Datasheet
2
VIN VIN MIN LIM _ MIN
MIN VOUT
V V 4 R P
R I V
2
± - ´ ´
´ = =
2
VIN VIN MIN LIM _ MIN
MIN
V V 4 R P
I
2 R
± - ´ ´
=
´
2
MIN VIN LIM _ MIN
R I V I P 0´ - ´ + =
2
LOAD LIM LOAD VIN
ON
2
LIM
LIM
C P C V
t = +
2 P
2 I
´ ´
´
´
TPS2421-1
TPS2421-2
www.ti.com
SLUS907H –JANUARY 2009–REVISED JANUARY 2014
APPLICATION INFORMATION
Startup
Large inrush current occurs when power is applied to discharged capacitors and load. During the inrush period,
the TPS2421 device operates in power limit (or SOA protect mode) managing the current as V
VOUT
rises. In SOA
protect mode, the internal MOSFET power dissipation ([V
VIN
– V
VOUT
] x I
VOUT
) is regulated at 5W typical while the
fault timer starts and C
CT
ramps up. As the charge builds on C
LOAD
, the current increases towards I
LIM
. When the
capacitor is fully charged, I
VOUT
drops to the dc load value, the fault timer stops, and C
CT
ramps down. In order
for the TPS2421 device to start properly, the fault timer duration must exceed C
LOAD
start up time, t
ON
. Start-up
time without additional dc loading is calculated using Equation 5 where P
LIM
= 5 W (typical).
(5)
When the load has a resistive component in addition to C
LOAD
, the fault time must be extended because the
resistive load current is unavailable to charge C
LOAD
. Use Table 2 and Table 3 to predict start-up time in the
presence of resistive dc loading.
Refer to the TPS2421 Design Calculator Tool (SLUC427) for assistance with design calculations.
Table 2. Start up Time (ms) with DC Loading: V
IN
= 5 V, P
LIM
= 3 W, I
LIM
= 5 A
R
LOAD_
(Ω) C
LOAD_
= 100 µF C
LOAD_
= 220 µF C
LOAD_
= 470 µF C
LOAD_
= 1000 µF
1000 0.43 0.95 2.03 4.33
10 0.5 1.11 2.36 5.03
5 0.61 1.34 2.87 6.1
3 0.91 2 4.28 9.11
2.5 1.31 2.88 6.14 13.07
Table 3. Start up Time (ms) with DC Loading: V
IN
= 12 V, P
LIM
=3W, I
LIM
= 5 A
R
LOAD_
(Ω) C
LOAD_
= 100 µF C
LOAD_
= 220 µF C
LOAD_
= 470 µF C
LOAD_
= 1000 µF
10000 2.46 5.41 11.56 24.59
100 2.67 5.87 12.55 26.69
50 2.93 6.45 13.79 29.34
15 6.7 14.74 31.5 67.01
13 11.68 25.69 54.87 116.75
Maximum Allowable Load to Ensure Successful Start up
The power limiting function of the TPS2421 device provides very effective protection for the internal FET. As
expected, there is a supply voltage dependent maximum allowable load required for successful startup. Loads
above this can cause the output to shut off due to CT timeout or thermal shutdown because V
VOUT
hangs at an
intermediate voltage below V
IN
. The equation for maximum load (or R
MIN
is derived using the circuit equations for
V
VOUT
as a function of V
VIN
, R
LOAD
, P
LIM
, and the result is quadratic in form.
(6)
(7)
(8)
When R
LOAD
< R
MIN
, the numerical result for V
VOUT
is real (V
VIN
2
– 4 × R
LOAD
× P
LIM
> 0) and less than V
VIN
meaning the circuit will not start (CT or thermal shutdown). When R
LOAD
> R
MIN
, the numerical result for V
VOUT
is
imaginary (V
VIN
2
– 4 × R
LOAD
x P
LIM
< 0) and the circuit will start (V
VOUT
= V
VIN
). Ensure that R
LOAD
is > R
MIN
per
Equation 10.
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