PM6644 datasheet(18/35 Pages) STMICROELECTRONICS

18 / 35 page

Device description

PM6644

18/35

Doc ID 023203 Rev 1

2.1.5

Current sensing and current limit

The PM6644 implements a positive valley current limit to protect the application from an

overcurrent fault. The inductor current is sensed during the OFF-time TOFF by measuring

the voltage drop across the integrated low-side MOSFET using the MOSFET RDS(on) as a

lossless sensing element. The voltage drop is then compared with a fixed voltage threshold

so that the inductor (or low-side MOSFET) trip current of the comparator is about 350 mA

(minimum value).

A new switching cycle cannot start until the inductor current goes lower than the 350 mA

current limit threshold. As a result, the device can work with a maximum inductor RMS

current ILRMS (max) equal to:

Equation 6

where

∆IL is the inductor current ripple.

Figure 22.

Current waveforms in current limit conditions

Figure 21.

Inductor current in skip mode

TSWCCM

Load> ∆IL/2

∆IL

Load< ∆IL/2

∆IL

TSWDCM

AM11781v1

LRMS max

()

350mA

∆

--------

Time

maximum DC current

350mA valley current

limit threshold

Inductor current

Current

AM11780v1

PM6644 Fiches technique(PDF) 18 Page - STMicroelectronics