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S1F71200 Series
S1F70000 Series EPSON 4–45
Technical Manual
S1F71200
Series
Description of Operation
S1F71200 is a switching regulator (step-up) and series
regulator (step-down) control IC of pulse width modu-
lation (PWM) system. When external parts such as
transistor, inductor, capacitor, diode and resistor are
connected, S1F71200 can constitute a step-up/step-
down DC/DC converter.
When the input voltage is lower than the specified volt-
age value, S1F71200 raises the voltage to the set volt-
age (selectable at the V
C pin) with the switching regula-
tor and lowers the step-up voltage with the series regu-
lator to stabilize the output voltage. On the other hand,
when the input voltage is higher than the specified volt-
age value, S1F71200 stops the operation of the switch-
ing regulator (step-up) and operates the series regulator
(step-down) only. Switching operations with input
voltage makes constant voltage outputs possible all the
time.
Operation of Switching Regulator
S1F71200 monitors voltage at the V
SW pin, i.e., the out-
put voltage of the step-up switching regulator, and con-
trols pulse width of the switching transistor. When the
voltage at the V
SW pin drops below the step-up set volt-
age, S1F71200 changes the output level of the error am-
plifier and increases the on duty of the switching
transistor for control.
On the other hand, when the voltage at the V
SW pin rises
over the step-up set voltage, S1F71200 reduces the on
duty for control. When voltages higher than the step-up
set voltage is constantly supplied to the V
SW pin,
S1F71200 stops operation of the step-up switching.
When the switching stops completely, the input voltage
is supplied to the V
SW pin through the coil and the di-
ode. (The voltage at the V
SW pin comes to the one ob-
tained by reducing V
F of the diode from the input volt-
age.)
Step-up Set Voltage
Step-up set voltage can be selected at the V
C pin.
1 V
C = High VDD1 level :
Step-up set voltage = 6.0V (S1F71200M0A0)
4.3V (S1F71200M0B0)
2 V
C = Low VSS level :
Step-up set voltage = 5.5V (S1F71200M0A0)
3.8V (S1F71200M0B0)
In case of output stability and heavy load like ripple, 1
is superior, bur for the conversion efficiency at step-up
operation, 2 is superior. Characteristics vary with ap-
plicable external parts or voltage and load conditions.
So, select a proper voltage after sufficient evaluation.
The V
C pin cannot be used in the open state.
Soft Start
When a capacitor is connected to the SSCAP pin,
S1F71200 can set a soft start operation to prevent inrush
current at the time when the power is turned on. The
SSCAP pin is at the V
SS level when the power is turned
off (POFFX=V
SS). When the operation state is set
(POFFX=V
DD1), the soft start capacitor starts being
charged and the voltage at the SSCAP pin rises slowly.
The step-up output voltage rises gradually as the volt-
age at the SSCAP pin rises. This operation is carried
out when the power is turned on, when the power off is
released or when the overcurrent detection is released
and restrains surge current and output overshoot.
FUNCTIONAL DESCRIPTIONS