PM6681ATR datasheet(25/47 Pages) STMICROELECTRONICS

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PM6681A

Device description

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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 s oft start time

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

any overshoot on the output:

Equation 10

Figure 37.

Soft-start waveforms

When a switching section is turned off (EN1/EN2 pins low), the controller enters in soft-end

mode. The output capacitor is discharged through an internal 18

Ω P-MOSFET switch;

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

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

7.8

Gate drivers

The integrated high-current drivers allow to use different power MOSFETs. The high side

driver MOSFET uses a bootstrap circuit which is indirectly supplied by LDO5 output. The

BOOT and PHASE pins work respectively as supply and return rails for the HS driver.

The low side driver uses the internal LDO5 output for the supply rail and PGND pin as return

rail.

An important feature of the gate drivers is the adaptive anti-cross conduction protection,

which prevents high side and low side MOSFETs from being on at the same time. When the

out

Lvalley

INT

∆

≥

EN1/EN2

Current limit threshold

Switching

Time

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