Description of Operation
S1F71200 is a switching regulator (step-up) and series regulator (step-down) control IC of pulse width modulation (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 voltage value, S1F71200 raises the voltage to the set voltage (selectable at the VC pin) with the switching regulator and lowers the step-up voltage with the series regulator to stabilize the output voltage. On the other hand, when the input voltage is higher than the specified voltage value, S1F71200 stops the operation of the switching 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 VSW pin, i.e., the output voltage of the step-up switching regulator, and controls pulse width of the switching transistor. When the voltage at the VSW pin drops below the step-up set volt- age, S1F71200 changes the output level of the error amplifier and increases the on duty of the switching transistor for control.
On the other hand, when the voltage at the VSW 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 VSW pin, S1F71200 stops operation of the step-up switching. When the switching stops completely, the input voltage is supplied to the VSW pin through the coil and the di- ode. (The voltage at the VSW pin comes to the one obtained by reducing VF of the diode from the input volt- age.)
Step-up Set Voltage
Step-up set voltage can be selected at the VC pin.
1 VC = High VDD1 level :
Step-up set voltage = 6.0V (S1F71200M0A0) 4.3V (S1F71200M0B0)
2VC = 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 applicable external parts or voltage and load conditions. So, select a proper voltage after sufficient evaluation. The VC 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 VSS level when the power is turned off (POFFX=VSS). When the operation state is set (POFFX=VDD1), 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 voltage 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.
