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IRS20955(S)PbF
+B
Q1
OC
REF
OCREF
5.1V
R4
0.5mA
OCSET
-
+
OC
VS
OUT
R5
OC Comparator
COM
LO
LO
Q2
IRS20955
-B
Figure 13. Low-Side Over-Current Sensing
R 5 =
? 10 k Ω
=
? 10 k Ω
Low-Side Over-Current Setting
Let the low-side MOSFET have an R DS(ON) of 100
m ? and set the current trip level to 30 A. V OCSET is
given by:
V OCSET = I TRIP+ x R DS(ON) = 30 A x 100 m Ω = 3.0 V
Choose R4+R5=10 k Ω to properly load the VREF
pin.
V OCSET
V REF
3 . 0 V
5 . 1 V
= 5 . 8 k Ω
where V REF = 5.1 V
High-side current sensing is based on the
measurement of V DS across the high-side MOFET
during high-side turn on through pins CSH and VS.
In order to avoid triggering OCP from overshoot, a
blanking interval inserted after HO turn on disables
over-current detection for 450 ns.
In contrast to low-side current sensing, the threshold
at which the CSH pin engages 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 required
to block high voltages from feeding into the CSH pin
while the high-side is off. Due to a forward voltage
drop of 0.6 V across D1, the minimum threshold
required for high-side over-current protection is 0.6
Based on the E-12 series of resistor values, choose
R5 to be 5.6 k Ω and R4 to be 3.9 k Ω to complete
the design.
V.
V CSH =
R 3
R 2 + R 3
? ( V DS ( HIGHSIDE ) + V F ( D 1 ) )
In general, R DS(ON) has a positive temperature
coefficient that needs to be considered when setting
the threshold level. Variations in R DS(ON) will affect
the selection of external or internal component
values.
where V DS(HIGH SIDE) = the drain to source voltage of
the high-side MOSFET during high-side turn on
V F(D1) = 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) of the high-side
MOSFET. V CSH can be rewritten as:
High-Side Over-Current Sensing
For positive load currents, high-side over-current
V CSH =
R 3
R 2 + R 3
? ( R DS ( ON ) ? I D + V F ( D 1 ) )
sensing also monitors the load condition and shuts
down the switching operation if the load current
exceeds the preset trip level.
www.irf.com
Note: The reverse blocking diode D1 is forward
biased by a 10 k Ω resistor R1 when the high-side
MOSFET is on.
13
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