Datasheet

8
7
VDD
ISNS
R
ILIM
6
GDRV
V
IN
R
F1
R
F2
C
F
TPS40200
SWIN
O
f
fV
V
C
f2f1
RR ´
f2f1
RR +
÷
´
£
2F
2F1F
ILIM
ILIM
OC
R
RR
R
V
=
I
+
´
TPS40200-Q1
SLUS739E –SEPTEMBER 2006–REVISED JULY 2013 www.ti.com
If necessary, a small R-C filter can be added to the current sensing network to reduce nuisance tripping due to
noise pickup. This filter can also be used to trim the overcurrent trip point to a higher level with the addition of a
single resistor (see Figure 31). The nominal overcurrent trip point using the circuit of Figure 31 is described as:
Where:
I
OC
= overcurrent trip point, peak current in the inductor
V
ILIM
= overcurrent threshold voltage for the TPS40200,
typically 100 mV
R
ILIM
= value of the current sense resistor in Ω
R
F1
and R
F2
= values of the scaling resistors in Ω
The value of the capacitor is determined by the nominal pulse width of the converter and the values of the
scaling resistors R
F1
and R
F2
. It is best not to have the time constant of the filter longer than the nominal pulse
width of the converter, otherwise a substantial increase in the overcurrent trip point occurs. Using this constraint,
the capacitor value may be bounded by the following:
Where:
C
f
= value of the current limit filter capacitor in F
V
O
= output voltage of the converter
V
IN
= input voltage to the converter
f
SW
= converter switching frequency
R
f1
and R
f2
= values of the scaling resistors in Ω
NOTE: The current-limit resistor and its associated circuitry can be eliminated, and pins 7 and 8 shorted. However, the result
of this may result in damage to the part or PCB in the event of an overcurrent event.
Figure 31. Current-Limit Adjustment
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