Datasheet

2F
2F1F
ILIM
ILIM
OC
R
RR
R
V
=
I
+
´
( )
( )
OUT
IN SW
F
F1 F2
F1 F2
V
V f
C
R R
R R
æ ö
ç ÷
´
è ø
£
´
+
7ISNS
TPS40200
8
6
VDD
GDRV
R
F2
C
F
R
F1
R
ILIM
V
IN
UDG-05071
TPS40200
www.ti.com SLUS659F –FEBRUARY 2006–REVISED MARCH 2012
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 27. The nominal overcurrent trip point using the circuit of Figure 27 is described in
Equation 3.
where
• I
OC
is the overcurrent trip point, peak current in the inductor
• V
ILIM
is the overcurrent threshold voltage for the TPS40200, typically 100 mV
• R
ILIM
is the value of the current sense resistor in Ω
• R
F1
and R
F2
are the values of the scaling resistors in Ω (3)
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 Equation 4.
where
• C
F
is the value of the current limit filter capacitor in F
• V
OUT
is the output voltage of the converter
• V
IN
is the input voltage to the converter
• f
SW
is the converter switching frequency
• R
F1
and R
F2
are the values of the scaling resistors in Ω (4)
Figure 27. Current Limit Adjustment
NOTE
The current limit resistor and its associated circuitry can be eliminated and the ISNS pin
(pin 7) and the VDD pin (pin 8) are shorted. The result of this however, may result in
damage to the device or PC board during an overcurrent event.
Copyright © 2006–2012, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Link(s): TPS40200