PM6675STR datasheet(32/53 Pages) STMICROELECTRONICS

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Device description

PM6675S

32/53

It must be taken into account that the current limit circuit actually regulates the inductor

valley current. This means that RILIM must be calculated to set a limit threshold given by the

maximum DC output current plus half of the inductor ripple current:

Equation 21

The PM6675S provides also a fixed negative current limit to prevent excessive reverse

inductor current when the switching section sinks current from the load in forced-PWM (3rd

quadrant working conditions). This negative current limit threshold is measured between

PHASE and PGND pins, comparing the drop magnitude on PHASE pin with an internal

110 mV fixed threshold.

7.1.6

POR, UVLO and soft-start

The PM6675S 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 > 1V).

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 PM6675S 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 41).

Figure 41.

Soft-start waveforms

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

doesn't reach the under voltage threshold within soft-start duration, the UVP condition is

detected and 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

during the soft-start (Figure 42 shows two examples).

DSon

ILIM

100

⋅

SWEN

Current limit threshold

Switching output

Time

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