Datasheet

T FLT
T
10 μA
C = t ,
1.35 V
therefore,
10 μA
C = 7 ms 52 nF
1.35 V
´
´ =
ISS
FLT ON
GATE
FLT
5.9 V C
t t ,
I
therefore,
5.9 V 2040 pF
t 0.614 ms 1.22 ms
20 μA
´
= +
´
= + =
ON
t =
2
OUT VCC(MAX) OUT VCC(MAX)
OUT LIM
LIM LIM VCC(MAX)
2
LIM LIM
LIM
C V C V
C P
if P I V
2 P I
2 I
´ ´
´
+ - < ´
´
´
OUT VCC(MAX)
LIM LIM VCC(MAX)
LIM
C V
if P I V
I
´
> ´
( )
( )
2
ON
2
therefore,
470 μF 12 V
470 μF 29.3 W 470 μF 12 V
t 0.614 ms
2 29.3 W 12 A
2 12 A
´
´ ´
= + - =
´
´
TPS24710, TPS24711
TPS24712, TPS24713
SLVSAL2E –JANUARY 2011–REVISED NOVEMBER 2013 www.ti.com
STEP 4. Choose Output Voltage Rising Time, t
ON
, C
T
The maximum output voltage rise time, t
ON
, set by the timer capacitor C
T
must suffice to fully charge the load
capacitance C
OUT
without triggering the fault circuitry. Equation 10 defines t
ON
for two possible inrush cases.
Assuming that only the load capacitance draws current during start-up,
(10)
The next step is to determine the minimum fault-timer period. In Equation 10, the output rise time is t
ON
. This is
the amount of time it takes to charge the output capacitor up to the final output voltage. However, the fault timer
uses the difference between the input voltage and the gate voltage to determine if the TPS24710/11/12/13 is still
in inrush limit. The fault timer continues to run until V
GS
rises 5.9 V (for V
VCC
= 12 V) above the input voltage.
Some additional time must be added to the charge time to account for this additional gate voltage rise. The
minimum fault time can be calculated using Equation 11,
(11)
where C
ISS
is the MOSFET input capacitance and I
GATE
is the minimum gate sourcing current of
TPS24710/11/12/13, or 20 μA. Using the example parameters in Equation 11 and the CSD16403Q5 data sheet
(SLPS201) leads to a minimum fault time of 1.22 ms. This time is derived considering the tolerances of C
OUT
,
C
ISS
, I
LIM
, P
LIM
, I
GATE
, and V
VCC(MAX)
. The fault timer must be set to a value higher than 1.22 ms to avoid turning
off during start-up, but lower than any maximum fault time limit determined by the SOA curve (see Figure 35)
derated for operating junction temperature.
For this example, select 7 ms to allow for variation of system parameters such as temperature, load, component
tolerance, and input voltage. The timing capacitor is calculated in Equation 4 as 52 nF. Selecting the next-highest
standard value, 56 nF, yields a 7.56-ms fault time (see Equation 12).
(12)
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