PM6670ASTR datasheet(32/54 Pages) STMICROELECTRONICS

32 / 54 page

Device description

PM6670AS

32/54

Referring to Figure 34, 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 PM6670AS 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

120 mV fixed threshold.

7.1.6

POR, UVLO and soft-start

The PM6670AS 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 (4.2 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 start-up

reducing the input/output surge currents. At the beginning of start-up, the PM6670AS

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 35).

Figure 35. Soft-start waveforms

DSon

ILIM

100

⋅

Current limit threshold

Switching output

Time

PM6670ASTR Fiches technique(PDF) 32 Page - STMicroelectronics