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FAN5631 Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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FAN5631 Datasheet(HTML) 8 Page - Fairchild Semiconductor |
8 / 13 page © 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5631/FAN5632 Rev. 1.0.2 8 Block Diagram UVLO + - SHUTDOWN CONTROL LOGIC CONFIGURATION PULSE_SKIP SHORT_CKT. + - + - + - THERMAL SHUTDOWN IN OUT Vref RAMP FB FB OUTPUT 150mV 1V 0.5* INPUT VOLTAGE REF. SOFT START Vref RAMP OSCILLATOR (2MHz) C+ C- ENABLE V IN GND. D R I V E R S 0.25SW1 0.25SW1 0.25SW4 0.25SW4 0.5SW4 0.25SW3 0.25SW3 0.5SW3 0.5SW2 0.25SW2 0.25SW2 0.5SW1 FB VOUT Figure 14. Block Diagram Detailed Description The FAN5631 / FAN5632 switched capacitor DC/DC converter automatically configures switches to achieve high efficiency and provides a regulated output voltage by means of pulse skipping, pulse frequency modulation (PFM). An internal soft-start circuit prevents excessive inrush current from the supply. Each switch is split into three segments. Based on the values of VIN, VOUT, and IOUT; an internal circuit determines the number of segments used to reduce current spikes. Step-Down Charge Pump Operation When VIN ≥ 2 × VOUT/9, the 2:1 configuration shown in Figure 15 is enabled. The factor 0.9 is used instead of 1 to account for the effect of resistive losses across the switches and to accommodate hysteresis in the voltage detector comparator. Two-phase, non-overlapping clock signals are generated to drive four switches. When switches 1 and 3 are on, switches 2 and 4 are off and CB is charged. When switches 2 and 4 are on, 1 and 3 are off and charge is transferred from CB to COUT. When VIN ≥ 2 × VOUT/9, the 1:1 configuration shown in Figure 16 is enabled. In the 1:1 configuration, switch 3 is always off and the switch 4 is always on. At 1.6V output setting, the configuration changes from 2:1 to 1:1 at VIN=3.56V. At 1.3V output setting, the change occurs at VIN=3.06V. Pulse-skipping PFM and Fractional Switch Operation When the regulated output voltage reaches its upper limit, the switches are turned off and the output voltage reaches its lower limit. In a step-down 2:1 mode of operation, with 1.6V output as an example; when the output reaches about 1.62V (upper limit), the control logic turns off all switches: switching stops completely. This is pulse-skipping mode. Since the supply is isolated from the output, the output voltage drops. Once the output is dropped to about 1.58V (lower limit), the device returns to regular switching mode with one quarter of each switch turning on first. Another quarter of each switch is turned on if VOUT cannot reach regulation by the third charge cycle. Full switch operation occurs only during star-up or under heavy- load condition, when half switch operation cannot achieve regulation within seven charge cycles. Soft-Start The soft-start feature limits inrush current when the device is initially powered up and enabled. The reference voltage is used to control the rate of the output voltage ramp-up to its final value. Typical start-up time is 1ms. Since the rate of the output voltage ramp- up is controlled by an internally generated slow ramp, pulse-skipping occurs and inrush current is automatically limited. |
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