Datasheet

SET
PROG
LIM SENSE IMON
PROG
R
84375
R
P R R
therefore,
84375 51.1
R 51.45 k
29.3 W 0.002 1430
= ´
´
W
= ´ = W
´ W W
( )
( ) ( )
(
)
2
J(MAX)2 MAX DS(on) CA A(MAX)
LIM
JC
2
LIM
T I r R T
P 0.8 ,
R
therefore,
130 C 12 A 0.002 51 C / W 1.8 C / W 50 C
P 0.8 29.3 W
1.8 C / W
q
q
é ù
- ´ ´ +
ë û
£ ´
é ù
° - ´ W ´ ° - ° + °
ê ú
ë û
£ ´ =
°
( )
J(MAX) A(MAX)
DS(on)(MAX)
2
MAX JA
DS(on)(MAX)
2
T T
r ,
I R
therefore,
150 C 50 C
r 13.6 m
12 A 51 C / W
q
-
=
´
° - °
= = W
´ °
TPS24720
www.ti.com SLVSAL1C –MARCH 2011–REVISED SEPTEMBER 2013
Next select the on-resistance of the transistor, r
DS(on)
. The maximum on-resistance must not generate a voltage
greater then the minimum power-good threshold voltage of 140 mV. Assuming a current limit of 12 A, a
maximum r
DS(on)
of 11.67 mΩ is required. Also consider the effect of r
DS(on)
on the maximum operating
temperature T
J(MAX)
of the MOSFET. Equation 6 computes the value of r
DS(on)(MAX)
at a junction temperature of
T
J(MAX)
. Most manufacturers list r
DS(on)(MAX)
at 25°C and provide a derating curve from which values at other
temperatures can be derived. Compute the maximum allowable on-resistance, r
DS(on)(MAX)
, using Equation 6.
(6)
Taking these factors into consideration, the TI CSD16403Q5 was selected for this example. This transistor has a
V
GS(MAX)
rating of 16 V, a V
DS(MAX)
rating of 25 V, and a maximum r
DS(on)
of 2.8 mΩ at room temperature. During
normal circuit operation, the MOSFET can have up to 10 A flowing through it. The power dissipation of the
MOSFET equates to 0.24 W and an 9.6°C rise in junction temperature. This is well within the data sheet limits for
the MOSFET. The power dissipated during a fault (e.g., output short) is far larger than the steady-state power.
The power handling capability of the MOSFET must be checked during fault conditions.
STEP 3. Choose Power-Limit Value, P
LIM
, and R
PROG
MOSFET M
1
dissipates large amounts of power during inrush. The power limit P
LIM
of the TPS24720 should be
set to prevent the die temperature from exceeding a short-term maximum temperature, T
J(MAX)2
. The short-term
T
J(MAX)2
could be set as high as 150°C while still leaving ample margin to the usual manufacturer’s rating of
175°C. Equation 7 is an expression for calculating P
LIM
,
(7)
where R
θJC
is the junction-to-case thermal resistance of the MOSFET, r
DS(on)
is the its resistance at the maximum
operating temperature, and the factor of 0.8 represents the tolerance of the constant-power engine. For an
ambient temperature of 50°C, the calculated maximum P
LIM
is 29.3 W. From Equation 1, a 53.6-kΩ, 1% resistor
is selected for R
PROG
(see Equation 8).
(8)
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