PM6680 datasheet(28/49 Pages) STMICROELECTRONICS

28 / 49 page

Device description

PM6680

28/49

Where RSNS is the sensing element (RDSon)

PM6680 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.

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:

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

Figure 35.

Soft start waveforms

DSon

NEG

120

out

Lvalley

INT

∆

≥

EN1/EN2

Current limit threshold

Switching output

Time

PM6680 데이터시트(PDF) 28 Page - STMicroelectronics