 7A with all the functional blocks off.
BLOCK DIAGRAM
VCC SNS GATE OUT
50mV
+ - +
IA
CHARGE PUMP
14V FB VA
-
SHDN FLT AOUT OC 1.25V AUXILIARY AMPLIFIER SHDN RESTART OUT CONTROL LOGIC GATEOFF FLT OV EN
IN+ 1.35V VCC 0.5V ITMR
- + +
2A
1.25V TMR
-
GND
4356mp12 BD
+ - + -
1.25V
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LT4356MP-1/LT4356MP-2 OPERATION
Some power systems must cope with high voltage surges of short duration such as those in automobiles. Load circuitry must be protected from these transients, yet high availability systems must continue operating during these events. The LT4356 is an overvoltage protection regulator that drives an external N-channel MOSFET as the pass transistor. It operates from a wide supply voltage range of 4V to 80V. It can also be pulled below ground potential by up to 60V without damage. The low power supply requirement of 4V allows it to operate even during cold cranking conditions in automotive applications. The internal charge pump turns on the N-channel MOSFET to supply current to the loads with very little power loss. Two MOSFETs can be connected back to back to replace an inline Schottky diode for reverse input protection. This improves the efficiency and increases the available supply voltage level to the load circuitry during cold crank. Normally, the pass transistor is fully on, powering the loads with very little voltage drop. When the supply voltage surges too high, the voltage amplifier (VA) controls the gate of the MOSFET and regulates the voltage at the source pin to a level that is set by the external resistor divider from the OUT pin to ground and the internal 1.25V reference. A current source starts charging up the capacitor connected at the TMR pin to ground. If the voltage at the TMR pin, VTMR, reaches 1.25V, the FLT pin pulls low to indicate impending turn-off due to the overvoltage condition. The pass transistor stays on until the TMR pin reaches 1.35V, at which point the GATE pin pulls low turning off the MOSFET. The potential at the TMR pin starts decreasing as soon as the overvoltage condition disappears. When the voltage at the TMR pin reaches 0.5V the GATE pin begins rising, turning on the MOSFET. The FLT pin will then go to a high impedance state. The fault timer allows the loads to continue functioning during short transient events while protecting the MOSFET from being damaged by a long period of supply overvoltage, such as a load dump in automobiles. The timer period varies with the voltage across the MOSFET. A higher voltage corresponds to a shorter fault timer period, ensuring the MOSFET operates within its safe operating area (SOA). The LT4356 senses an overcurrent condition by monitoring the voltage across an optional sense resistor placed between the VCC and SNS pins. An active current limit circuit (IA) controls the GATE pin to limit the sense voltage to 50mV. A current is also generated to start charging up the TMR pin. This current is about 5 times the current generated during an overvoltage event. The FLT pin pulls low when the voltage at the TMR pin reaches 1.25V and the MOSFET is turned off when it reaches 1.35V. A spare amplifier (SA) is provided with the negative input connected to an internal 1.25V reference. The output pulldown device is capable of sinking up to 2mA of current allowing it to drive an LED or opto coupler. This amplifier can be configured as a linear regulator controller driving an external PNP transistor or a comparator function to monitor voltages. A shutdown pin turns off the pass transistor and reduces the supply current to less than 7A for the LT4356-1. The supply current drops down to 60A while keeping the internal reference and the auxiliary amplifier active for the LT4356-2 version during shutdown.
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LT4356MP-1/LT4356MP-2 APPLICATIONS INFORMATION
The LT4356 can limit the voltage and current to the load circuitry during supply transients or overcurrent events. The total fault timer period should be set to ride through short overvoltage transients while not causing damage to the pass transistor. The selection of this N-channel MOSFET pass transistor is critical for this application. It must stay on and provide a low impedance path from the input supply to the load during normal operation and then dissipate power during overvoltage or overcurrent conditions. The following sections describe the overcurrent and the overvoltage faults, and the selection of the timer capacitor value based on the required warning time. The selection of the N-channel MOSFET pass transistor is discussed next. Auxiliary amplifier, reverse input, and the shutdown functions are covered after the MOSFET selection. External component selection is discussed in detail in the Design Example section. Overvoltage Fault The LTC4356 limits the voltage at the OUT pin during an overvoltage situation. An internal voltage amplifier regulates the GATE pin voltage to maintain a 1.25V threshold at the FB pin. During this period of time, the power MOSFET is still on and continues to supply current to the load. This allows uninterrupted operation during short overvoltage transient events. When the voltage regulation loop is engaged for longer than the time-out period, set by the timer capacitor connected from the TMR pin to ground, an overvoltage fault is detected. The GATE pin is pulled down to the OUT pin by a 150mA current. After the fault condition has disappeared and a cooldown period has transpired, the GATE pin starts to pull high again. This prevents the power MOSFET from being damaged during a long period of overvoltage, such as during load dump in automobiles. Overcurrent Fault The LT4356 features an adjustable current limit that protects against short circuits or excessive load current. During an overcurrent event, the GATE pin is regulated to limit the current sense voltage across the VCC and SNS pins to 50mV. An overcurrent fault occurs when the current limit circuitry has been engaged for longer than the time-out delay set by the timer capacitor. The GATE pin is then immediately pulled low by a 10mA current to GND turning off the MOSFET. After the fault condition has disappeared and a cooldown period has transpired, the GATE pin is allowed to pull back up and turn on the pass transistor. Fault Timer The LT4356 includes an adjustable fault timer pin. Connecting a capacitor from the TMR pin to ground sets the delay timer period before the MOSFET is turned off. The same capacitor also sets the cooldown period before the MOSFET is allowed to turn back on after the fault condition has disappeared. Once a fault condition, either overvoltage or overcurrent, is detected, a current source charges up the TMR pin. The current level varies depending on the voltage drop across the drain and source terminals of the power MOSFET(VDS), which is typically from the VCC pin to the OUT pin. This scheme takes better advantage of the available Safe Operating Area (SOA) of the MOSFET than would a fixed timer current. The timer function operates down to VCC = 5V across the whole temperature range.
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LT4356MP-1/LT4356MP-2 APPLICATIONS INFORMATION
Fault Timer Current The timer current starts at around 2A with 0.5V or less of VDS, increasing linearly to 50A with 75V of VDS during an overvoltage fault (Figure 1). During an overcurrent fault, it starts at 4A with 0.5V or less of VDS but increases to 260A with 80V across the MOSFET (Figure 2). This arrangement allows the pass transistor to turn off faster during an overcurrent event, since more power is dissipated during this condition. Refer to the Typical Performance Characteristics section for the timer current at different VDS in both overvoltage and overcurrent events.
VTMR(V) ITMR = 5A 1.35 1.25 VDS = 75V (ITMR = 50A) VDS = 10V (ITMR = 8A) ITMR = 5A
When the voltage at the TMR pin, VTMR, reaches the 1.25V threshold, the FLT pin pulls low to indicate the detection of a fault condition and provide warning to the load of the impending power loss. In the case of an overvoltage fault, the timer current then switches to a fixed 5A. The interval between FLT asserting low and the MOSFET turning off is given by: t WARNING = CTMR * 100mV 5A
0.50 tFLT = 15ms/F tWARNING = 20ms/F tFLT = 93.75ms/F TOTAL FAULT TIMER = tFLT + tWARNING tWARNING = 20ms/F
TIME
4356mp12 F01
Figure 1. Overvoltage Fault Timer Current
VTMR(V) 1.35 1.25 VDS = 80V (ITMR = 260A) VDS = 10V (ITMR = 35A)
0.50 tFLT = 2.88ms/F
tWARNING = 0.38ms/F tFLT = 21.43ms/F
TIME
TOTAL FAULT TIMER = tFLT + tWARNING
tWARNING = 2.86ms/F
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Figure 2. Overcurrent Fault Timer Current
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LT4356MP-1/LT4356MP-2 APPLICATIONS INFORMATION
This fixed early warning period allows the systems to perform necessary backup or house keeping functions before the power supply is cut off. After VTMR crosses the 1.35V threshold, the pass transistor turns off immediately. Note that during an overcurrent event, the timer current is not reduced to 5A after VTMR has reached 1.25V threshold, since it would lengthen the overall fault timer period and cause more stress on the power MOSFET. As soon as the fault condition has disappeared, a 2A current starts to discharge the timer capacitor to ground. When VTMR reaches the 0.5V threshold, the internal charge pump starts to pull the GATE pin high, turning on the MOSFET. The TMR pin is then actively regulated to 0.5V until the next fault condition appears. The total cooldown timer period is given by: tCOOL * 0.85V C = TMR 2A voltage N-channel MOSFETs. For systems with VCC less than 8V, a logic level MOSFET is required since the gate drive can be as low as 4.5V. The SOA of the MOSFET must encompass all fault conditions. In normal operation the pass transistor is fully on, dissipating very little power. But during either overvoltage or overcurrent faults, the GATE pin is servoed to regulate either the output voltage or the current through the MOSFET. Large current and high voltage drop across the MOSFET can coexist in these cases. The SOA curves of the MOSFET must be considered carefully along with the selection of the fault timer capacitor. Transient Stress in the MOSFET During an overvoltage event, the LT4356 drives a series pass MOSFET to regulate the output voltage at an acceptable level. The load circuitry may continue operating throughout this interval, but only at the expense of dissipation in the MOSFET pass device. MOSFET dissipation or stress is a function of the input voltage waveform, regulation voltage and load current. The MOSFET must be sized to survive this stress. Most transient event specifications use the model shown in Figure 3. The idealized waveform comprises a linear ramp of rise time tr, reaching a peak voltage of VPK and exponentially decaying back to VIN with a time constant of t. A common automotive transient specification has constants of tr = 10s, VPK = 80V and = 1ms. A surge condition known as "load dump" has constants of tr = 5ms, VPK = 60V and = 200ms.
MOSFET Selection The LT4356 drives an N-channel MOSFET to conduct the load current. The important features of the MOSFET are on-resistance RDS(ON), the maximum drain-source voltage V(BR)DSS, the threshold voltage, and the SOA. The maximum allowable drain-source voltage must be higher than the supply voltage. If the output is shorted to ground or during an overvoltage event, the full supply voltage will appear across the MOSFET. The gate drive for the MOSFET is guaranteed to be more than 10V and less than 16V for those applications with VCC higher than 8V. This allows the use of standard threshold
VPK
VIN
tr
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Figure 3. Prototypical Transient Waveform
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LT4356MP-1/LT4356MP-2 APPLICATIONS INFORMATION
MOSFET stress is the result of power dissipated within the device. For long duration surges of 100ms or more, stress is increasingly dominated by heat transfer; this is a matter of device packaging and mounting, and heatsink thermal mass. This is analyzed by simulation, using the MOSFET thermal model. For short duration transients of less than 100ms, MOSFET survival is increasingly a matter of safe operating area (SOA), an intrinsic property of the MOSFET. SOA quantifies the time required at any given condition of VDS and ID to raise the junction temperature of the MOSFET to its rated maximum. MOSFET SOA is expressed in units of watt-squared-seconds (P2t). This figure is essentially constant for intervals of less than 100ms for any given device type, and rises to infinity under DC operating conditions. Destruction mechanisms other than bulk die temperature distort the lines of an accurately drawn SOA graph so that P2t is not the same for all combinations of ID and VDS. In particular P2t tends to degrade as VDS approaches the maximum rating, rendering some devices useless for absorbing energy above a certain voltage. Calculating Transient Stress To select a MOSFET suitable for any given application, the SOA stress must be calculated for each input transient which shall not interrupt operation. It is then a simple matter to chose a device which has adequate SOA to survive the maximum calculated stress. P2t for a prototypical transient waveform is calculated as follows (Figure 4). Let a = VREG - VIN b = VPK - VIN (VIN = Nominal Input Voltage) Then 1 ( b - a )3 tr + 2 3 2 b P t = ILOAD 1 2 b 2 2 2a ln + 3a + b - 4ab a 2
VPK
VREG VIN tr
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Figure 4. Safe Operating Area Required to Survive Prototypical Transient Waveform
Typically VREG VIN and >> tr simplifying the above to P2 t = 1 2 ILOAD 2 ( VPK - VREG ) 2 (W 2s)
For the transient conditions of VPK = 80V, VIN = 12V, VREG = 16V, tr = 10s and = 1ms, and a load current of 3A, P2t is 18.4W2s--easily handled by a MOSFET in a D-pak package. The P2t of other transient waveshapes is evaluated by integrating the square of MOSFET power versus time. Calculating Short-Circuit Stress SOA stress must also be calculated for short-circuit conditions. Short-circuit P2t is given by: P2t = (VIN * VSNS/RSNS)2 * tTMR (W2s) where, VSNS is the SENSE pin threshold, and tTMR is the overcurrent timer interval. For VIN = 14.7V, VSNS = 50mV, RSNS = 12m and CTMR = 100nF P2t is 6.6W2s--less than the transient SOA , calculated in the previous example. Nevertheless, to account for circuit tolerances this figure should be doubled to 13.2W2s. Limiting Inrush Current and GATE Pin Compensation The LT4356 limits the inrush current to any load capacitance by controlling the GATE pin voltage slew rate. An external capacitor can be connected from GATE to ground to slow down the inrush current further at the expense of slower turn-off time. The gate capacitor is set at: C1 = IGATE(UP) IINRUSH * CL
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12
LT4356MP-1/LT4356MP-2 APPLICATIONS INFORMATION
The LTC4356 does not need extra compensation components at the GATE pin for stability during an overvoltage or overcurrent event. With transient input voltage step faster than 5V/s, a gate capacitor, C1, to ground is needed to prevent self enhancement of the N-channel MOSFET. The extra gate capacitance slows down the turn-off time during fault conditions and may allow excessive current during an output short event. An extra resistor, R1, in series with the gate capacitor can improve the turn-off time. A diode, D1, should be placed across R1 with the cathode connected to C1 as shown in Figure 5.
Q1 D1 IN4148W R3 R1 C1 GATE LT4356
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*4.7 INPUT
2N2905A OR BCP53
OUTPUT
R6 100k
D1* BAV99 14 AOUT LT4356S
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* OPTIONAL FOR CURRENT LIMIT
Figure 6. Auxiliary LDO Output with Optional Current Limit
Reverse Input Protection A blocking diode is commonly employed when reverse input potential is possible, such as in automotive applications. This diode causes extra power loss, generates heat, and reduces the available supply voltage range. During cold crank, the extra voltage drop across the diode is particularly undesirable. The LT4356 is designed to withstand reverse voltage without damage to itself or the load. The VCC, SNS, and SHDN pins can withstand up to 60V of DC voltage below the GND potential. Back-to-back MOSFETs must be used to eliminate the current path through their body diodes (Figure 7). Figure 8 shows the approach with a P-channel MOSFET in place of Q2.
VIN 12V D2* SMAJ58CA RSNS 10m Q2 IRLR2908 Q1 IRLR2908 VOUT 12V, 3A CLAMPED AT 16V
Figure 5
Auxiliary Amplifier An uncommitted amplifier is included in the LT4356 to provide flexibility in the system design. With the negative input connected internally to the 1.25V reference, the amplifier can be connected as a level detect comparator with external hysteresis. The open collector output pin, AOUT, is capable of driving an opto or LED. It can also interface with the system via a pull-up resistor to a supply voltage up to 80V. The amplifier can also be configured as a low dropout linear regulator controller. With an external PNP transistor, such as 2N2905A, it can supply up to 100mA of current with only a few hundred mV of dropout voltage. Current limit can be easily included by adding two diodes and one resistor (Figure 6).
Q3 2N3904
R4 R5 10 1M
R3 10
D1 1N4148 7 SNS VCC
R1 59k R7 10k 5 GATE 4 OUT FB
8
2 R2 4.99k
LT4356S 9 14 16 SHDN AOUT IN+ GND 12 *DIODES INC. TMR 1 CTMR 0.1F FLT EN
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10 11
Figure 7. Overvoltage Regulator with N-channel MOSFET Reverse Input Protection
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13
LT4356MP-1/LT4356MP-2 APPLICATIONS INFORMATION
VIN 12V D2* SMAJ58CA RSNS 10m Q2 Si4435 D1 1N5245 15V R6 10k 7 SNS 8 VCC R3 10 5 GATE 4 OUT FB 2 R2 4.99k 9 14 16 LT4356S SHDN AOUT IN
+
Q1 IRLR2908
VOUT 12V, 3A CLAMPED AT 16V
RSNS 10m VIN D2 SMAJ58A
Q1 IRLR2908 CL* 22F
R1 59k 8 R4 383k 9 16 R5 100k FLT 10 11 UNDERVOLTAGE 14 AOUT VCC SHDN IN+
R3 10 7 5 4 SNS GATE OUT FB 2
R1 59k
R2 4.99k LT4356S EN GND 12 TMR 1 FLT
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VCC DC-DC CONVERTER SHDN GND
11 10 FAULT
GND 12 *DIODES INC.
TMR 1 CTMR 0.1F
EN
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*SANYO 25CE22GA CTMR 47nF
Figure 9. Overvoltage Regulator with Low-Battery Detection
Figure 8. Overvoltage Regulator with P-channel MOSFET Reverse Input Protection
Shutdown The LT4356 can be shut down to a low current mode when the voltage at the SHDN pin goes below the shutdown threshold of 0.6V. The quiescent current drops to 7A for the LT4356-1 and 60A for the LT4356-2. The SHDN pin can be pulled up to VCC or below GND by up to 60V without damaging the pin. Leaving the pin open allows an internal current source to pull it up and turn on the part while clamping the pin to 2.5V. The leakage current at the pin should be limited to no more than 1A if no pull-up device is used to help turn it on. Supply Transient Protection The LT4356 is 100% tested and guaranteed to be safe from damage with supply voltages up to 80V. Nevertheless, voltage transients above 100V may cause permanent damage. During a short-circuit condition, the large change in current flowing through power supply traces and associated wiring can cause inductive voltage transients which could exceed 100V. To minimize the voltage transients, the power trace parasitic inductance should be minimized by using wide traces. A small surge suppressor, D2, in Figure 9, at the input will clamp the voltage spikes.
A total bulk capacitance of at least 22F low ESR electrolytic is required close to the source pin of MOSFET Q1. In addition, the bulk capacitance should be at least 10 times larger than the total ceramic bypassing capacitor on the input of the DC/DC converter. Layout Considerations To achieve accurate current sensing, Kelvin connection to the current sense resistor (RSNS in Figure 9) is recommended. The minimum trace width for 1oz copper foil is 0.02" per amp to ensure the trace stays at a reasonable temperature. 0.03" per amp or wider is recommended. Note that 1oz copper exhibits a sheet resistance of about 530/square. Small resistances can cause large errors in high current applications. Noise immunity will be improved significantly by locating resistive dividers close to the pins with short VCC and GND traces. Design Example As a design example, take an application with the following specifications: VCC = 8V to 14V DC with transient up to 80V, VOUT 16V, current limit (ILIM) at 5A, low battery detection at 6V, and 1ms of overvoltage early warning (Figure 9). First, calculate the resistive divider value to limit VOUT to 16V during an overvoltage event: VREG = 1.25V * (R1 + R2) = 16V R2
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14
LT4356MP-1/LT4356MP-2 APPLICATIONS INFORMATION
Set the current through R1 and R2 during the overvoltage condition to 250A. R2 = 1.25V = 5k 250A Finally, calculate R4 and R5 for the 6V low battery threshold detection: 6V = 1.25V * (R4 + R5) R5
Choose 4.99k for R2. R1 =
Choose 100k for R5. = 58.88k R4 =
(16V - 1.25V ) * R2
1.25V
(6V - 1.25V ) * R5
1.25V
= 380k
The closest standard value for R1 is 59k. Next calculate the sense resistor, RSNS, value: RSNS = 50mV 50mV = = 10m ILIM 5A 1ms * 5A = 50nF 100mV
Select 383k for R4. The pass transistor, Q1, should be chosen to withstand the output short condition with VCC = 14V. The total overcurrent fault time is: tOC = 47nF * 0.85V = 0.878ms 45.5A
CTMR is then chosen for 1ms of early warning time: CTMR =
The power dissipation on Q1 equals to: P= 14V * 50mV = 70W 10m
The closest standard value for CTMR is 47nF .
These conditions are well within the Safe Operating Area of IRLR2908.
TYPICAL APPLICATIONS
Wide Input Range 5V to 28V Hot Swap with Undervoltage Lockout
RSNS 20m VIN R6 118k C1 47nF 9 14 16 R7 49.9k GND 12 TMR 1 8 VCC SHDN AOUT IN+ LT4356S-1 FLT EN
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Q1 SUD50N03-10 VOUT 100F R3 10
7 SNS
5 GATE
4 OUT FB 2
10 11
CTMR 1F
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15
LT4356MP-1/LT4356MP-2 TYPICAL APPLICATIONS
24V Overvoltage Regulator Withstands 150V at VIN
VIN 24V Q1 IRF640 R9 1k 1W 7 SNS 8 D2* SMAT70A 9 10 11 SHDN FLT EN GND 12 *DIODES INC. TMR 1
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VOUT CLAMPED AT 32V
R3 10 5 GATE 4 OUT FB 2
R1 118k
VCC
R2 4.99k LT4356S
CTMR 0.1F
Overvoltage Regulator with Undervoltage Lockout
RSNS 20m VIN D2* SMAJ58A R3 10 5 GATE Q1 IRLR2908 VOUT CLAMPED AT 16V
R6 280k
R4 1M
8 VCC
7 SNS
4 OUT FB 2
R1 59k
R5 1M
9 14 16
SHDN AOUT IN+ LT4356S-2
R2 4.99k
R7 100k GND 12 TMR 1
FLT EN
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10 11
*DIODES INC.
CTMR 0.1F
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16
LT4356MP-1/LT4356MP-2 TYPICAL APPLICATIONS
Overvoltage Regulator with Low Battery Detection and Output Keep Alive During Shutdown
1k 0.5W RSNS 10m Q1 IRLR2908 VOUT 12V, 4A CLAMPED AT 16V D1 1N4746A 18V 1W
VIN 12V D2* SMAJ58A R4 402k
R3 10
8
7 SNS VCC
5 GATE
Q2 VN2222 4 OUT FB 2
R1 294k
16 R5 105k 9
LT4356S IN+ SHDN GND 12 TMR 1 CTMR 0.1F AOUT FLT EN
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R2 VDD 24.9k R6 47k 14 10 11 LBO
*DIODES INC.
2.5A, 48V Hot Swap with Overvoltage Output Regulation at 72V and UV Shutdown at 35V
VIN 48V D2* SMAT70A RSNS 15m Q1 FDB3632 R4 140k R6 100k VOUT 48V 2.5A CL 300F C1 6.8nF D1 1N4714 BV = 33V 9 R8 47k LT4356S 10 11 *DIODES INC. FB 2 8 VCC SHDN 7 5 SNS GATE 4 OUT 16 IN+ R7 1M R5 4.02k
R3 10
R1 226k R2 4.02k
FLT EN GND 12 TMR 1 AOUT
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14
PWRGD
CTMR 0.1F
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17
LT4356MP-1/LT4356MP-2 TYPICAL APPLICATIONS
2.5A, 28V Hot Swap with Overvoltage Output Regulation at 36V and UV Shutdown at 15V
VIN 28V D2* SMAT70A RSNS 15m Q1 FDB3632 R4 113k R6 27k VOUT 28V 2.5A CL 300F C1 6.8nF D1 1N4700 BV = 13V 9 R8 47k LT4356S 10 11 *DIODES INC. FB 2 8 VCC SHDN 7 5 SNS GATE 4 OUT 16 IN+ R7 1M R5 4.02k
R3 10
R1 110k R2 4.02k
FLT EN GND 12 TMR 1 AOUT
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14
PWRGD
CTMR 0.1F
Overvoltage Regulator with Reverse Input Protection Up to -80V
VIN 12V Q2 IRLR2908 D2* R4 SMAJ58CA 10 8 D1 1N4148 R7 10k VCC 7 SNS RSNS 10m Q1 IRLR2908 VOUT 12V, 3A CLAMPED AT 16V
Q3 2N3904
R5 1M
R3 10 5 GATE 4 OUT FB
2
R1 59k R2 4.99k
LT4356S 9 14 16 *DIODES INC. SHDN AOUT IN+ GND 12 TMR 1 CTMR 0.1F FLT EN
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10 11
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18
LT4356MP-1/LT4356MP-2 PACKAGE DESCRIPTION
MS Package 10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661)
0.889 0.127 (.035 .005)
5.23 (.206) MIN
3.20 - 3.45 (.126 - .136) 3.00 0.102 (.118 .004) (NOTE 3)
0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT
10 9 8 7 6
0.497 0.076 (.0196 .003) REF
0.254 (.010) GAUGE PLANE
DETAIL "A" 0 - 6 TYP
4.90 0.152 (.193 .006)
3.00 0.102 (.118 .004) (NOTE 4)
12345 0.53 0.152 (.021 .006) DETAIL "A" 0.18 (.007) SEATING PLANE 0.17 -0.27 (.007 - .011) TYP 0.1016 0.0508 (.004 .002)
MSOP (MS) 0307 REV E
1.10 (.043) MAX
0.86 (.034) REF
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.50 (.0197) BSC
S Package 16-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.045 .005 .050 BSC N 16 15 14 .386 - .394 (9.804 - 10.008) NOTE 3 13 12 11 10 9
.245 MIN
N .160 .005 .228 - .244 (5.791 - 6.197) 1 2 3 N/2 N/2 .150 - .157 (3.810 - 3.988) NOTE 3
.030 .005 TYP
RECOMMENDED SOLDER PAD LAYOUT 1
.010 - .020 45 (0.254 - 0.508)
2
3
4
5
6
7
8
.053 - .069 (1.346 - 1.752)
0 - 8 TYP
.008 - .010 (0.203 - 0.254)
.004 - .010 (0.101 - 0.254)
.016 - .050 (0.406 - 1.270)
NOTE: 1. DIMENSIONS IN
.014 - .019 (0.355 - 0.483) TYP
.050 (1.270) BSC
S16 0502
INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LT4356MP-1/LT4356MP-2 TYPICAL APPLICATION
Overvoltage Regulator with Linear Regulator Up to 100mA
Q2 2N2905A RSNS 10m Q1 IRLR2908 C5 10F VOUT 12V, 3A CLAMPED AT 16V R1 59k 2 R2 4.99k 14 9 LT4356S AOUT SHDN GND 12 TMR 1 16 IN+ FLT EN
4356mp12 TA09
2.5V, 100mA
VIN 12V D2* SMAJ58A
R6 100k
7 SNS 8 VCC
R3 10 5 GATE
4 OUT FB
R4 249k R5 249k
C3 47nF
10 11
*DIODES INC.
CTMR 0.1F
RELATED PARTS
PART NUMBER LT1641-1/LT1641-2 LTC1696 LTC1735 LTC1778 LTC2909 LTC2912/LTC2913 LTC2914 DESCRIPTION Positive High Voltage Hot SwapTM Controllers Overvoltage Protection Controller High Efficiency Synchronous Step-Down Switching Regulator No RSENSETM Wide Input Range Synchronous Step-Down Controller Triple/Dual Inputs UV/OV Negative Monitor Single/Dual UV/OV Voltage Monitor Quad UV/OV Monitor COMMENTS Active Current Limiting, Supplies From 9V to 80V ThinSOTTM Package, 2.7V to 28V Output Fault Protection, 16-Pin SSOP Up to 97% Efficiency, 4V VIN 36V, 0.8V VOUT (0.9)(VIN), IOUT Up to 20A Pin Selectable Input Polarity Allows Negative and OV Monitoring Ads UV and OV Trip Values, 1.5% Threshold Accuracy For Positive and Negative Supplies 4V VIN 36V, 0.8V VOUT 14V 4V VIN 36V, 0.8V VOUT 10V, 80A Quiescent Current Single Channel LTC3827/LTC3827-1 4V VIN 60V, 1.23V VOUT 36V, 120A Quiescent Current Dual 180 Phased Controllers, VIN 4V to 24V, 97% Duty Cycle, 4mm x 4mm QFN-28, SSOP-28 Packages Foldback Current Limiting, Open-Circuit and Overcurrent Fault Output, Up to 80V Supply Wide Operating Range 8.5V to 80V External N-channel MOSFETs Replace ORing Diodes, 0V to 18V Operation Controls Two N-channel MOSFETs, 1s Turn-Off, 80V Operation Controls Two N-channel MOSFETs, 0.5s Turn-Off, 80V Operation
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LTC3727/LTC3727-1 2-Phase, Dual, Synchronous Controller LTC3827/LTC3827-1 Low IQ, Dual, Synchronous Controller LTC3835/LTC3835-1 Low IQ, Synchronous Step-Down Controller LT3845 Low IQ, Synchronous Step-Down Controller LT3850 LT4256 LTC4260 LTC4352 LTC4354 LTC4355 Dual, 550kHz, 2-Phase Sychronous Step-Down Controller Positive 48V Hot Swap Controller with Open-Circuit Detect Positive High Voltage Hot Swap Controller with ADC and I2C Ideal MOSFET ORing Diode Negative Voltage Diode-OR Controller Positive Voltage Diode-OR Controller
Hot Swap, No RSENSE and ThinSOT are trademarks of Linear Technology Corporation.
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 0509 * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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