Datasheet

I
LR
=
(V
IN
- V
OUT
) x t
on
L
LMR24210
SNVS738G –OCTOBER 2011–REVISED APRIL 2013 www.ti.com
Figure 20. Shutdown Implementation
Current Limit
Current limit detection is carried out during the off-time by monitoring the re-circulating current through the
synchronous MOSFET. Referring to Simplified Functional Block Diagram, when the main MOSFET is turned off,
the inductor current flows through the load, the PGND pin and the internal synchronous MOSFET. If this current
exceeds 1.8A, the current limit comparator toggles, and as a result disabling the start of the next on-time period.
The next switching cycle starts when the re-circulating current falls back below 1.8A (and the voltage at the FB
pin is below 0.8V). The inductor current is monitored during the on-time of the synchronous MOSFET. As long as
the inductor current exceeds 1.8A, the main MOSFET will remain inhibited to achieve current limit. The operating
frequency is lower during current limit due to a longer off-time.
Figure 21 illustrates an inductor current waveform. On average, the output current I
OUT
is the same as the
inductor current I
L
, which is the average of the rippled inductor current. In case of current limit (the current limit
portion of Figure 21), the next on-time will not initiate until the current drops below 1.8A (assume the voltage at
the FB pin is lower than 0.8V). During each on-time the current ramps up an amount equal to:
(6)
During current limit, the LMR24210 operates in a constant current mode with an average output current I
OUT(CL)
equal to 1.8A + I
LR
/ 2.
Figure 21. Inductor Current - Current Limit Operation
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