PM6680ATR datasheet(24/48 Pages) STMICROELECTRONICS

24 / 48 page

Device description

PM6680A

24/48

7.6

Current limit

The current-limit circuit employs a "valley" current-sensing algorithm. During the conduction

time of the low side MOSFET the current flowing through it is sensed. The current-sensing

element is the low side MOSFET on-resistance (Figure 33).

An internal 100 µA current source is connected to CSENSE pin and determines a voltage

drop on RCSENSE. If the voltage across the sensing element is greater than this voltage

drop, the controller doesn't initiate a new cycle. A new cycle starts only when the sensed

current goes below the current limit.

Since the current limit circuit is a valley current limit, the actual peak current limit is greater

than the current limit threshold by an amount equal to the inductor ripple current.

Moreover the maximum DC load is equal to the valley current limit plus half of the inductor

ripple current:

Equation 6

The output current limit depends on the current ripple, as shown in Figure 34:

Figure 34.

Current waveforms in current limit conditions

Figure 33.

Rsense sensing technique

LGATE

HGATE

CSENSE

PHASE

Rcsense

RDSon

(max)

Lvalley

LOAD

∆

PM6680ATR Datasheet(PDF) 24 Page - STMicroelectronics