PM6680A datasheet(25/48 Pages) STMICROELECTRONICS

25 / 48 page

PM6680A

Device description

25/48

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 = 100 µA, 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 8

Where RSNS is the sensing element(RDSon)

PM6680A 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 120 mV.

The negative valley-current limit INEG (if the device works in PWM mode) is given by:

Equation 9

7.7

Soft start and soft end

Each switching section is enabled separately by asserting high EN1/EN2 pins respectively.

In order to realize the soft start, at the startup the overcurrent threshold is set 25 % of the

nominal value and the undervoltage protection (see related sections) is disabled. The

controller starts charging the output capacitor working in current limit. The overcurrent

threshold is increased from 25 % to 100 % of the nominal value with steps of 25 % every

700 µs (typ.). After 2.8 ms (typ.) the undervoltage protection is enabled. The soft start time is

not programmable. A minimum capacitor CINT is required to ensure a soft start without any

overshoot on the output:

Equation 10

CSENSE

DS on

()

Lvalley

Icsense

---------------------------------------------

SNS

CSENSE

OFF

CSENSE

Lvalley

∆

⎥

⎦

⎤

⎢

⎣

⎡

∆

100

DSon

NEG

120

out

Lvalley

INT

∆

≥

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