S-8340A25AFT-T2-X Datasheet, PDF (17/55 Pages) Seiko Instruments Inc – SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-UP, 600 kHz, PWM CONTROL OR PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.4.0_01
S-8340/8341 Series
 Operation
1. Switching Control Method
1. 1 PWM Control (S-8340 Series)
The S-8340 Series is a DC-DC converter using a pulse width modulation method (PWM).
In conventional PFM DC-DC converters, pulses are skipped when the output load current is low, causing a
fluctuation in the ripple frequency of the output voltage, resulting in an increase in the ripple voltage.
The switching frequency does not change, although the pulse width changes from 0 to 82% corresponding to each
load current in the S-8340 Series. The ripple voltage generated from switching can thus be eliminated easily
through a filter. When the pulse width is 0% (when no load is applied or the input voltage is high), pulses are
skipped and the current consumption is low.
1. 2 PWM/PFM Switching Control (S-8341 Series)
The S-8341 Series is a DC-DC converter that automatically switches between a pulse width modulation method
(PWM) and a pulse frequency modulation method (PFM) depending on the load current.
The S-8341 Series operates under PWM control with the pulse duty changing from 27 to 82% (A and B types) and
from 21 to 82% (C and D types) in a high output load current area.
The S-8341 Series operates under PFM control with the pulse duty fixed at 27% (A and B types) and at 21% (C and
D types) in a low load current area, and pulses are skipped according to the load current. The oscillation circuit
thus oscillates intermittently so that the resultant lower self current consumption prevents a reduction in the
efficiency at a low load current. The switching point from PWM control to PFM control depends on the external
devices (coil, diode, etc.), and input and output voltage values. The S-8341 Series is an especially highly efficient
DC-DC converter at an output load current around 1 mA.
2. Soft-Start Function
The S-8340/8341 Series has a built-in soft-start circuit.
This circuit enables the output voltage (VOUT) to rise gradually over the specified soft-start time (tSS) to suppress the
overshooting of the output voltage and the rush current from the power supply when the power is switched on or the
ON/OFF pin is changed to “H”.
Generally, a rush current flows to an output capacitor through an inductor and a diode in the step-up circuit
immediately after the power is turned on as shown in Figure 9. Note that the soft-start function of this IC,
however, does not limit this current.
3V
O u t p u t vo lta g e
(1 V /d iv)
0V
S -8 3 40 A 3 3 A FT (V _IN = 0 → 1 .9 V , R _L = 3 0 0 k Ω )
1 .5 A
R u s h cu r re n t
(0. 5 A /d iv)
0A
t (2 m s /d iv )
Figure 9 Waveforms of Output Voltage and Rush Current at Soft-Start
17