PM6675AS datasheet(28/48 Pages) STMICROELECTRONICS

28 / 48 page

Device description

PM6675AS

28/48

7.1.6

POR, UVLO and soft-start

The PM6675AS automatically performs an internal startup sequence during the rising

phase of the analog supply of the device (AVCC). The switching controller remains in a

stand-by state until AVCC crosses the upper UVLO threshold (4.25 V typ.), keeping active

the internal discharge MOSFETs (only if AVCC > 1 V).

The soft-start allows a gradual increase of the internal current limit threshold during startup

reducing the input/output surge currents. At the beginning of start-up, the PM6675AS

current limit is set to 25 % of nominal value and the under voltage protection is disabled.

Then, the current limit threshold is sequentially brought to 100 % in four steps of

approximately 750

µs (figure 13).

Figure 31.

Soft-start waveforms

After a fixed 3 ms total time, the soft-start finishes and UVP is released: if the output voltage

doesn't reach the

under voltage lower threshold within soft-start duration, the UVP condition

is detected; the device performs a soft end and latches off. Depending on the load

conditions, the inductor current may or may not reach the nominal value of the current limit

(Figure 32 on page 29 shows two examples).

SWEN

Current limit threshold

Switching output

Time

PM6675AS Fiches technique(PDF) 28 Page - STMicroelectronics