Datasheet

Device description PM6641
28/47 Doc ID 13510 Rev 3
7.6 Outputs soft-start
The soft-start function of each switching regulator is achieved by ramping up the SS pin
voltage with a constant slew rate dV/dt.
When the switching section is enabled (EN high), the SS pin constant current charges the
capacitor connected between SS and ground pins.
The SS voltage is used as reference of the switching regulator and the output voltage of the
converter follows the ramp of the SS voltage. When the SS pin voltage is higher than 0.8 V,
the error amplifier uses the internal 0.8 V ±1% reference to regulate the output voltage.
Figure 31. SW regulator programmable soft-start
During the soft-start period the current limit is set to the nominal value.
The dV/dt slope is set by charging the external capacitor with a 10 µA current. The
capacitance values has to be of the order of magnitude of 10 nF for a 1 msec soft-start
duration, as pointed out by the following formula:
Equation 6
During the soft-start the output under voltage management is not enabled, whereas the
output over voltage, the current limit and the thermal overheat are always monitored.
When the first switching regulator is turned on the output soft-start begins after an additional
delay of about 180 µs, due to PM6641 initializing and fuses reading.
SS_1Sx
SS DO
VREF
Iss
SW
Regulator
nF5.12
V8.0
ms1A10
V
tI
C =
⋅μ
=
Δ
Δ⋅
=