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SC4808B-2 Datasheet(PDF) 10 Page - Semtech Corporation |
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SC4808B-2 Datasheet(HTML) 10 Page - Semtech Corporation |
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10 / 28 page  10 © 2009 Semtech Corp. www.semtech.com SC4808B-2 POWER MANAGEMENT age divider provides a 3X amplification of the CS signal. This is used for comparison to the external error amplifier signal. If an external resistor is connected from CS to the current sense resistor, the internal current source will pro- vide a programmable slope compensation. The value of the resistor will determine the level of compensation. At higher compensation levels, voltage mode of operation can be achieved. The error amplifier signal at the FB pin will be used in conjunction with the CS signal to achieve regula- tion. Two levels of undervoltage lockout are also available. The LUVLO (line under voltage lockout) pin via an external re- sistive divider will program the undervoltage lockout level. During the LUVLO, the driver outputs are disabled and the softstart is reset. Once VCC has exceeded the UVLO (VCC 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 1.0V. 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. In a single con- troller operation, SYNC should be grounded or connected to an external synchronization clock within the SYNC fre- quency range (see page 3). In the Bi-phase operation mode a very unique oscillator is utilized to allow two SC4808B-2 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. THEORY OF OPERATION The SC4808B-2 is a versatile double ended, high speed, low power, pulse width modulator that is optimized for ap- plications requiring minimum space. The device contains all of the control and drive circuity re- quired for isolated or non isolated power supplies where an external error amplifier is used. A fixed oscillator fre- quency (up to 1MHz) can be programmed by an external RC network. The SC4808B-2 is a peak current or voltage mode controller, depending on the amount of slope compensation, programmable with only one external resistor. The cycle by cycle peak current limit prevents core saturation when a transformer is used for isolation while the overcurrent circuitry initiates the softstart cycle. The SC4808B-2 dual output drive stages are arranged in a push-pull configuration. Both outputs switch at half the oscillator frequency using a toggle flip flop. The dead time between the two outputs is programmable depending on the values of the timing capacitor and resistors, thus limiting each output stage duty cycle to less than 50%. The SC4808B-2 also provides flexibility with programmable LUVLO thresholds, with built-in hysteresis. SUPPLY A single supply, VCC is used to provide the bias for the internal reference, oscillator, drivers, and logic circuitry of SC4808B-2. To ensure proper operation during start up, VCC slew rate of less than 10V/mS is recommended. PWM CONTROLLER SC4808B-2 is a double ended PWM controller that can be used in voltage or current mode applications. The SC4808B-2 provides a 4.4V VCC UVLO, and a 3.125V ref- erence. The oscillator frequency is programmed by a resis- tor and a capacitor network connected to an external refer- ence provided by the SC4808B-2. The two outputs, OUTA and OUTB, are 180 degrees out of phase and run at half of the oscillator frequency. An external error amplifier will provide the error signal to the FB pin of the SC4808B-2. The current sense input and internal slope compensation are both provided via the CS pin. The current sense input from a sense resistor is used for the peak current and overcurrent comparators. An internal 1 to 3 feedback volt- Application Information |
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