S1F71100 Series

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 reference 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 external 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 transistor 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 VSS 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 according to the capacitance value of a capacitor to be

connected.

The voltage at the SSCAP pin drops to the VSS 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 VDD pin). When the supply voltage drops below a detected voltage value, the low-voltage protection circuit works and stops the switching operation of the output transistor. This low-voltage protection circuit links to the soft start circuit and drops the voltage at the SSCAP pin to the VSS level during detection of low voltage.

The low-voltage protection circuit has hysteresis. When the supply voltage returns to more than the release 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 ISENSE 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 VDD level and turns off the output transistor. It links to the soft start circuit, and the soft start circuit controls output voltage rise after detection of overcurrent.

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S1F70000 Series Technical Manual