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SI9121
New Product
Vishay Siliconix
High-Voltage, Non-Isolated Buck-Boost Converter for ISDN Digital Phones
FEATURES
D Fixed +5-V or +3.3-V Output D Integrated Floating Feedback Amplifier D On-Chip 70-V, 1.5-W N-Channel MOSFET Switch D Integrated High Voltage Start-Up Circuit, with VCC Regulator D D D D D -10-V to -60-V Input Voltage Range 95-kHz PWM Operation Integrated Soft-Start and Oscillator High Efficiency Over Full Load Range Under Voltage Lockout D Current Mode Control D Hiccup Mode Short Circuit Protection D Thermal Shutdown D SOIC-8 Narrow-Body Package
DESCRIPTION
The SI9121 simplifies the -48-V to +5-V or +3.3-V converter design for ISDN application by integrating the floating feedback error amplifier providing direct output voltage regulation. This approach eliminates the need for an external shunt regulator. The SI9121 also integrates a high voltage depletion mode MOSFET which allows the converter to be powered directly from the high input bus voltage without requiring an external start-up circuit. Combined with simple magnetic design due to its non-isolated topology, the SI9121 provides a one-chip solution for complete ISDN power supply. In order to reduce external component count, the SI9121 has a fully integrated 95-kHz oscillator and soft-start circuit.
The SI9121 is available in SOIC-8 pin package, and is available in either +5-V or +3.3-V fixed output options (SI9121DY-5 or SI9121DY-3, respectively). In order to satisfy the stringent ambient temperature requirements in many applications, the SI9121 is rated to the industrial temperature range of -40_C to 85_C.
FUNCTIONAL BLOCK DIAGRAM
GND
VCC Regulator VCC
Reference Generator VOUT BYPASS LX Control VOUT +5 V/400 mA or +3.3 V/400 mA
COMP
VNEG
CS
VNEG -48 V
Document Number: 71112 S-00802--Rev. B, 08-May-00
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1
SI9121
Vishay Siliconix
New Product
ABSOLUTE MAXIMUM RATINGS (ALL VOLTAGES REFERENCED TO GND = 0 V)
VNEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -63 V VCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VNEG -0.3 V to VCC + 0.3 V VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VNEG + 13.2 V ILX (peak current ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 A VOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 V Bypass, CS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VNEG -0.3 V to VCC +0.3 V (VLX - VCS ) internal power MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 V Storage Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -65 to 150_C Operating Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150_C Power Dissipation (Package)a 8-Pin SOIC (Y Suffix)b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.25 W Thermal Impedance (QJA)a 8-Pin SOIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 _C/W Notes a. Device mounted with all leads soldered or welded to PC board. b. Derate 10 mW/_C above 25_C.
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
RECOMMENDED OPERATING RANGE (ALL VOLTAGES REFERENCED TO GND = 0 V)
VNEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -10 V to -60 V VCC (internally regulated) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VNEG + 8.5 V VCC (externally supplied) . . . . . . . . . . . . . . . . VNEG + 9.5 V to VNEG + 12.0 V Digital Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 V to VCC
RECOMMENDED EXTERNAL COMPONENTS ( SEE TYPICAL APPLICATIONS CIRCUIT )
L = 68 mH, COUT = 220 mF // 0.1 mF, CIN = 33 mF, CBYPASS = 0.1 mF, CVCC = 1 mF, RSENSE = 0.25 W, 0.5 W
SPECIFICATIONSa (All Voltages Are With Respect To VNEG Unless Otherwise Specified)
Test Conditions (Internally Regulated) Unless Otherwise Specified Parameter Symbol
VNEG = -10 to -60 V
Limits
-40 to 85_C
Tempb
Minc
Typd
Maxc
Unit
Output Voltage (with respect to GND = 0 V)
+5-V Converter +3.3-V Converter VOUT 10 mA Line Regulation (with respect to GND = 0 V)
Line Regulation -60 V v VNEG v -40 V Full 1 %
VCC (Internal Regulator)
VCC Bias Voltage VCC Full 7.5 8.5 9.5 V
UVLO
Under Voltage Lockout Hysteresis VCC - VNEG DV Turn-On Full Room 6.6 7.6 0.6 8.7 V
Soft-Start
Error Amplifier Start-Up Current ISS VOUT = 0 V Room 10 mA
Oscillator
Switching Frequency fOSC Room 80 95 110 kHz
Error Amplifier
Transconductance Clamp Voltage gm VCL Internal Error Amplifier Output Clamp Voltage Room Room 10 15 3.5 20 umho V
Current Limit
Threshold Voltage VCS Full 0.57 0.67 0.77 V
MOSFET Switch
N-Channel MOSFET rDS(on) Room 1.5 2.5 W
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Document Number: 71112 S-00802--Rev. B, 08-May-00
SI9121
New Product
Vishay Siliconix
SPECIFICATIONSa (All Voltages Are With Respect To VNEG Unless Otherwise Specified)
Test Conditions (Internally Regulated) Unless Otherwise Specified Parameter Supply
Supply Current (Internally Regulater) Supply Current (External VCC Applied) VOUT supply Current Start-Up Current IGND ICC IOUT ISTART GND to VNEG VCC to VNEG +10 V; VNEG >-20 V VOUT to VNEG VCC = 0 V Full Full Full Full 1.2 1.5 0.2 5 1.5 2.0 0.3 30 mA A
Limits
-40 to 85_C
Symbol
VNEG = -10 to -60 V
Tempb
Minc
Typd
Maxc
Unit
Thermal Shutdown
Thermal Shutdown Temperature Thermal Hysteresis TOTP THYS 170 25 _C
Efficiency
Efficiency 400-mA Output, VNEG = -48 V +5 V +3.3 V Room Room 77 73 %
Notes a. Refer to PROCESS OPTION FLOWCHART for additional information. b. Room = 25_C, Full = as determined by the operating temperature suffix. c. The algebraic convention whereby the most negative value is a minimum and the most positive a maximum. d. Typical values are for DESIGN AID ONLY, not guaranteed nor subject to production testing.
DETAILED BLOCK DIAGRAM
4 VOUT 5 R1* - GM + R2* + - VREF 1.25 V + - - + H 10 mA VCC Soft Start 2 mA VNEG COMP
3.5 V
L
Hiccup Discharge Mode
GND
6
1.5 V
OSC 7 VCC 0.6 V - + Low Side Error Amp - + RQ S 3 BYPASS Bias/ Reference Circuit OCL + 0.67 V - 50% Max. PWM Duty Cycle
OTP 8 LX
-VIN
1 - + CS 0.6-V Hysteresis *R1 and R2 are internal voltage setting resistors used to set output voltage to fixed 3.3 V or 5 V.
- + 2 VNEG Document Number: 71112 S-00802--Rev. B, 08-May-00 8.5 V + -
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SI9121
Vishay Siliconix
New Product
TYPICAL CHARACTERISTICS (INTERNALLY REGULATED, 25_C UNLESS NOTED)
5-V VOUT vs. Temperature
5.15 3.45
3.3-V VOUT vs. Temperature
5.10
3.40
5.05 V OUT (V) V OUT (V) -20 0 20 40 60 80 100
3.35
5.00
3.30
4.95
3.25
4.90
3.20
4.85 -40
3.15 -40
-20
0
20
40
60
80
100
Temperature (_C)
Temperature (_C)
rDS(on) vs. Temperature
2.5 105
Frequency vs. Temperature
r DS(on) - On-Resistance ( W )
2.0 Frequency (kHz) -20 0 20 40 60 80 100
100
1.5
95
1.0
90
0.5 -40
85 -40
-20
0
20
40
60
80
100
Temperature (_C)
Temperature (_C)
Supply Current vs. VNEG
1.4
Supply Current (I GND )
1.3 25_C 1.2 85_C -40_C 1.1
1 -60
-50
-40
-30
-20
-10
VNEG - (V)
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Document Number: 71112 S-00802--Rev. B, 08-May-00
SI9121
New Product
Vishay Siliconix
TYPICAL CHARACTERISTICS (INTERNALLY REGULATED, 25_C UNLESS NOTED)
90 80 70 Efficiency (%)
Output Load vs. Efficiency (SI9121DY-3) No VCC Winding
VIN = 10 V
90 80 70 Efficiency (%) 60 50 40 30 20 10 0 10
Output Load vs. Efficiency (SI9121DY-5) No VCC Winding
10 VIN
60 50 40 30 20 10 0 10 VIN = 48 V VIN = 60 V
48 VIN
60 VIN
100 IOUT (mA)
1000
100 IOUT (mA)
1000
90 80 70
Output Load vs. Efficiency (SI9121DY-3) With VCC Winding
VNEG = -10 V
90 80 70
Output Load vs. Efficiency (SI9121DY-5) With VCC Winding
48 VIN 10 VIN 60 VIN
Efficiency (%)
VNEG = -48 V
Efficiency (%)
60 50 40 30 20 10 0 10
60 50 40 30 20 10 0 10
VNEG = -60 V
100 IOUT (mA)
1000
100 IOUT (mA)
1000
Document Number: 71112 S-00802--Rev. B, 08-May-00
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SI9121
Vishay Siliconix
PIN CONFIGURATION ORDERING INFORMATION
SOIC-8
CS VNEG BYPASS COMP 1 2 3 4 Top View 8 LX VCC GND VOUT
New Product
Part Number
SI9121DY-5-T1 SI9121DY-3-T1
Temperature Range
Package
Tape and Reel
SI9121
7 6 5
-40 to 85_C 40 SI9121DY-5 SI9121DY-3 Bulk
Eval Kit
SI9121DB-5 SI9121DB-3
Temperature Range
-10 to 70_C
Board Type
Surface Mount and Thru-Hole Th H l
PIN DESCRIPTION
Pin Number
1 2 3 4 5 6 7 8
Name
CS VNEG BYPASS COMP VOUT GND VCC LX Negative supply voltage (-10 V to -60 V)
Function
Current sense pin to detect the inductor current for current mode control and over current protection +1.5-V bandgap reference. Decouple with 0.1 mF capacitor. Compensation node to stabilize the converter Output voltage feedback connected to the PWM summing comparator Low impedance system ground Internally generated supply voltage for the internal circuit and MOSFET drive circuit. Decouple with an external bypass capacitor. Inductor connection node
DETAILED DESCRIPTION
Start-Up The UVLO circuit prevents the internal circuits from turning on if VCC is less than 7.6 V (typical) above the negative supply voltage at VNEG pin. With a typical hysteresis of 0.6 V, the controller is continuously powered on until the VCC voltage drops below 7.0 V. This hysteresis prevents the converter from oscillating during the start-up phase and unintentionally locking up the system. Once (VCC - VNEG) exceeds the UVLO threshold the internal reference, oscillator, and soft-start circuits are enabled. Soft-Start PWM Mode and Current Limit The SI9121 has an on-chip soft-start circuit which utilizes the error amplifier external compensation network to ramp the output NMOS transistor current limit which, in turn, allows the output voltage to rise gradually without excessive overshoot. The soft-start circuit is enable once the VCC voltage exceeds the UVLO threshold. For the recommended frequency
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compensation components (see Typical Application Circuit) the soft-start time is approximately 10 ms. Oscillator The oscillator is designed to operate at a nominal frequency of 95 kHz with no external components. The 95-kHz operating frequency allows the converter to operate in PWM mode during the full load condition even though the duty cycle is very low. The 95kHz switching frequency also allows the converter to operate at optimal efficiency without a large output inductor and capacitor.
The SI9121 is a current-mode converter designed to operate in PWM mode. It features pulse-by-pulse peak current limiting such that when the peak current sensed voltage on the CS pin is greater than 0.67 V the switch is turned off for the remainder of the clock cycle.
Document Number: 71112 S-00802--Rev. B, 08-May-00
6
SI9121
New Product
DETAILED DESCRIPTION
Bypass The bypass voltage of the SI9121 is set at a particular positive reference relative to the VNEG pin. The bypass voltage is used to set an accurate voltage and bias current for the on-chip oscillator and soft-start circuits. The 0.1-mF ceramic decoupling capacitor is recommended between the bypass and VNEG. No other components should be connected to this pin. Error Amplifier With a -48-V bus voltage, the converter is referenced to the -48-V (VNEG) node as its system ground. However, the +5-V or +3.3-V output is referenced to the GND pin, which is normally connected to 0 V. To regulate this output voltage, the SI9121 has an on-chip error amplifier which continuously monitors the output voltage and compares it to a reference voltage. This difference signal is level-shifted to the low side circuit to control the power switch duty-cycle and hence the regulation of the output voltage. Frequency compensation for the error amplifier is achieved by connecting an external network between the COMP pin and the VNEG pin. VCC Regulator VCC is an internally generated bias supply voltage which should be externally bypassed with a 0.1-mF capacitor connected to the negative supply voltage, VNEG. No load current should be drawn from the VCC pin. VCC may be supplied from an external source of 9.5-V to 12-V referenced to VNEG. In this configuration, the internal VCC regulator is disabled when the VCC receives 9.5-V, or greater, above VNEG. Thermal Shutdown The SI9121 also includes thermal shutdown which shuts down the device when junction temperature exceeds 170_C due to over heating. In thermal shutdown once the die temperature cools to below 145_C the regulator is enabled. If the die temperature is excessive due to high package power If an output short-circuit occurs, the device immediately enters a cycle-by-cycle current limiting mode. As long as the thermal shutdown is not activated then the SI9121 automatically determines whether the internal hiccup timer should be started by monitoring the COMP pin. If VCOMP exceeds an upper threshold voltage (approximately 3.5 V) the timer is started and the external network at the COMP pin is discharged by an internal 2-mA current sink until VCOMP reaches approximately 1.5 V. (Note: all voltages are with respect to VNEG). At this point, the circuit reverts to the normal soft-start mode, whereby the COMP network is charged by its internal soft-start 10-mA current source and the circuit will attempt to start up in the normal manner. However, if the output short circuit is still present, the converter will again enter the cycle-by-cycle current limiting mode until the COMP pin voltage reaches 3.5 V, whereupon this cycle repeats until the short circuit is removed. The duty cycle imposed by the hiccup timer allows the SI9121 to handle continuous short-circuit without damage as long as the recommended component values shown in the Typical Application Circuit are used. MOSFET Switch The low-side n-channel MOSFET switch is integrated to provide optimum performance and to minimize the overall converter size. The typical 1.5-W rDS(on) of the MOSFET allows the converter to deliver up to 2 W of output power. dissipation the regulator's thermal circuit will continue to pulse the regulator on and off. This is called thermal cycling. Hiccup Mode Short Circuit Protection In addition to the thermal shutdown and the cycle-by-cycle current limiting features already described, the SI9121 has a built-in hiccup mode timer to handle a continuous output short-circuit and to automatically restart the device when the short-circuit is removed.
Vishay Siliconix
Document Number: 71112 S-00802--Rev. B, 08-May-00
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SI9121
Vishay Siliconix
TYPICAL APPLICATION CIRCUIT
New Product
GND
L1 68 mH + 6 GND 3 7 2 + 33 mF 80 V C4 0.1 mF C5* 0.1 mF C1 1 mF C6 BYPASS VCC VNEG LX VOUT COMP CS 5 4 1 8
220 mF C2
0.1 mF* C3
10MQ100N
VOUT +5 V/400 mA or +3.3 V/400 mA
SI9121DY-5 or SI9121DY-3
R1 0.25 W 1/ W 2
180 pF C7
R2 75 kW (27 kW for SI9121DY-3) + 0.1 mF C8
VNEG -10 to -60 V *Optional
FIGURE 1. Typical Applications Circuit
Silk Screen
GND 68 mH Np C2* 0.1 mF
T1
GND
C1 33 mF 80 V
+
Ns
C7 220 mF 10 V +
C8* 0.1 mF
D2 BAS21 1 CS 2 3 4 C5 0.1 mF VNEG BYPASS COMP COIL VCC GND VOUT 7 6 5 8
D1 10MQ100N
+VOUT
+5 V @ 400 mA
R1 0.25 1/ W 2
C3 0.1 mF
C4 180 pF
SI9121DY-5
C6 1 mF
R2 75 kW (27 kW for SI9121DY-3) -VIN
*Optional
-10 to -60 V
FIGURE 2. SI9121 Application with External Vcc Through Winding
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Document Number: 71112 S-00802--Rev. B, 08-May-00

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