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STEP-UP OR DOWN, SINGLE-COIL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
S-8460
Rev.1.2_00
10
Seiko Instruments Inc.
1. 6 PWM Control
The S-8460 is a DC-DC converter using a pulse width modulation method (PWM) and features a low current
consumption. In conventional PFM DC-DC converters, pulse 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 83% at step-up operation
and 0 to 78% in step-up operation. When the pulse width is 0% (when there is no load or the input voltage is
high), current consumption is low since pulses are skipped.
2. Soft-Start Function
The S-8460 has a built-in soft-start circuit. This circuit enables the output voltage to rise gradually over the
specified soft-start time to suppress the overshooting of the output voltage when the power is switched on or the
ON/OFF pin is changed from “L” to “H”.
The soft-start time (tSS) can be set by an external capacitor (CSS).
The time needed for the output voltage (VOUT) to reach 95% of the setting value of the output voltage is
approximately expressed by the following equation.
tSS (ms)
= 0.0026 × CSS (pF)
0
10
20
30
40
50
60
70
80
90
100
0
5000
10000
15000
20000
External capacitor (CSS) [pF]
Soft
-start
time
(t
SS
)[ms]
Figure 11 Soft-Start Time
The value for CSS should be selected to give enough margin to the soft-start time against the power supply rise
time. If the soft-start time is short, possibility for output overshoot, input current rush, and malfunction of the IC
increases.
3. ON/OFF Pin (Shutdown Pin)
When the ON/OFF pin is set to “L”, the EXT1 pin voltage becomes equal to the LX voltage and the voltage of the
EXT2 and EXT3 pins becomes VSS level to turn the external MOS FET off. At the same time, the S-8460 stops all
the internal circuit and suppresses the current consumption down to approximately 0.5
A. The power supply
(VL), the CSS pin, and CPRO pin become VSS level. Electrical isolation between power input side VIN and output
side VOUT is thus possible when the S-8460 is in halt state.
The ON/OFF pin is configured as shown in Figure 12 and is not either pulled up or pulled down. So, do not use it
with it in a floating state. When the ON/OFF pin is not used, connect it to the VIN pin.