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SC412AMLTRT Datasheet(PDF) 8 Page - Semtech Corporation |
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SC412AMLTRT Datasheet(HTML) 8 Page - Semtech Corporation |
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8 / 22 page  8 © 2006 Semtech Corp. www.semtech.com SC412A POWER MANAGEMENT SC412A Synchronous Buck Controller The SC412A is a synchronous power supply controller which simplifies the task of designing a power supply suitable for powering low voltage circuits. Battery and +5V Bias Supplies The SC412A requires an external +5V bias supply in ad- dition to the battery. If stand-alone capability is required, the +5V supply can be generated with an external linear regulator. Pseudo-Fixed-Frequency Constant On-Time PWM Controller The PWM control method is a constant-on-time, pseudo- fixed-frequency PWM controller, see Figure 1. The ripple voltage seen across the output capacitor’s ESR provides the PWM ramp signal, eliminating the need for a current sense resistor. The on-time is determined by an internal one-shot whose period is proportional to output voltage, and inversely proportional to input voltage. A separate one-shot sets the minimum off-time (typically 350ns). The typical operating frequency is 325kHz. It is possible to raise or lower the operating frequency with external components, refer to the section on Switching Frequency Variations. Q1 Q2 L COUT VBAT ESR + CIN VOUT FB Threshold 0.75V VOUT/FB Ripple VPHASE VPHASE TON Figure 1. On-Time One-Shot (T ON) The internal on-time one-shot comparator has two inputs. One input looks at the output voltage via the VOUT pin, while the other input samples the input voltage via the LX pin and converts it to a proportional current which charges an internal on-time capacitor. The TON time is the time required for this capacitor to charge from zero volts to VOUT, thereby making the on- time directly proportional to output voltage and inversely proportional to input voltage. This implementation results in a fairly constant switching frequency without the need of a clock generator. The internal frequency is optimized for 325kHz. The general equation for the on-time is: TON (nsec) = 2560 • (VOUT/VBAT) + 35 VOUT Voltage Selection Output voltage is regulated by comparing VOUT as seen through a resistor divider to the internal 0.75V reference, see Figure 2. The output voltage is set by the equation: VOUT = 0.75 • (1 + R1/R2) R1 R2 VOUT To FB pin Figure 2. Applications Information |
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