Not recommended for new designs. IRS20954SPBF
Please use IRS20957SPBF.
www.irf.com 14
Figure 13: Low-Side Over-Current Sensing
Since the sensed voltage of V
S
is compared with the voltages fed to the OCSET
pin, the required voltage of OCSET with
respect to COM for a trip level I
TRIP+
is:
V
OCSET
= V
DS(LOW-SIDE)
= I
TRIP+
x R
DS(ON)
In order to neglect the input bias current of OCSET pin, it is recommended to use 10 kΩ total for R4 and R5 to drain 0.5 mA
through the resistors.
High-side Over-Current Sensing
For the positive load current, high-side over-current sensing monitors over load condition by measuring V
DS
with CSH and Vs
pins and shutdown the operation. The CSH pin is to detect the drain-to-source voltage refers to the V
S
pin which is the source
of the high-side MOSFET. In order to neglect overshoot ringing at the switching edges, CSH sensing circuitry starts monitoring
after the first 300 ns the HO is on by blanking the signal from CSH pin.
In contrast to the low-side current sensing, the threshold of CSH pin to engage OC protection is internally fixed at 1.2 V. An
external resistive divider R2 and R3 can be used to program a higher threshold.
An external reverse blocking diode, D1, is to block high voltage feeding into the CSH pin while high-side is off. By subtracting a
forward voltage drop of 0.6 V at D1, the minimum threshold which can be set in the high-side is 0.6 V across the drain to
source.
With the configuration in Fig. 14, the voltage in CSH is:
()
)1()(
32
3
DFHIGHSIDEDSCSH
VV
R
V +⋅
+
=
Where:
V
DS(HIGH-SIDE)
is drain to source voltage of the high-side MOSFET in its ON state
VF
(D1)
is the forward drop voltage of D1
Since V
DS(HIGH-SIDE)
is determined by the product of drain current I
D
and R
DS(ON)
in the high-side MOSFET. V
CSH
can be written
as:
()
)1()(
32
3
DFDONDSCSH
VIR
R
V +⋅⋅
+
=
1
3
2
−
+
=
OCH
FDS
Vth
VV
R
R
The reverse blocking diode D1 is forward biased by a 10 kΩ resistor R1 when the high-side MOSFET is on.
