Datasheet

( )
VCC SENSE
SENSE
LIM
SENSE
V
R ,
I
therefore,
25 mV
R 2 m
12 A
-
=
= » W
LOADPROTECTION
0.1 Fμ 0.1 Fμ
B0440-02
M
1
12-VMainBusSupply
TIMER
OUT
GATE
VCC
SENSE
GND
TPS2471x
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
TPS24710, TPS24711
TPS24712, TPS24713
SLVSAL2E –JANUARY 2011–REVISED NOVEMBER 2013 www.ti.com
DESIGN EXAMPLE: POWER-LIMITED START-UP
This design example assumes a 12-V system voltage with an operating tolerance of ±2 V. The rated load current
is 10 A, corresponding to a dc load of 1.2 Ω. If the current exceeds 12 A, then the controller should shut down
and then attempt to restart. Ambient temperatures may range from 20°C to 50°C. The load has a minimum input
capacitance of 470 μF. Figure 34 shows a simplified system block diagram of the proposed application.
This design procedure seeks to control the junction temperature of MOSFET M
1
under both static and transient
conditions by proper selection of package, cooling, r
DS(on)
, current limit, fault timeout, and power limit. The design
procedure further assumes that a unit running at full load and maximum ambient temperature experiences a brief
input-power interruption sufficient to discharge C
OUT
, but short enough to keep M
1
from cooling. A full C
OUT
recharge then takes place. Adjust this procedure to fit your application and design criteria.
Figure 34. Simplified Block Diagram of the System Constructed in the Design Example
STEP 1. Choose R
SENSE
From the TPS24710/11/12/13 electrical specifications, the current-limit threshold voltage, V
(VCC – SENSE)
, is around
25 mV. A resistance of 2 mΩ is selected for the peak current limit of 12 A, while dissipating only 200 mW at the
rated 10-A current (see Equation 5). This represents a 0.17% power loss.
(5)
STEP 2. Choose MOSFET M
1
The next design step is to select M
1
. The TPS24710/11/12/13 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 across the transistor.
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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