Datasheet

V
OUT
- Output Voltage - V
0
10
30
60
I
OUT
- Output Current - A
OUTPUT CURRENT
vs
OUTPUT VOLTAGE (V
VCC
= 12 V)
12 8 4 0
50
2610
20
40
REF
4 5
PROG
V
R R ( 1) 140.6kΩ
V
= ´ - =
LIM
PROG
LIM
P
249
V 0.498V
10 I 10 50
= = =
´ ´
( )
2
J(MAX2) CA LIMIT(NOM) DSON A(MAX)
LIM
C
0.7 T R I R T
P 249
R
é ù
´ - ´ ´ -
ë û
= =
J
W
q
q
TPS2492
TPS2493
SLUSA65C –JULY 2010–REVISED JANUARY 2013 www.ti.com
3. Choose the Power Limit P
LIM
and the PROG Resistors, R
4
and R
5
M1 dissipates large amounts of power during power-up or output short circuit. Power limit, P
LIM
should be set to
prevent the M1 die temperature from exceeding a short term maximum temperature, T
J(MAX2)
. Short term T
J(MAX2)
may be set as high as 150°C (specified on FET datasheet) while still leaving ample margin for the typical
manufacturer's rating of 175°C. The R4 and R5 resistors set V
PROG
, programming the FET power dissipation.
Assume that R
θJA
is 10 °C/W, R
θJC
is 0.2 °C/W, and R
θCA
is 9.8 °C/W for the device we chose above. P
LIM
can
be estimated as follows:
(9)
Where R
θCA
is the M1 plus PCB case-to-ambient thermal resistance, R
θJC
is M1 junction-to-case thermal
resistance, R
DSON
is M1 channel resistance at the maximum operating temperature, and the factor of 0.7
accounts for the tolerance of the constant power engine. In this case we know that power limit is less than I
LIMIT
x
V
IN
and that power limit will control operation during a short circuit.
It is often advantageous to use a transient value of R
θJC
to get a usable solution, that is a V
PROG
within the
recommended range. If a current/power limited startup is used, transient R
θJC
should be based on the TIMER
period (see below). FET manufacturers typically provide transient thermal resistance in graphic format on their
datasheet. Additional information can be found in SLVA158.
The following equations calculate V
PROG
and R4 using an assumed R
5
= 20 kΩ.
(10)
(11)
Choose R
4
= 140 kΩ. The recommended minimum V
PROG
is 0.4 V. This is based on tolerance and accuracy of
the constant power engine making very low power-limited designs highly variable. Some suggestions to get
larger P
LIM
values are to start with a low static operating junction temperature, and to utilize the transient thermal
impedance (energy absorbing nature) of the package.
The output I vs. V
OUT
curve for this configuration is shown in Figure 20.
Figure 20. TPS2492/93 Power and Current Limit Curve
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