PM6681ATR datasheet(23/47 Pages) STMICROELECTRONICS

23 / 47 page

PM6681A

Device description

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The low side switch is turned on until the output voltage crosses about Vreg+1 %. Then the

high side MOSFET is turned on for a fixed on time period. Afterwards the low side switch is

enabled until the inductor current reaches the zero-crossing threshold. This keeps the

switching frequency higher than 33 kHz. As a consequence of the control, the regulated

voltage can be slightly higher than Vreg (up to 1 %).

If, due to the load, the frequency is higher than 33 kHz, 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 35).

Figure 35.

RDS(on) sensing technique

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 output current 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 36:

(max)

Lvalley

LOAD

∆

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