S-8333CAAA-I8T1U Datasheet, PDF (10/44 Pages) Seiko Instruments Inc – PWM CONTROL SWITCHING REGULATOR CONTROLLER

STEP-UP, FOR LCD BIAS SUPPLY, 1-CHANNEL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
S-8333 Series
Rev.4.2_01
 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
diode (SD) and VOUT output, the VOUT voltage when the VIN voltage is applied with the SW OFF stays 0 V.
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.
SD
SW
V_OUT
L
PWM
Comparator
0.5 V
RFB1
V_IN
M1
EXT
+
0V
−
FB
C_L
Error amplifier
+
CC
Error amplifier
− _{reference voltage RFB2}
RZ
V_ref
CZ
Figure 7
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