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SP6121 Datasheet(PDF) 4 Page - Sipex Corporation |
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SP6121 Datasheet(HTML) 4 Page - Sipex Corporation |
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4 / 16 page  Date: 5/25/04 SP6121 Low Voltage, Synchronous Step Down PWM Controller © Copyright 2004 Sipex Corporation 4 FUNCTIONAL DIAGRAM + - - + - + Synchronous Driver PWM Logic S Q R Reset Dominant S Q R ISET VCC ISENSE Reference 5 4 3 6 1.25V UVLO FAULT GND 2 7 NDRV 8 DRIVER ENABLE RESET Dominant PWM COMP FAULT + - X 3.3 PDRV PDRV VCC 5µA 300mV SHUTDOWN GM ERROR AMP Over Current (Gated S&H) 2.79V ON 2.69V OFF 30µA (3300 ppm/°C) COMP SHUTDOWN F = 500kHz 750mV RAMP 0.4V/ms SOFTSTART VFB COMP 1 - + - + 1V 530mV (3300 ppm/°C) + - General Overview The SP6121 is a constant frequency, voltage mode, synchronous PWM controller designed for low voltage, DC/DC step down converters. It is intended to provide complete control for a high power, high efficiency, precisely regulated output voltage from a highly integrated 8-pin solution. The internal free-running oscillator accurately sets the PWM frequency at 500kHz without requiring any external elements and allows the use of physically small, low value external com- ponents without compromising performance. A transconductance amplifier is used for the error amplifier, which compares an attenuated sample of the output voltage with a precision reference voltage. The output of the error amplifier (COMP), is compared to a 0.75V peak-to-peak ramp waveform to provide PWM control. The COMP pin provides access to the output of the error amplifier and allows the use of external components to stabilize the voltage loop. High efficiency is obtained through the use of synchronous rectification. Synchronous regula- tors replace the catch diode in the standard buck converter with a low RDS(ON) N-channel MOSFET switch allowing for significant effi- ciency improvements. The SP6121 includes two fast MOSFET drivers with internal non-overlap circuitry and drives a complementary pair of power transistors, P-channel on the high side, and N-channel on the low side. The use of a P- channel high side device minimizes complexity and external component count by eliminating the need for a charge pump that would otherwise be required to fully enhance an N-channel de- vice. It also allows inherent 100% duty cycle for low dropout operation in the event of a low input supply voltage condition. The SP6121 includes an internal 0.4V/ms soft- start circuit that provides controlled ramp up of the output voltage, preventing overshoot and inrush current at power up. Current limiting is implemented by monitoring the voltage drop across the RDS(ON) of the high side P-channel MOSFET while it is conducting, thereby eliminating the need for an external sense resistor. The over-current comparator has a built-in threshold of 160mV that can be pro- grammed to higher values using a single exter- nal resistor, connected to the ISET pin, whose THEORY OF OPERATION |
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