S-1137 Datasheet, PDF (21/47 Pages) Seiko Instruments Inc – HIGH RIPPLE-REJECTION LOW DROPOUT SOFT-START FUNCTION CMOS VOLTAGE REGULATOR

HIGH RIPPLE-REJECTION LOW DROPOUT SOFT-START FUNCTION CMOS VOLTAGE REGULATOR
Rev.2.1_01
S-1137 Series
7. Soft-start function
The S-1137 Series has a soft-start circuit to suppress the rush current (IRUSH) and overshoot of the output voltage
generated at power-on or at the time when the ON / OFF pin is set to ON.
Immediately after power-on or after the ON / OFF pin is set to ON, the output voltage slowly rises. The time that the
output voltage rises to 99% (tSS) is adjustable by a capacitor (CSS) externally connected between the SSC and VSS
pins. When CSS = 1.0 nF, the time is 0.7 ms typ. At the time when the ON / OFF pin is set to OFF, the electric charge
charged in the external capacitor (CSS) will be discharged by the discharge shunt transistor.
The soft-start time (tSS) is defined by time consonants of the built-in constant current (approx. 1 μA) and the external
capacitor (CSS):
tSS (ms) = soft-start coefficient × CSS (nF)
The soft-start coefficient (25°C) is as follows.
Min. : 0.4, Typ. : 0.7, Max. : 1.2
Input
voltage
(V_IN)
SSC pin
voltage
(V_SSC)
approx.0.7 V
Output
voltage
(V_OUT)
V_OUT × 0.99
Soft-start time (tSS)
Figure 20
The rush current can be suppressed greatly by the soft-start function.
As the value of the external capacitor is larger, the rush current decreases more.
Figure 21 shows the waveform example of the rush current.
Input
voltage
(VIN)
t [ms]
Rush
current
(IRUSH)
Without soft-start : about 400 mA at the peak
With soft-start : about 20 mA at the peak
t [ms]
Figure 21 Waveform Example of Rush Current
21