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| Final Electrical Specifications LTC1649 3.3V Input High Power Step-Down Switching Regulator Controller May 1998 FEATURES s s s s DESCRIPTION The LTC(R)1649 is a high power, high efficiency switching regulator controller optimized for use with very low supply voltages. It operates from 2.7V to 5V input, and provides a regulated output voltage from 1.26V to 2.5V at up to 20A load current. A typical 3.3V to 2.5V application features efficiency above 90% from 1A to 10A load. The LTC1649 uses a pair of standard 5V logic-level N-channel external MOSFETs, eliminating the need for expensive P-channel or super-low-threshold devices. The LTC1649 shares its internal switching architecture with the LTC1430, and features the same 1% line, load and temperature regulation characteristics. Current limit is user-adjustable without requiring an external low-value sense resistor. The LTC1649 uses a 200kHz switching frequency and voltage mode control, minimizing external component count and size. Shutdown mode drops the quiescent current to below 10A. The LTC1649 is available in the 16-pin narrow SO package. , LTC and LT are registered trademarks of Linear Technology Corporation. s s s s High Power 3.3V to 1.XV-2.XV Switching Regulator Controller: Up to 20A Output All N-Channel External MOSFETs Provides 5V MOSFET Gate Drive with 3.3V Input Constant Frequency Operation Minimizes Inductor Size Excellent Output Regulation: 1% Over Line, Load and Temperature Variations High Efficiency: Over 90% Possible No Low-Value Sense Resistor Needed Available in 16-Lead SO Package APPLICATIONS s s s s 3.3V Input Power Supply for Low Voltage Microprocessors and Logic Low Input Voltage Power Supplies High Power, Low Voltage Regulators Local Regulation for Multiple Voltage Distributed Power Systems TYPICAL APPLICATION VIN 3.3V MBR0530 50k 1F Q1, Q2 IRF7801 TWO IN PARALLEL PVCC1 22 PVCC2 VCC IMAX LTC1649 SHUTDOWN 10F RC 7.5k CC 0.01F C1 220pF 0.1F G1 1k IFB G2 FB VIN C+ 1F C- CPOUT Q3 IRF7801 R1 12.4k + SHDN COMP SS GND + MBR0530 10F 0.33F IRF7801 = INTERNATIONAL RECTIFIER MBRO530 = MOTOROLA Figure 1. 3.3V to 2.5V, 15A Converter 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. U U U + CIN 3300F LEXT 1.2H VOUT 2.5V @15A + COUT 4400F R2 12.7k 1649 Schem 1 LTC1649 ABSOLUTE MAXIMUM RATINGS (Note 1) PACKAGE/ORDER INFORMATION TOP VIEW G1 1 PVCC1 2 GND 3 FB 4 SHDN 5 SS 6 VIN 7 C- 8 16 G2 15 PVCC2 14 VCC 13 IFB 12 IMAX 11 COMP 10 CPOUT 9 C+ Supply Voltage VIN ........................................................................................... 6V VCC ........................................................................................... 9V PVCC1, 2 ................................................................................ 13V Input Voltage IFB ........................................................................ -0.3V to 18V C+, C- .................................................. -0.3V to (VIN + 0.3V) All Other Inputs ........................-0.3V to (VCC + 0.3V) Operating Temperature Range ..................... 0C to 70C Storage Temperature Range .................. -65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C ORDER PART NUMBER LTC1649CS S PACKAGE 16-LEAD PLASTIC SO TJMAX = 150C, JA = 110C/ W Consult factory for Industrial and Military grade parts. ELECTRICAL CHARACTERISTICS SYMBOL VIN VFB VCPOUT IIN IPVCC1, 2 fCP fOSC VIH VIL IIN gmV gmI IIMAX ISS tr, tf tNOV DCMAX PARAMETER Minimum Supply Voltage Feedback Voltage Charge Pump Output Voltage Supply Current (VIN) Supply Current (PVCC1, 2) Internal Charge Pump Frequency Internal PWM Oscillator Frequency SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Current Error Amplifier Transconductance ILIM Amplifier Transconductance IMAX Sink Current Soft Start Source Current Driver Rise/Fall Time Driver Non-Overlap Time Maximum Duty Cycle VIN = 3.3V, TA = 25C unless otherwise noted. (Note 2) MIN q q q q CONDITIONS Figure 1 (Note 3) Figure 1 Figure 1 VSHDN = VCC, ILOAD = 0 VSHDN = 0V PVCC = 5V, VSHDN = VCC (Note 4) VSHDN = 0V ICPOUT = 20mA (Note 5) q q q q TYP 1.265 5 3 10 1.5 0.1 700 MAX 1.28 5.2 5 25 UNITS V V V mA A mA A kHz 2.7 1.25 4.8 140 2.4 200 260 0.8 0.01 650 1300 1 Mho Mho 16 -16 250 250 A A ns ns % (Note 6) VIMAX = VCC VSS = 0V PVCC1 = PVCC2 = 5V PVCC1 = PVCC2 = 5V VCOMP = VCC 25 q q 8 -8 12 -12 80 130 95 The q denotes specifications which apply over the full operating temperature range. Note 1: Absolute Maximum Ratings are those values beyond which the life of a part may be impaired. Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground unless otherwise specified. Note 3: Maximum Duty Cycle limitations will limit the output voltage obtainable at very low supply voltages. Note 4: Supply current at PVCC1 and PVCC2 is dominated by the current needed to charge and discharge the external MOSFET gates. This current will vary with the operating voltage and the external MOSFETs used. Note 5: Under normal operating conditions, the charge pump will skip cycles to maintain regulation and the apparent frequency will be lower than 700kHz. Note 6: The ILIM amplifier can sink but not source current. Under normal (not current limited) operation, the ILIM output current will be zero. 2 U W U U WW W kHz V V A LTC1649 PIN FUNCTIONS G1 (Pin 1): Driver Output 1. Connect this pin to the gate of the upper N-channel MOSFET, Q1. This output will swing from PVCC1 to GND. G1 will always be low when G2 is high. In shutdown, G1 and G2 go low. PVCC1 (Pin 2): Power VCC for Driver 1. This is the power supply input for G1. G1 will swing from PVCC1 to GND. PVCC1 must be connected to a potential of at least VIN + VGS(ON)(Q1). This potential can be generated using a simple charge pump connected to the switching node between the two external MOSFETs as shown in Figure 1. GND (Pin 3): System Ground. Connect to a low impedance ground in close proximity to the source of Q2. The system signal and power grounds should meet at only one point, at the GND pin of the LTC1649. FB (Pin 4): Feedback. The FB pin is connected to the output through a resistor divider to set the output voltage. VOUT = VREF [1 + (R1/R2)]. SHDN (Pin 5): Shutdown, Active Low. A TTL compatible LOW level at SHDN for more than 50s puts the LTC1649 into shutdown mode. In shutdown, G1, G2, COMP and SS go low, and the quiescent current drops to 25A max. CPOUT remains at 5V in shutdown mode. A TTL compatible HIGH level at SHDN allows the LTC1649 to operate normally. SS (Pin 6): Soft Start. An external capacitor from SS to GND controls the startup time and also compensates the current limit loop, allowing the LTC1649 to enter and exit current limit cleanly. VIN (Pin 7): Charge Pump Input. This is the main low voltage power supply input. VIN requires an input voltage between 3V and 5V. Bypass VIN to ground with a 1F ceramic capacitor located close to the LTC1649. C- (Pin 8): Flying Capacitor, Negative Terminal. Connect a 1F ceramic capacitor from C- to C+. C+ (Pin 9): Flying Capacitor, Positive Terminal. CPOUT (Pin 10): Charge Pump Output. CPOUT provides a regulated 5V output to provide power for the internal switching circuitry and gate drive for the external MOSFETs. CPOUT should be connected directly to PVCC2 in most applications. At least 10F of reservoir capacitance to ground is required at CPOUT. This requirement can usually be met by the bypass capacitor at PVCC2. COMP (Pin 11): External Compensation. The COMP pin is connected directly to the output of the internal error amplifier and the input of the PWM generator. An RC network is used at this node to compensate the feedback loop to provide optimum transient response. IMAX (Pin 12): Current Limit Set. IMAX sets the threshold for the internal current limit comparator. If IFB drops below IMAX with G1 on, the LTC1649 will go into current limit. IMAX has an internal 12A pull-down to GND. The voltage at IMAX can be set with an external resistor to the drain of Q1 or with an external voltage source. IFB (Pin 13): Current Limit Sense. Connect to the switched node at the source of Q1 and the drain of Q2 through a 1k resistor. The resistor is required to prevent voltage transients at the switched node from damaging the IFB pin. IFB can be taken up to 18V above GND without damage. VCC (Pin 14): Internal Power Supply. VCC provides power to the feedback amplifier and switching control circuits. VCC is designed to run from the 5V supply provided by CPOUT. VCC requires a 10F bypass capacitor to GND. PVCC2 (Pin 15): Power VCC for Driver 2. This is the power supply input for G2. G2 will swing from PVCC2 to GND. PVCC2 must be connected to a potential of at least VGS(ON)(Q2). This voltage is usually supplied by the CPOUT pin. PVCC2 requires a bypass capacitor to GND; this capacitor also provides the reservoir capacitance required by the CPOUT pin. G2 (Pin 16): Driver Output 2. Connect this pin to the gate of the lower N-channel MOSFET, Q2. This output will swing from PVCC2 to GND. G2 will always be low when G1 is high. In shutdown, G1 and G2 go low. U U U 3 LTC1649 APPLICATIONS INFORMATION The LTC1649 shares the bulk of its circuitry with the LTC1430, and many of the applications shown in the LTC1430 datasheet apply equally to the LTC1649. The significant difference is the LTC1649 includes an onboard charge pump to boost a 3.3V input supply to a level adequate to fully enhance 5V logic-level external MOSFETs. In exchange, the LTC1649 gives up the adjustable frequency control and the fixed 3.3V output options of the LTC1430. The LTC1649 runs at an internally fixed 200kHz clock frequency, and requires an external resistor divider at the FB pin to set the output voltage. The LTC1649 features an internal charge pump that provides a regulated 5V output at the CPOUT pin with supply voltages at the VIN pin as low as 2.7V. This output is used to power the LTC1649 output drivers to provide 5V drive to the external MOSFETs. A typical application has the CPOUT pin connected directly to the PVCC2 pin, to the VCC pin through an RC filter, and to the PVCC1 pin via a simple charge pump at the SW pin, as shown in Figure 1 on the first page of this data sheet. The CPOUT pin can typically provide 5V at 50mA with a 3.3V supply at VIN, enough to drive multiple external MOSFETs in parallel. Input voltages below 3.3V reduce the current capacity at CPOUT, reducing the total gate capacitance the LTC1649 can drive. The circuit shown in Figure 1 on the first page of this data sheet will continue to adequately drive typical external MOSFETs with as little as 2.7V at VIN. The LTC1649 features a shutdown mode that reduces supply current below 25A when the SHDN pin is taken low. In shutdown, the external MOSFET drivers both go low, keeping the external MOSFETs off and isolating the output from the input supply. The CPOUT pin remains regulated at 5V in shutdown, and can be used as a keepalive supply if desired. Note that any current drawn from the CPOUT pin adds to the quiescent current in shutdown, and subtracts from the current available to drive the external MOSFETs if the load remains connected while the LTC1649 is active. As with any switching regulator controller, the LTC1649 is sensitive to layout and external component parasitics, and requires a carefully designed PCB layout to provide optimum performance. External component selection is also critical to proper operation. Switching regulators are notorious for unusual modes of operation and erratic regulation when built with poorly chosen components. The LTC1430 datasheet provides extensive advice on proper switching regulator controller layout and design, and the LTC Applications Department is always ready to help at (408) 432-1900. Recommended component lists and sample layouts are available to aid in PCB design. RELATED PARTS PART NUMBER LTC1430 LTC1553 LTC1517-5 DESCRIPTION High Power Step-Down Switching Regulator Controller High Power Switching Regulator with Digital Output Voltage Control Micropower, Regulated 5V Charge Pump in a 5-Pin SOT-23 Package COMMENTS 5V to 1.X - 3.X @10A 1.8V to 3.5V Supply for Pentium(R)II Low Power 3.3V to 5V Step-Up Converter Pentium is a registered trademark of Intel Corporation. 4 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com U W U U 1649i LT/TP 0598 4K * PRINTED IN USA (c) LINEAR TECHNOLOGY CORPORATION 1998 |
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