PM6670S View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'PM6670S' PDF : 53 Pages View PDF
Device description
PM6670S
7.1.6
Referring to Figure 36, the RDS(on) 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
ICL
= 120μA ⋅ RILIM
RDSon
The PM6670S 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.
POR, UVLO and soft-start
The PM6670S 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 start-up
reducing the input/output surge currents. At the beginning of start-up, the PM6670S 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
Switching output
32/54
Current limit threshold
S5
Doc ID 14432 Rev 4
Time
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