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FAN5631/FAN5632 Rev. 1.0.2

Block Diagram

Figure 4. 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 the Pulse Fre-

quency Modulation (PFM) pulse-skipping mode. An internal

soft-start circuit prevents excessive in-rush current drawn from

the supply. The switches are split into three segments. Based on

mines the number of segments to be used to reduce current

spikes.

Step-Down Charge Pump Operation

5, is enabled. The factor 0.9 is used instead of 1 in order 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

is enabled. In the 1:1 configuration switch 3 is always OFF and

switch 4 is always ON. At the 1.6V output setting the configura-

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. Considering a step-down 2:1 mode of operation, 1.6V out-

put 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 will drop. Once

the output drops to about 1.58V (lower limit), the device will

return to regular switching mode with one quarter of each switch

turning on first. Another quarter of each switch will be turned on

cycle. Full switch operation occurs only during startup or under

heavy load condition, when a half switch operation cannot

achieve regulation within seven charge cycles.

Soft-Star

t

The soft-start feature limits in-rush current when the device is ini-

tially powered up and enabled. The reference voltage is used to

UVLO

+

-

SHUTDOWN

CONTROL

LOGIC

CONFIGURATION

PULSE_SKIP

SHORT_CKT.

+

-

+

-

+

-

THERMAL

SHUTDOWN

IN

OUT

Vref. RAMP

FB

OUTPUT

150mV

1V

0.5* INPUT

VOLTAGE

REF.

SOFT START

Vref. RAMP

OSCILLATOR

(2MHz)

C+

C-

ENABLE

VIN

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