PM6685 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

MFG CO.

PM6685

Dual step-down main supply controller with auxilary voltages for notebook system power

STMicroelectronics 

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Device description

Equation 8

INEG

=

120mV

RDSon

PM6685

7.7

Soft start and soft end

Each switching section is enabled separately by asserting high EN5/EN3 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

700µs (typ.). After 2.8ms (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 9

CINT

≥

6uA

ILvalley + ∆IL

× Cout

4

2

Figure 35. Soft start waveforms

Switching output

Current limit threshold

EN5/EN3

Time

When a switching section is turned off(EN5/EN3 pins low), the controller enters in soft end

mode. The output capacitor is discharged through an internal 16Ω P-MOSFET switch; when

the output voltage reaches 0.3V, the low-side MOSFET turns on, keeping the output to

ground. The soft end time also depends on load condition.

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