Datasheet

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 T
OFF
by measuring
the voltage drop across the integrated low-side MOSFET using the MOSFET R
DS(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 I
LRMS
(max) equal to:
Equation 6
where ∆I
L
is the inductor current ripple.
Figure 22. Current waveforms in current limit conditions
Figure 21. Inductor current in skip mode
TSW
CCM
Load> ∆IL/2
∆IL
IL
Load< ∆IL/2
t
t
IL
∆IL
TSW
DCM
AM11781v1
I
LRMS
max()350mA
I
L
∆
2
------- -+=
Time
maximum DC current
350mA valley current
limit threshold
Inductor current
Current
AM11780v1