FAN5602 Fairchild Semiconductor, FAN5602 Datasheet - Page 10

• 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
Switch Array Modes
Light-Load Operation
The power transistors used in the charge pump are very
large in size. The dynamic loss from the switching the
power transistors is not small and increases its propor-
tion of the total power consumption as the load gets light.
To save power, the FAN5602 switches, when the load is
less than 10mA, from constant frequency to pulse-skip-
ping mode (PFM) for modes 2:3(3:2), 1:2(2:1) and
1:3(3:1), except mode 1:1. In PFM mode, the linear loop
is disabled and the error amplifier is turned off. A PFM
comparator is used to setup an upper threshold and a
lower threshold for the output. When the output is lower
than the lower threshold, the oscillator is turned on and
the charge pump starts working and keeps delivering
charges from the input to the output until the output is
higher than the upper threshold. The oscillator shuts off
power transistors and delivers the charge to the output
from the output capacitor. PFM operation is not used for
Mode 1:1, even if at light load. Mode 1:1 is designed as
an LDO with the oscillator off. The power transistors at
LDO mode are not switching and therefore do not have
the dynamic loss.
Switching
(I
(I
which is monitored all the time.
LOAD
LOAD
Figure 23. Mode 3 (1:2 or 2:1) All Switches Set for
<10mA)
>10mA) is automatic, based on the load current,
C1
Phase 1 and Reverse State for Phase 2
from
C1-
C1+
Figure 21. Mode 1 (1:1)
and
MID
S1A
S2A
S3A
S4A
S1A
S2A
linear
GND
TOP
from
MID
TOP
operation
S1B
S2B
S3B
S4B
PFM
C1-
C1+
C2+
C2-
to
to
C1
linear
PFM
C2
mode
mode
10
Short Circuit
When the output voltage is lower than 150mV, the
FAN5602 enters short-circuit condition. In this condition,
all power transistors are turned off. A small transistor
shorting the input and the output turns on and charges
the output. This transistor stays on as long as the V
<150mV. Since this transistor is very small, the current
from the input to the output is limited. Once the short at
the output is eliminated, this transistor is large enough to
charge the output higher than 150mV and the FAN5602
enters soft-start period.
Soft Start
The FAN5602 uses a constant current, charging a low-
pass filter to generate a ramp. The ramp is used as refer-
ence voltage during the startup. Since the ramp starts at
zero and goes up slowly, the output follows the ramp and
inrush current is restricted. When the ramp is higher than
bandgap voltage, the bandgap voltage supersedes ramp
as reference and the soft start is over. The soft start
takes about 500µs.
Thermal Shutdown
The FAN5602 goes to thermal shutdown if the junction
temperature is over 150°C with 15°C hysteresis.
Figure 22. Mode 2 (2:3 or 3:2) All Switches Set for
Figure 24. Mode 4 (1:3 or 3:1) All Switches Set for
C1
C1
Phase 1 and Reverse State for Phase 2
Phase 1 and Reverse State for Phase 2
C1+
C1-
C1-
C1+
S3A
S1A
S2A
S1A
S2A
S4A
TOP
TOP
GND
S5
MID
MID
S5
S1A
S2B
S3B
S4B
GND
S3B
S4B
C2+
C2-
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C1+
C1-
C2
C2
OUT