PM6685 datasheet(28/52 Pages) STMICROELECTRONICS

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Device description

PM6685

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Equation 8

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

700

µs (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

Figure 35. Soft start waveforms

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 16

Ω 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.

DSon

NEG

120

out

Lvalley

INT

∆

≥

EN5/EN3

Current limit threshold

Switching output

Time

PM6685 Datasheet(PDF) 28 Page - STMicroelectronics