FAN5602 Fairchild Semiconductor, FAN5602 Datasheet - Page 9

• Manufacturer Part Number: FAN5602
• Description: The FAN5602 is a universal switched capacitor DC/DC converter capable of step-up or step-down operation
• Manufacturer: Fairchild Semiconductor

FAN5602 Rev. 1.5.3
© 2005 Fairchild Semiconductor Corporation
Functional Description
FAN5602 is a high-efficiency, low-noise switched capaci-
tor DC/DC converter capable of step-up and step-down
operations. It has seven built-in switch configurations.
Based on the ratio of the input voltage to the output volt-
age, the FAN5602 automatically reconfigures the switch
to achieve the highest efficiency. The regulation of the
output is achieved by a linear regulation loop, which
modulates the on-resistance of the power transistors so
that the amount of charge transferred from the input to
the flying capacitor at each clock cycle is controlled and
is equal to the charge needed by the load. The current
spike is reduced to minimum. At light load, the FAN5602
automatically switches to Pulse Frequency Modulation
(PFM) mode to save power. The regulation at PFM mode
is achieved by skipping pulses.
Linear Regulation Loop
The FAN5602 operates at constant frequency at load
higher than 10mA. The linear regulation loop consisting
of power transistors, feedback (resistor divider), and
error amplifier is used to realize the regulation of the out-
put voltage and to reduce the current spike. The error
amplifier takes feedback and reference as inputs and
generates the error voltage signal. The error voltage sig-
nal is then used as the gate voltage of the power transis-
tor and modulates the on-resistance of the power
transistor and, therefore, the charge transferred from the
input to the output is controlled and the regulation of the
output is realized. Since the charge transfer is controlled,
the FAN5602 has a small ESR spike.
Switch Array
Switch Configurations
The FAN5602 has seven built-in switch configurations,
including 1:1, 3:2, 2:1 and 3:1 for step-down and 2:3, 1:2
and 1:3 for step-up.
When 1.5 x V
Figure 21 is used. In this mode, the internal oscillator is
turned off. The power transistors connecting the input
and the output become pass transistors and their gate
voltages are controlled by the linear regulation loop, the
rest of power transistors are turned off. In this mode, the
FAN5602 operates exactly like a low dropout (LDO) regu-
lator and the ripple of the output is in the micro-volt range.
When 1.5 x V
shown in Figure 22 is used. In the charging phase, two
flying capacitors are placed in series and each capacitor
is charged to a half of the input voltage. In pumping
phase, the flying capacitors are placed in parallel. The
OUT
IN
> V
> V
OUT
IN
> V
> V
OUT
IN
, the 2:3 mode (step-up)
, the 1:1 mode shown in
9
input is connected to the bottom the capacitors so that
the top of the capacitors is boosted to a voltage that
equals V
of the capacitors to the output, one can ideally charge
the output to 3/2 x V
needed V
resistance to drop some voltage. Boosting the voltage of
the top of the capacitors to 3/2 x V
the bottom of the capacitors, boosts the power efficiency
3/2 times. In 2:3 mode, the ideal power efficiency is
V
= 4V, the ideal power efficiency is 100%.
When 2 x V
shown in Figure 23 is used. Both in the charging phase
and in pumping phase, two flying capacitors are placed
in parallel. In charging phase, the capacitors are charged
to the input voltage. In the pumping phase, the input volt-
age is placed to the bottom of the capacitors. The top of
the capacitors is boosted to 2 x V
top of the capacitors to the output, one can ideally
charge the output to 2 x V
top of the capacitors to 2V
2 times. In 1:2 mode, the ideal power efficiency is V
x V
ideal power efficiency is 100%.
When 3 x V
shown in Figure 24 is used. In charging phase, two flying
capacitors are placed in parallel and each is charged to
V
placed in series and the input is connected to the bottom
of the series connected capacitors. The top of the series
connected capacitors is boosted to 3 x V
power efficiency is boosted 3 times and is equal to V
3V
ideal power efficiency is 100%. By connecting the output
to the top of the series connected capacitors, one can
charge the output to 3 x V
The internal logic in the FAN5602 monitors the input and
the output compares them, and automatically selects the
switch configuration to achieve the highest efficiency.
The step-down modes 3:2, 2:1, and 3:1 can be under-
stood by reversing the function of V
above discussion.
The built-in modes improve power efficiency and extend
the battery life. For example, if V
needs a minimum V
minimum battery voltage is extended to 1.7V.
OUT
IN
IN.
. In the pumping phase, the two flying capacitors are
IN
/1.5 x V
. For example, V
For example, V
IN
OUT
/2 + V
IN
IN
IN
, the linear regulation loop adjusts the on-
> V
> V
. For example, if V
IN
OUT
OUT
, i.e., 3/2 x V
IN
> 1.5 x V
IN
IN
IN
> 2 x V
= 2.5V. By built-in 1:3 mode, the
. If 3/2 x V
= 1V, V
= 2V, V
IN
IN
IN
.
. Boosting the voltage on the
boosts the power efficiency
IN
IN
IN
, the 1:3 mode (step-up)
OUT
OUT
, the 1:2 mode (step-up)
. By connecting the top
IN
IN
IN
OUT
IN
= 2V, V
. By connecting the
= 3 x V
= 2 x V
IN
is higher than the
by connecting V
and V
= 5V, mode 1:2
OUT
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IN
IN
IN
. The ideal
OUT
= 3V, the
= 4V, the
= 2 x V
in the
OUT
OUT
IN
IN
/2
/