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-EP
www.ti.com SGLS371A –JANUARY 2007–REVISED JANUARY 2013
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 also can be used to trim the overcurrent trip point to a higher level with the addition of a
single resistor. See Figure 28. The nominal overcurrent trip point using the circuit of Figure 28 is described as:
(3)
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: .
(4)
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 PC board in the event of an overcurrent event.
Figure 28. Current-Limit Adjustment
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