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SC4905AMSTR Datasheet(PDF) 10 Page - Semtech Corporation |
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SC4905AMSTR Datasheet(HTML) 10 Page - Semtech Corporation |
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10 / 20 page  SC4905A/B POWER MANAGEMENT PRELIMINARY 2002 Semtech Corp. www.semtech.com 10 GATE DRIVERS OUT is a CMOS gate drive output stage that is supplied from REF and provides a peak source/sink current of about 1mA. The output stage is capable of driving the logic input of external MOSFET Drivers and is switched at the oscillator frequency. When the voltage on the RC pin is rising,, the output is high. It should be noted that if high speed/high current drivers such as the SC1301 are used, careful layout must be fol- lowed in order to minimize stary inductance, which might cause negative voltages at the output of the drivers. This negative voltage can be clamped to reasonable level by placing a small Schottky diode directly at the output of the driver as shown below. VDD U1 5+"' # 4 5 3 2 10 1 6 7 8 9 DMAX RC VFF FB REF VDD SYNC ILIM GND OUT C22 VDD REF Mosfet Gate D11 U2 5+ ! ) 1 4 3 2 5 C18 D8 C17 R13 SOFT START During start up of the converter, the discharged output capacitor, and the load current demand large supply cur- rent requirements. To avoid this a soft start scheme is usually implemented where the duty cycle of the regula- tor is gradually increased from 0% until the soft start duration is elapsed. Programmable soft start duration can implemented exter- nally by utilizing a simple external circuitry shown below. SC4905 U1 4 5 3 2 1 6 7 8 9 DMAX RC VFF FB REF VDD SYNC ILIM GND OUT 56.2K Csoft start REF MOCD207 3 4 6 5 C30 R22 1.1k Approximate soft start duration can be calculated as be- low: 22 R C T SoftStart SoftStart × ≅ Application Information (Cont.) FEED BACK The error signal from the output of an external Error am- plifier such as SC431 or SC4431 is applied to the invert- ing input of the PWM comparator at the FB pin either directly or via an opto coupler for the Isolated applica- tions. For best stability keep the FB trace length as short as possible. C35 22n, 16V R23 5.1k Secondary Supply C31 22pF C30 NA R25 100k C36 4.7nF 8HAB U6 5+""! 1 2 4 5 REF C33 680pF Vout C32 1nF R24 3.74k C34 0.1u,16V R26 80.2k R29 NA MOCD207 1 2 8 7 3 4 6 5 0 FB R27 9.1k R22 1.1k The signal at the FB pin is then compared to the ramp signal from the RC pin and the OUT gate drive signal is generated. Voltages below 600mV at the FB pin, will produce a 0% duty cycle at the OUT drive. Maximum duty cycle is pro- duced when VFB-600mV>VFF. The FB signal range is from 600mv to 4V. OVER CURRENT A pulse by pulse current limit is provided by the SC4905. The current information is sensed at the ILIM pin and com- pared to a peak current limit level of 200mV. If the 200mV limit is exceeded, the OUT pulse is terminated. D11 D15 Rsense C18 C17 U1 5+"' # 4 5 3 2 10 1 6 7 8 9 DMAX RC VFF FB REF VDD SYNC ILIM GND OUT M1 C22 R17 D7 D8 C26 VDD VDD R13 0 REF R11 U2 5+ ! ) 1 4 3 2 5 R30 |
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