FAN5601 Datasheet, PDF (10/14 Pages) Fairchild Semiconductor – Regulated Step-Down Charge Pump DC/DC Converter

PRODUCT SPECIFICATION
Block Diagram
FAN5601
V _IN
VOLTAGE
REF.
OSCILLATOR
(2MHz)
IN _{- CONFIGURATION}
OUT ₊
SOFT START
Vref RAMP
Vref RAMP _{- PULSE_SKIP}
FB ₊
CONTROL
LOGIC
D
R
I
V
E
R
OUTPUT _- SHORT_CKT.
S
150mV ₊
0.5* INPUT
-
1V ₊
UVLO
SHUTDOWN
THERMAL
SHUTDOWN
ENABLE
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
GND.
Detailed Description
The FAN5601 switched capacitor DC/DC converter auto-
matically configures switches to achieve a 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 drawn
from the supply. Each switch is split into three segments.
Based on the values of V_IN, V_OUT and I_OUT, an internal cir-
cuitry determines the number of segments to be used to
reduce current spikes.
Step-Down Charge Pump Operation
When V_IN ≥ 2.22 × V_OUT, a 2:1 configuration shown in
Fig.1(A) 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 detec-
tor 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_B is charged. When
switches 2 and 4 are ON, switches 1 and 3 are OFF, charge is
transferred from C_B to C_OUT.
When V_IN <2.22 × V_OUT, a 1:1 configuration shown in
Fig. 1(B) 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 V_IN =
3.56V. At 1.3V output setting the change occurs at V_IN =
3.06V
.
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 completely. This is pulse-skipping mode.
Since the supply is isolated from the output, the output volt-
age will drop. Once the output is dropped to about
1.58V(lower limit), the device will return to regular switch-
ing mode with one quarter of each switch turning on first.
Another quarter of each switch will be turned on if V_OUT
cannot reach regulation by the time of arrival of the third
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REV. 1.0.4 4/4/05