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SC4905BMSTR Datasheet(PDF) 7 Page - Semtech Corporation |
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SC4905BMSTR Datasheet(HTML) 7 Page - Semtech Corporation |
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7 / 20 page  SC4905A/B POWER MANAGEMENT PRELIMINARY 2002 Semtech Corp. www.semtech.com 7 This feed forward action provides an immediate duty cycle adjustment while maintaining a constant oscillator fre- quency. A maximum duty cycle can be programmed by connect- ing a resistor divider from the VFF to the DMAX pin. The scaling of the VFF signal will set the maximum duty cycle percentage. An external error amplifier will provide the error signal to the FB pin of the SC4905. A current sense input is provided via the ILIM pin. The current sense input from a sense resistor is used for the peak current limit comparator. Once VDD has exceeded the UVLO (VDD under voltage lock out) limit, the internal reference, oscillator, drivers and logic are powered up. SYNC is a positive edge triggered input with a threshold set to 0.5*VFF. By connecting a faster external control signal to the SYNC pin, the internal oscillator frequency will be synchronized to the positive edge of the external control signal. In a single controller operation, SYNC could be grounded or connected to an external synchronization clock within the SYNC frequency range (see page 3). 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. e c i v e DO L V U c c V . p y Te g a tl o V e c n e r e f e R A 5 0 9 4 C S V 5 . 4V 4 B 5 0 9 4 C S V 5 . 2 1V 5 THEORY OF OPERATION The SC4905 is a versatile 10 pin BICMOS primary side voltage mode controller optimized for applications requir- ing minimum space such as isolated DC-DC and off-line switching power supplies. The device contains all of the control and drive circuity required for isolated or non-isolated power supplies, where an external error amplifier is used. Fixed oscillator frequency up to 1MHz can be programmed by an exter- nal RC network. The SC4905 is a voltage mode controller, utilizing a feed forward scheme to accommodate for any variations in the input supply voltage resulting in a duty cycle adjustment. This feed forward action results in an improved dynamic performance of the converter. The SC4905 also provides a programmable maximum duty cycle to prevent core saturation when a transformer is used. As an added level of protection, SC4905 provides a cycle by cycle peak current limit during an over current condition. SUPPLY A single supply, VDD is used to provide the bias for the internal reference, oscillator, drivers, and logic circuitry of the SC4905. PWM CONTROLLER The SC4905 is a BICMOS primary side voltage mode controller for use in isolated DC-DC and off-line switch- ing power supplies. It is a highly integrated solution, re- quiring few external components. The device features a high speed oscillator with integrated feed forward compensation, accurately programmable maximum duty cycle, voltage mode of operation, line volt- age monitoring, supply UVLO, low start-up current, low voltage current limit threshold and user accessible refer- ence. Two voltage options are available for the SC4905. The SC4905A version has a typical VDD under voltage of 4V, and a 4V reference, while the SC4905B version provides a 12V VDD UVLO, and a 5V reference. The Oscillator frequency is programmed by a resistor and a capacitor network connected to the line supply voltage . Any variations in the input supply voltage result in a duty cycle adjustment, provided by the change of the oscillator peak voltage via the VFF pin. Application Information |
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