Datasheet

SET
IMON
LIM SENSE
IMON
0.675 V R
R ,
I R
therefore,
0.675 V 51.1
R 1437
12 A 2 m
´
=
´
´ W
= = W
´ W
R
SET
R
IMON
LOADPROTECTION
0.1 Fμ 0.1 Fμ
B0440-01
M
1
12-VMainBusSupply
TIMER
OUT
GATE
VCC
SENSE
GND
TPS24720
Specifications(atOutput):
PeakCurrentLimit=12 A
NominalCurrent=10 A
R
1.2
LOAD
W
C
470 F
OUT
μ
C
T
R
SENSE
R
GATE
SET
IMON
TPS24720
SLVSAL1C –MARCH 2011–REVISED SEPTEMBER 2013 www.ti.com
Figure 37. Simplified Block Diagram of the System Constructed in the Design Example
STEP 1. Choose R
SENSE
, R
SET
, and R
IMON
The recommended range of the current-limit threshold voltage, V
(VCC – SENSE)
, extends from 10 mV to 42 mV.
Values near the low threshold of 10 mV may be affected by system noise. Values near the upper threshold of
42 mV may be too close to the minimum fast-trip threshold voltage of 52 mV. Values near the middle of this
range help minimize both concerns.
To achieve high efficiency, the power dissipation in R
SENSE
must be kept to a minimum. A R
SENSE
of 2 mΩ
develops a voltage of 24 mV at the specified peak current limit of 12 A, while dissipating only 200 mW at the
rated 10-A current. This represents a 0.17% power loss.
For best performance, a current of approximately 0.5 mA (referring to the RECOMMENDED OPERATING
CONDITIONS table) should flow into the SET pin and out of the IMON pin when the TPS24720 is in current limit.
The voltage across R
SET
nominally equals the voltage across R
SENSE
, or 24 mV. Dividing 24 mV by 0.5 mA gives
a recommended value of R
SET
of 48 Ω. A 51.1-Ω, 1% resistor was chosen. Using Equation 3, the value of R
IMON
must equal 1437 Ω, or as near as practically possible. A 1.43-kΩ, 1% resistor was chosen.
(5)
STEP 2. Choose MOSFET M
1
The next design step is to select M
1
. The TPS24720 is designed to use an N-channel MOSFET with a gate-to-
source voltage rating of 20 V.
Devices with lower gate-to-source voltage ratings can be used if a Zener diode is connected so as to limit the
maximum gate-to-source voltage the transistor sees.
The next factor to consider is the drain-to-source voltage rating, V
DS(MAX)
, of the MOSFET. Although the
MOSFET only sees 12 V dc, it may experience much higher transient voltages during extreme conditions, such
as the abrupt shutoff that occurs during a fast trip. A TVS may be required to limit inductive transients under such
conditions. A transistor with a V
DS(MAX)
rating of at least twice the nominal input power-supply voltage is
recommended regardless of whether a TVS is used or not.
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