PM6670AS View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'PM6670AS' PDF : 54 Pages View PDF
PM6670AS
Device description
Figure 34. Current sensing scheme
VIN
PPMM66667700AS
HGATE
PHASE
CSNS
R 100µA· ILIM
LGATE
PGND
VOUT
I R· VALLEY DSon
An internal 120 μA current source is connected to CSNS pin that is also the non-inverting
input of the positive current limit comparator. When the voltage drop developed across the
sensing parameter equals the voltage drop across the programming resistor RILIM, the
controller skips subsequent cycles until the overcurrent condition is detected or the output
UV protection latches off the device (see Section 7.1.11: Over voltage and under voltage
protections on page 35).
7.1.6
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
ICL
= 120μA ⋅ RILIM
RDSon
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.
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).
Doc ID 14436 Rev 2
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