PM6685 datasheet(26/52 Pages) STMICROELECTRONICS

26 / 52 page

Device description

PM6685

26/52

If, due to the load, the frequency is higher than 33kHz, the device works like in skip mode.

No-audible skip mode reduces audio frequency noise that may occur in pulse skip mode at

very light loads, keeping the efficiency higher than in PWM mode.

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)

Figure 33. RDSON sensing technique

An internal 100

µA ∆I

L current source (ICSENSE) 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 output current is equal to the valley current limit plus half of the inductor ripple

current:

Equation 5

The output current limit depends on the current ripple, as shown in Figure 34 on page 27:

LGATE

HGATE

CSENSE

PHASE

Rcsense

RDSon

(max)

Lvalley

LOAD

∆

PM6685 Datasheet(PDF) 26 Page - STMicroelectronics