Datasheet

I
LR
=
(V
IN
- V
OUT
) x t
on
L
LMR24220
SNVS737E –OCTOBER 2011–REVISED APRIL 2013 www.ti.com
Current Limit
Current limit detection is carried out during the off-time by monitoring the re-circulating current through the
synchronous MOSFET. Referring to the 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 2.8A, 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 2.8A (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 2.8A, 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 20 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 20), the next on-time will not initiate until the current drops below 2.8 (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 LMR24220 operates in a constant current mode with an average output current I
OUT(CL)
equal to 2.8A + I
LR
/ 2.
However, due to thermal limitations, the device may not support load currents greater than 2A for extended
periods.
Figure 20. Inductor Current - Current Limit Operation
N-Channel MOSFET and Driver
The LMR24220 integrates an N-Channel main MOSFET and an associated floating high voltage main MOSFET
gate driver. The gate drive circuit works in conjunction with an external bootstrap capacitor C
BST
and an internal
high voltage diode. C
BST
connecting between the BST and SW pins powers the main MOSFET gate driver during
the main MOSFET on-time. During each off-time, the voltage of the SW pin falls to approximately -1V, and C
BST
charges from V
CC
through the internal diode. The minimum off-time of 260 ns provides enough time for charging
C
BST
in each cycle.
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