PM6670 datasheet(31/53 Pages) STMICROELECTRONICS

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PM6670

Device description

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Referring to Figure 36, the RDSon sensing technique allows high efficiency performance

without the need for an external sensing resistor. The on-resistance of the MOSFET is

affected by temperature drift and nominal value spread of the parameter itself; this must be

considered during the RILIM setting resistor design.

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 22

The PM6670 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

120mV fixed threshold.

7.1.6

POR, UVLO and Soft Start

The PM6670 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 (4.2V 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 start-up

reducing the input/output surge currents. At the beginning of start-up, the PM6670 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 37).

Figure 37. Soft-start waveforms

DSon

ILIM

100

⋅

Current limit threshold

Switching output

Time

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