IR3651SPBF
06/08/2011
Over-Current Protection
The over current protection is performed by
sensing current through the R
DS(on)
of low side
MOSFET. This method enhances the converter’s
efficiency and reduce cost by eliminating a
current sense resistor. As shown in figure 6, an
external resistor (R
SET
) is connected between
OCSet pin and the drain of low side MOSFET
(Q2) which sets the current limit set point.
The internal current source develops a voltage
across R
SET
. When the low side MOSFET is
turned on, the inductor current flows through the
Q2 and results a voltage which is given by:
The OCP circuit starts sampling current
approximately 200ns before the low gate drive
turns off. The OCSet pin is internally clamped to
about 1.5V during deadtime to prevent false
trigging, figure 8 shows the OCSet pin during one
switching cycle.
Fig. 6: Connection of over current sensing resistor
Fig. 7: 3uA current source for discharging
soft-start capacitor during hiccup
An over current is detected if the OCSet pin goes
below ground. This trips the OCP comparator
and cycles the soft start function in hiccup mode.
The hiccup is performed by charging and
discharging the soft-start capacitor in certain
slope rate. As shown in figure 7 a 3uA current
source is used to discharge the soft-start
capacitor.
The OCP comparator resets after every soft start
cycles, the converter stays in this mode until the
overload or short circuit is removed. The
converter will automatically recover.
)2) --(I(R)R(IV
LDS(on)OCSetOCSetOCSet
∗−∗=
0)I(R)R(IV
LDS(on)OCSetOCSetOCSet
=∗
∗=
)3 --(
R
IR
II
onDS
OCSetOCSet
criticalLSET
)(
)(
∗
==
Fig. 8: OCset pin during normal condition
Ch1: Inductor point, Ch2:Ldrv, Ch3:OCSet
The value of R
SET
should be checked in an actual
circuit to ensure that the over current protection
circuit activates as expected. The IR3651 current
limit is designed primarily as disaster preventing,
"no blow up" circuit, and doesn't operate as a
precision current regulator.
The critical inductor current can be calculated by
setting:
L1
R
SET
IR3651
OCSet
I
OCSET
V
OUT
Hiccup
Control
Q1
Q2
i
I51II
L
ocriticalLSET
+== *.
)(
11
SS / SD
10uA
3uA
Hiccup
3
