AN2833 Datasheet, PDF (19/32 Pages) STMicroelectronics – Dual step-down controller with auxiliary voltages for industrial system power

AN2833
Test setup and performance summary
11.2
Power-up
As shown in Figure 8 power-up starts when the input voltage is applied and the voltage on
the SHDN pin is above the SHDN on threshold (1.5 V). First LDO5 goes up with a masking
time of about 4 ms. If the LDO5 output is above the UVLO threshold at this time, the device
enters standby mode and the adjustable internal linear regulator LDO is turned on.
Figure 8. REF, LDO5 and LDO power-up
AM01443v1
11.3
Soft-start and shutdown waveforms
Figure 9, 10, 11 and 12 show respectively the soft-start and shutdown waveforms.
The PM681A has an independent internal digital soft-start for each switching section.
During the soft-start phase the internal current limit increases from 25% to 100% with steps
of 25% to avoid that the inductor current rises abruptly.
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