Datasheet

2
IN IN MIN LIM_ MIN
MIN OUT
V V 4 R P
R I V
2
± - ´ ´
´ = =
2
IN IN MIN LIM _ MIN
MIN
V V 4 R P
I
2 R
± - ´ ´
=
´
2
MIN IN LIM _ MIN
R I V I P 0´ - ´ + =
2
LOAD LIM LOAD IN
ON
2
LIM
LIM
C ×P C ×V
t = +
2×P
2×I
TPS2590
SLUS960G –JULY 2009–REVISED JANUARY 2014 www.ti.com
APPLICATION INFORMATION
Startup
Large inrush current occurs when power is applied to discharged capacitors and load. During the inrush period,
the TPS2590 device operates in power limit (or SOA protect mode) managing the current as VOUT rises. In SOA
protect mode, the internal MOSFET power dissipation ([V
IN
– V
OUT
] x I
OUT
) is regulated at 5 W 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
OUT
drops to the dc load value, the fault timer stops, and C
CT
ramps down. In order for
the TPS2590 device to start properly, the fault timer duration must exceed C
LOAD
startup time, t
ON
. Startup time
without additional dc loading can be determined using Equation 6 where P
LIM
= 5 W (typical).
(6)
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
. Table 1 and Table 2 can be used to predict start-up time in
the presence of resistive dc loading.
Refer to the TPS2590 Design Calculator Tool (SLUC398) for assistance with design calculations.
Table 1. 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 2. Start-up Time (ms) With DC Loading: V
IN
= 12 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
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 TPS2590 device provides very effective protection for the internal FET.
Expectedly, there is a supply voltage dependent maximum allowable load required for successful startup. Loads
above this can cause the output to shut off because of CT timeout or thermal shutdown because V
OUT
hangs at
an intermediate voltage below V
IN
. The equation for maximum load (or R
MIN
is derived using the circuit equations
for V
OUT
as a function of V
IN
, R
LOAD
, P
LIM
, and the result is quadratic in form.
(7)
(8)
(9)
When R
LOAD
< R
MIN
, the numerical result for V
OUT
is real (V
IN
2
– 4 × R
LOAD
× P
LIM
> 0) and less than V
IN
meaning
the circuit will not start (CT or thermal shutdown). When R
LOAD
> R
MIN
, the numerical result for V
OUT
is imaginary
(V
IN
2
– 4 × R
LOAD
x P
LIM
< 0) and the circuit will start (V
OUT
= V
IN
). Ensure that R
LOAD
is > R
MIN
per Equation 11.
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