FAN5601_05 FAIRCHILD [Fairchild Semiconductor], FAN5601_05 Datasheet - Page 10

• Manufacturer Part Number: FAN5601_05
• Description: Regulated Step-Down Charge Pump DC/DC Converter
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FAN5601 Rev. 1.0.5
Block Diagram
Detailed Description
The FAN5601 switched capacitor DC/DC converter automati-
cally configures switches to achieve a high efficiency and pro-
vides a regulated output voltage by means of pulse skipping,
pulse frequency modulation (PFM). An internal soft start circuit
prevents excessive inrush current drawn from the supply. Each
switch is split into three segments. Based on the values of V
V
segments to be used to reduce current spikes.
Step-Down Charge Pump Operation
When V
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 C
switches 1 and 3 are OFF, charge is transferred from C
C
When V
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 configura-
tion changes from 2:1 to 1:1 at V
ting the change occurs at V
OUT
OUT
.
and I
IN
IN
<2.22 × V
≥ 2.22 × V
OUT
SOFT START
V ref RAMP
VOLTAGE
, an internal circuitry determines the number of
REF.
B
is charged. When switches 2 and 4 are ON,
OUT
OUT
0.5* INPUT
, a 1:1 configuration shown in Fig. 1(B)
, a 2:1 configuration shown in Fig.1(A)
V ref RAMP
OUTPUT
150mV
IN
= 3.06V.
OUT
1V
FB
IN
IN
-
+
+
+
-
-
+
= 3.56V. At 1.3V output set-
-
SHORT_CKT.
CONFIGURATION
PULSE_SKIP
UVLO
ENABLE
SHUTDOWN
OSCILLATOR
CONTROL
(2MHz)
LOGIC
B
IN
to
,
10
SHUTDOWN
THERMAL
Pulse-skipping PFM and Fractional Switch
Operation
When the regulated output voltage reaches its upper limit, the
switches are turned off the output voltage reaches its lower limit.
Considering a step-down 2:1 mode of operation, 1.6V output as
an example, when the output reaches about 1.62V(upper limit),
the control logic turns off all switches. Switching stops com-
pletely. This is pulse-skipping mode. Since the supply is isolated
from the output, the output voltage will drop. Once the output is
dropped to about 1.58V(lower limit), the device will return to reg-
ular switching mode with one quarter of each switch turning on
first. Another quarter of each switch will be turned on if V
cannot reach regulation by the time of arrival of the third charge
cycle. Full switch operation occurs only during startup 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 ini-
tially 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 lim-
ited.
D
R
V
E
R
S
I
0.25SW1 0.25SW1
0.25SW3 0.25SW3 0.5SW3
0.25SW4
0.25SW2
GND.
0.25SW2
0.25SW4 0.5SW4
V IN
0.5SW1
0.5SW2
FB
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C+
V OUT
C-
OUT