S-8337AAAA-P8T1G Datasheet, PDF (11/51 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-UP, 1.2 MHz HIGH-FREQUENCY, PWM CONTROL SWITCHING REGULATOR CONTROLLER
Rev.4.0_01
S-8337/8338 Series
 Operation
1. Switching control method
PWM control (S-8337/8338 Series)
The S-8337/8338 Series is a DC-DC converter using a pulse width modulation method (PWM).
The pulse width of the S-8337/8338 Series varies from 0% to the maximum duty set by RDuty
depending on the load current (the pulse width of the S-8338 Series is fixed to 88%), 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 or when the ON/ OFF pin is switched to the “H” level. A reference voltage
adjustment method is adopted as the soft-start method. The following describes the soft-start
function.
The raising of the output voltage is controlled by slowly raising the reference voltage of the error
amplifier input from 0 V at power on as shown in Figure 7. 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.
The following explains the operation at power on (refer to Figure 8).
When VIN is raised from 0 V to 3.3 V, the VOUT voltage rises to a value close to VIN via the inductor L
and diode SD. This raises the voltage of the FB pin (VFB) by approximately 0.35 V (when RFB1 = 8.2
kΩ, RFB2 = 1.0 kΩ). Because the reference voltage rises from 0 V, the VFB voltage is higher than the
reference voltage while the voltage rises from 0 V to 0.35 V. During this period, the EXT output is low.
The EXT output is in the stepped-up status between high and low after the reference voltage reaches
0.35 V and VOUT is slowly raised in accordance with the rising of the reference voltage.
Once the reference voltage rises, the voltage cannot be reset (the reference voltage is 0 V) unless the
power supply voltage is the UVLO detection voltage or lower or the shutdown pin is the “L” level.
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
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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