Epson Power Supply S1F70000 - Manual (Page 191)

S1F71200 Series
4–46 EPSON
S1F70000 Series
Technical Manual
Overcurrent Protection Circuit
The overcurrent protection circuit functions when an
overcurrent flows through the current detection resistor
during the step-up operation and the voltage at the
I
SENSE pin rises over the set voltage value. When the
overcurrent protection function works, the transistor
drive pin SWO comes to the V
SS level and the switch-
ing transistor is turned off. Since the overcurrent pro-
tection circuit links to the soft start circuit, it drops the
voltage at the SSCAP pin to the V
SS level during
overcurrent detection. When the overcurrent detection
is released, the voltage at the SSCAP pin rises slowly
and the soft start operation starts.
Power lost at the current detection resistor becomes a
factor of lower efficiency. When the overcurrent pro-
tection circuit is not necessary, short-circuit the I
SENSE
pin to the V SS pin. The I SENSE pin cannot be used in an
open state.
Operation of Series Regulator
S1F71200 has a built-in series regulator control circuit.
When driving the PNP transistor as an external part,
S1F71200 reduces voltage supplied to the V
DD2 pin and
constitutes a series regulator. The V DD2 pin is generally
connected to the V SW pin of step-up output.
This series regulator operates monitoring voltage at the
V
O pin of step-up/step-down output. It controls the
drive current (base current of the PNP transistor) at the
EXO pin to stabilize voltage at the V
O pin.
Power Off Function
S1F71200 can control circuit operations according to
external signal control. When the POFFX pin is set to
the V
SS level, all circuits stop and their powers are
turned off. The current consumption at the power off
state is less than 1
µ
A. When the power is off, the SWO
pin is set to the V DD level, the EXO pin is set to the
V DD2 level and respective external transistors are
turned.
The voltage at the SSCAP pin comes to the V
SS level,
and the soft start circuit controls the step-up operation
output voltage is controlled with the soft start circuit af-
ter the power off state is released. The soft start circuit
controls step-up operations after power off release.
When being set to the V
DD level, the POFFX pin oper-
ates in general. The POFFX pin cannot be used in an
open state. When the power off function is not used,
connect the POFFX pin to the V
DD1 pin.