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IRS20955(S)PbF
CSH
Comparator
OC
+
-
CSH
VB
R3
R2
R1
D1
+B
1.2V
HO
HO
VS
Q1
OUT
Vcc
LO
Q2
IRS20955
-B
Figure 14. Programming High-Side Over-Current Threshold
High-Side Over-Current Setting
Deadtime Generator
Figure 14 demonstrates the typical circuitry used for
R 3 = 10 k Ω ?
high-side current sensing. In the following example,
the over-current protection level is set to trip at 30 A
using a MOSFET with an R DS(ON) of 100 m Ω . The
component values of R2 and R3 can be calculated
using the following formula:
Let R2 + R3=10 k Ω .
Vth OCH
V DS + V F
where V th,OCL = 1.2 V
V F = the forward voltage of reverse blocking
diode D1 = 0.6 V.
V DS@ID=30A = the voltage drop across the
high-side MOSFET when the MOSFET current is 30
A.
Therefore, V DS@ID=30A = I D x R DS(ON) = 30 A x 100 m Ω
=3V
Based on the formulas above, R2 = 6.8 k Ω and R3
= 3.3 k Ω .
Choosing the Right Reverse Blocking Diode
The selection of the appropriate reverse blocking
diode used in place of D1 depends on its voltage
rating and speed. To effectively block bus voltages,
the reverse voltage must be higher than the voltage
difference between +B and -B and the reverse
recovery time must be as fast as the boot strap
charging diode. A diode such as the Philips BAV21
W, a 200 V, 50 ns high speed switching diode, is
more than sufficient.
www.irf.com
Deadtime is a blanking period inserted between
high-side turn on and low-side turn on to prevent
shoot through. In the IRS20955S, an internal dead-
time generation block allows the user to select the
optimum deadtime from a range of preset values.
Selecting a preset deadtime through the DT/SD pin
voltage can easily be done through an external
voltage divider. This way of setting deadtime
prevents outside noise from modulating the
switching timing, which is critical to the audio
performances.
How to Determine Optimal Deadtime
The effective deadtime in an actual application
differs from the deadtime specified in this datasheet
due to the switching fall time, t f . . The deadtime value
in this datasheet is defined as the time period
between the beginning of turn-off on one side of the
switching stage and the beginning of turn-on on the
other side as shown in Figure 15. The fall time of
MOSFET gate voltage must be subtracted from the
deadtime value in the datasheet to determine the
effective deadtime of a Class D audio amplifier.
(Effective deadtime) = (Deadtime in datasheet) – t f .
14
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