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S1F71100 Series
4–36 EPSON S1F70000 Series
Technical Manual
FUNCTIONAL DESCRIPTIONS
Description of Operation
S1F71100 is a step-down switching regulator control IC
of load current detection type pulse width modulation
(PWM) system and is composed of an oscillator, a ref-
erence voltage circuit, an error amplifier, a PWM cir-
cuit, various protection circuits, etc.
S1F71100 can constitute a switching regulator, which
converters input voltages up to 12V into output voltages
of 3.3V, when external parts such as a power MOSFET,
a diode, an inductor and a capacitor are connected to it.
S1F71100 controls PWM by means of the two systems
of output voltage feedback and load current feedback.
The output voltage feedback system converts output
voltage values of the switching regulator into voltage
feedback signals through the error amplifier. And the
load current feedback system detects currents flown to
the external load current detection resistor as voltage
values at the load current detection pin and converts
them into current feedback signal in the internal circuit.
These two feedback signals control the switching duty
so that outputs from the switching regulator become
optimum all the time. When the output voltage reduces,
the output level of the error amplifier changes, and the
switching duty is controlled so that the switching on
duty of the output transistor increases.
Oscillator
Since S1F71100 has a built-in oscillator, and no exter-
nal part is necessary. The oscillation frequency is fixed
to 200kHz (Typ.). The oscillator stops its operation
when the power is turned off. When the low-voltage
protection circuit detects a low voltage, the output tran-
sistor is turned off, but the oscillation continues inside
the IC.
Soft Start
When a capacitor is connected to the SSCAP pin,
S1F71100 can set a soft start operation to prevent inrush
current at the time when the power is turned on. (Figure
6-1) The SSCAP pin is at the V
SS level when the power
is turned off. When the soft start operation begins, the
soft start capacitor starts being charged and the voltage
at the SSCAP pin rises slowly. The output voltage rises
gradually as the voltage at the SSCAP pin rises. This
operation controls the switching pulse width at the time
when the power is turned on and restrains surge current
and output overshoot. The soft start time can be set ac-
cording to the capacitance value of a capacitor to be
connected.
The voltage at the SSCAP pin drops to the V
SS level
when an overcurrent is detected, when a low voltage is
detected or when the power is turned off, and the soft
start operation begins.
SSCAP pin
Soft start circuit
Soft start
capacitor
Figure 6.1 Connection Diagram of SSCAP Pin
Low-voltage Protection Circuit
The low-voltage protection circuit monitors the supply
voltage (voltage at the V
DD pin). When the supply volt-
age drops below a detected voltage value, the low-volt-
age protection circuit works and stops the switching
operation of the output transistor. This low-voltage pro-
tection circuit links to the soft start circuit and drops the
voltage at the SSCAP pin to the V
SS level during detec-
tion of low voltage.
The low-voltage protection circuit has hysteresis.
When the supply voltage returns to more than the re-
lease voltage, the voltage at the SSCAP pin rises slowly
and the soft start circuit starts it operation, then the low-
voltage protection circuit resumes the switching opera-
tion.
Overcurrent Protection Circuit
When the current flowing through the current detection
resistor increases more than the set current value and
the voltage at the I
SENSE pin drops below a detected
voltage value, the overcurrent protection circuit oper-
ates. When the overcurrent protection circuit operates,
it maintains the SWO pin at the V
DD level and turns off
the output transistor. It links to the soft start circuit, and
the soft start circuit controls output voltage rise after de-
tection of overcurrent.