FAN5631(2006) 查看數據表（PDF） - Fairchild Semiconductor
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FAN5631 Datasheet PDF : 11 Pages
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Block Diagram
VIN
OSCILLATOR
(2MHz)
IN - CONFIGURATION
VOLTAGE
REF.
OUT +
D
R
SOFT START
Vref. RAMP
Vref. RAMP
FB
- PULSE_SKIP
+
CONTROL
LOGIC
I
V
E
R
OUTPUT - SHORT_CKT.
S
150mV +
0.5* INPUT
-
1V +
UVLO
SHUTDOWN
THERMAL
SHUTDOWN
0.25SW1 0.25SW1 0.5SW1
C+
0.25SW2 0.25SW2 0.5SW2
0.25SW3 0.25SW3 0.5SW3
VOUT
C-
0.25SW4 0.25SW4 0.5SW4
FB
ENABLE
GND.
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
the values of VIN, VOUT and IOUT, an internal circuitry deter-
mines the number of segments to be used to reduce current
spikes.
Step-Down Charge Pump Operation
When VIN ≥ 2 × VOUT/0.9, a 2:1 configuration, as shown in Fig.
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
are OFF and CB is charged. When switches 2 and 4 are ON,
switches 1 and 3 are OFF and charge is transferred from CB to
COUT.
When VIN < 2 × VOUT/0.9, a 1:1 configuration, as shown in Fig. 6
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-
tion changes from 2:1 to 1:1 at VIN = 3.56V. At the 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. 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
if VOUT cannot reach regulation by the time of the third charge
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-Start
The soft-start feature limits in-rush current when the device is ini-
tially powered up and enabled. The reference voltage is used to
7
FAN5631/FAN5632 Rev. 1.0.2
www.fairchildsemi.com
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