STEP-UP, FOR LCD BIAS SUPPLY, 1-CHANNEL, PWM CONTROL SWITCHING REGULATOR CONTROLLER

S-8333 Series

10

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Operation

1. Switching control method

1. 1 PWM control

The S-8333 Series is a DC-DC converter using a pulse width modulation method (PWM).

The pulse width of the S-8333 Series varies from 0% to the maximum duty set by RDuty depending on the load

current, but its switching frequency does not change. Consequently, the ripple voltage generated from switching

can be removed easily via a filter.

2. Soft-start function

For this IC, the built-in soft-start circuit controls the rush current and overshoot of the output voltage when

powering on. Reference voltage adjustment and maximum duty control methods are adopted as the soft-start

methods. The following describes the soft-start function at power application.

In the circuit where the input voltage is not directly output at shutdown by inserting a switch (SW) between the

Therefore, the voltage of the FB pin stays 0 V and the EXT output is in the step up status between the “H” and “L”

levels due to the maximum duty. The maximum duty at this time is approximately 7% and the rush current at

power application is controlled. The maximum duty soft start is accomplished by gradually increasing the duty

width up to the maximum duty set by the external resistor RDuty (refer to Figure 8).

The reference voltage of the error amplifier input also gradually increases from 0 V at the same time as the

maximum duty soft start. The increasing of the output voltage is controlled by turning the SW ON. The soft-start

function is realized by controlling the voltage of the FB pin so that it is the same potential as the reference voltage

that is slowly raised. A Rail-to-Rail amplifier is adopted as the error amplifier, which means that the voltage is

loop controlled so that it can be the same as the reference voltage.

Once the reference voltage rises, the voltage cannot be reset (the reference voltage is 0 V) unless making the

power supply voltage lower than the UVLO detection voltage. Conversely, when the power supply voltage rises

up to the reset voltage after it is lowered to the UVLO detection voltage or lower, the output voltage is stepped up

by the soft-start function.

PWM

Comparator

RFB2

RFB1

FB

SD

L

EXT

CC

RZ

CZ

M1

+

−

Error amplifier

0.5 V

0 V

+

−

Error amplifier

reference voltage

SW

Figure 7