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SC4905A Datasheet(PDF) 9 Page - Semtech Corporation |
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SC4905A Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 24 page  SC4905A/B POWER MANAGEMENT 2006 Semtech Corp. www.semtech.com 9 FEED FORWARD & MAXIMUM DUTY CYCLE The feed forward function provided by the SC4905 will improve the dynamic performance of the converter in response to the changes in the input supply. In voltage mode controllers without the voltage feed for- ward circuitry, any changes in the input supply will cause an error in the output voltage which is sensed by the error amplifier and eventually is translated to an adjustment in the duty cycle by the controller. This delay in the response will cause the slower dynamic performance of the con- verter. This problem is resolved by sensing the input supply line and making the adjustment in the duty cycle immediately at the PWM controller. R14 280k C27 220p, 16V R15 499k Shut Down Vin R16 2k R18 8.25k U1 SC4905 4 5 3 2 10 1 6 7 8 9 DMAX RC VFF FB REF VDD SYNC ILIM GND OUT The SC4905 uses the input supply line as the bias for the oscillator circuitry, and the VFF pin. Any changes in the line will cause the ramp peak voltage to be adjusted to the VFF pin voltage while maintaining the oscillator frequency unchanged. The VFF pin can also be used to shut down the SC4905 if it is pulled down to GND by an open collector circuitry. This can be useful for overvoltage protection or other control signals. The SC4905 also provides a programmable duty cycle, that can be set by an external voltage divider from the VFF pin. The ratio of the divider will determine the pro- grammed duty cycle allowed. VFF VDMAX % DutyCycle = If the application does not require an upper limit on the duty cycle, the VFF pin should be connected to the DMAX pin. In this mode, the duty cycle will be allowed to increase to the maximum limit of about 100%. SYNC/Bi-Phase operation In noise sensitive applications where synchronization of the oscillator frequency to a reference frequency is re- quired, the SYNC pin can accept the external clock.By connecting an external control signal to the SYNC pin, the Internal oscillator frequency will be synchronized to the positive edge of the external control signal. SYNC is a positive edge triggered input with a threshold set to 0.5*VFF. In a single controller operation, SYNC chould be grounded or connected to an external synchronization clock within the SYNC frequency (see page 3). Rosc2 VIN Cosc1 Cosc2 U2 SC4905 4 5 3 2 10 1 6 7 8 9 DMAX RC VFF FB REF VDD SYNC ILIM GND OUT Rosc1 U1 SC4905 4 5 3 2 10 1 6 7 8 9 DMAX RC VFF FB REF VDD SYNC ILIM GND OUT In the Bi-Phase operation mode a very unique oscillator is utilized to allow two SC4905 to be synchronized together and work out of phase. This feature is setup by simple connection of the SYNC input to the RC pin of the other part. The fastest oscillator automatically becomes the master, forcing the two PWMs to operate out of phase. This feature minimizes the input and output ripples, and reduces stress on the capacitors. Application Information (Cont.) |
Similar Part No. - SC4905A_06 |
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Similar Description - SC4905A_06 |
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