PM6641 datasheet(31/47 Pages) STMICROELECTRONICS

31 / 47 page

PM6641

Device description

31/47

The synchronous mode of operation provides the following total input current:

Equation 8

whereas by shifting the three regulator turn on pulses of 120 deg the resulting total input

current is given by

Equation 9

that is

times smaller than the one computed before.

The PM6641 SET_PH1 pin, if tied to AVCC, enables the synchronous switching regulators

high side MOSFET turn on, whereas if tied to ground enables the 120 deg phase shifting.

7.10

Peak current limit

The peak current limit performed by the PM6641 switching regulators allows to monitor,

cycle by cycle, the inductor current; this feature prevents IC wire bonding overheating and

failure.

If the current sensed on the monolithic high side MOSFET reaches the programmed current

limit the regulator starts behaving like a current generator, more than a voltage regulator.

Consequently, if the output load still increases the rail output capacitor discharges itself and

the regulator works as current generator until the Output Under Voltage occurs and the

regulator is latched off (see Chapter 7.11.2: Output under voltage on page 33 section for

details).

The pin 4 (CSNS) allows to select the right value for the peak current limit by inserting an

external resistor (RCSNS ) between this pin and ground. CSNS forces a constant voltage on

RCSNS resistor or, when tied to AVCC, enables the internal reference (equal to a 50 k

Ω

external resistor). A simple equation shows how to compute the right value for RCSNS in

order to decrease the peak current limit:

Equation 10

()

SYNC

CIN

∫

()

DELAY

CIN

∫

≅

REF

CSNS

⋅

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