PM6685 datasheet(27/52 Pages) STMICROELECTRONICS

27 / 52 page

PM6685

Device description

27/52

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 6

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(∆ICSENSE):

Equation 7

Where RSNS is the sensing element (RDSon).

PM6685 provides also a fixed negative peak current limit to prevent an excessive reverse

inductor current when the switching section sinks current from the load in PWM mode. This

negative current limit threshold is measured between PHASE and SGND pins, comparing

the magnitude drop on the PHASE node during the conduction time of the low side

MOSFET with an internal fixed voltage of 120mV.

If the current is sensed on the low side MOSFET, the negative valley-current limit INEG (if

the device works in PWM mode) is given by:

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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