PM6680 datasheet(27/49 Pages) STMICROELECTRONICS

27 / 49 page

PM6680

Device description

27/49

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 34:

Figure 34.

Current waveforms in current limit conditions

Being fixed the valley threshold, the greater the current ripple is, greater the DC output

current is The valley current limit can be set with resistor RCSENSE:

Equation 7

Where ICSENSE = 100uA, RDSon is the drain-source on resistance of the low side switch.

Consider the temperature effect and the worst case value in RDSon calculation.

The accuracy of the valley current threshold detection depends on the offset of the internal

comparator (

∆V

OFF) and on the accuracy of the current generator(∆VCSENSE):

Equation 8

(max)

Lvalley

LOAD

∆

Inductor current

Valley current threshold

DC current limit = maximum load

Current

Time

Maximum load current

is influenced by the

inductor current ripple

CSENSE

Lvalley

DSon

CSENSE

SNS

CSENSE

OFF

CSENSE

Lvalley

100

∆

⎥

⎦

⎤

⎢

⎣

⎡

∆

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