Datasheet
V
ON
+
1.35 V
ǒ
R2
R1)R2
Ǔ
V
OFF
+
1.25 V
ǒ
R2
R1)R2
Ǔ
C
T
+ 8.5 10
*6
t
ON
ǒ
1 ) C
OUT_TOL
) C
T_TOL
Ǔ
C
O
P
LIM
2 I
2
LIM
)
C
O
V
2
VCC(MAX)
2 P
LIM
if P
LIM
t I
LIM
V
VCC(MAX)
t
ON
+
C
O
V
VCC(MAX)
I
LIM
if P
LIM
w I
LIM
V
VCC(MAX)
V
PROG
+
P
LIM
10 I
LIM
where I
LIM
+
0.05
R
S
R4
R3 ) R4
+
V
PROG
V
REF
P
LIM
v 0.7
T
J(MAX)2
*
ƪ
ǒ
I
2
MAX
R
DSON
R
qCA
Ǔ
) T
A(MAX)
ƫ
R
qJC
R
DSON(MAX)
v
T
J(MAX)
* T
A(MAX)
R
qJA
I
2
MAX
TPS2490
TPS2491
www.ti.com
SLVS503D –NOVEMBER 2003–REVISED JULY 2012
(5)
where T
J(MAX)
is the desired maximum steady-state junction temperature (typically 125°C), and T
A(MAX)
is the
maximum ambient temperature. R
θJA
, the junction-to-ambient thermal resistance, depends upon the package
style chosen and the details of heat-sinking and cooling. Note the R
θJC
and R
θJA
for use below.
STEP 3. Choose P
LIM
, R3, R4
M1 dissipates large amounts of power during power-up or output short circuit. The power limit P
LIM
of the
TPS2940/91 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. An expression for calculating P
LIM
is:
(6)
where R
θJC
is M1 junction-to-case thermal resistance, R
DS(on)
is the channel resistance at the maximum operating
temperature, and the factor of 0.7 represents the tolerance of the constant power engine. Next calculate V
PROG
and the divider resistors R3 and R4. R3 must be greater than 4 kΩ, but it is recommended that 10 kΩ or greater
be used.
(7)
STEP 4. Choose t
ON
, C
T
The on-time, t
ON
, set by capacitor C
T
must suffice to fully charge the load capacitance C
O
without triggering the
fault circuitry. Assuming that only the load capacitance draws current during startup:
(8)
Using this value of t
ON
, C
T
is computed as:
(9)
where C
T_TOL
and C
OUT_TOL
are the tolerances associated with each capacitor. Assuming C
O
is a 20% tolerance
part, C
OUT_TOL
has a value of 0.2. This expression assures the worst case set of parts will always start.
STEP 5. Choose The Turn On Voltage, R1 and R2
Assuming that EN is used as an analog input, the turn-on voltage, V
ON
and turn-off voltage, V
OFF
are defined as:
(10)
Use caution in selecting large values of R1 and R2 because the leakage current causes errors in the threshold
voltages.
Copyright © 2003–2012, Texas Instruments Incorporated 15