Datasheet

( )
J(MAX) A(MAX)
DS(on)(MAX)
2
MAX JA
DS(on)(MAX)
2
T T
r ,
I R
therefore,
150 C 50 C
r 13.6 m
12 A 51 C / W
q
-
=
´
° - °
= = W
´ °
OUT VCC(MAX)
ON LIM LIM VCC(MAX)
LIM
ON
C V
t if P I V
I
therefore,
470 μF 12 V
t 0.47 ms
12 A
´
= > ´
´
= =
ISS
FLT ON
GATE
FLT
6 V C
t t ,
I
therefore,
6 V 2040 pF
t 0.47 ms 1.08 ms
20 μA
´
= +
´
= + =
TPS24700
TPS24701
www.ti.com SLVSAL3B – MARCH 2011– REVISED MAY 2011
Next select the on resistance of the transistor, r
DS(on)
. The maximum on-resistance must not generate a voltage
greater then the minimum power-good threshold voltage of 140 mV. Assuming a current limit of 12 A, a
maximum r
DS(on)
of 11.67 mΩ is required. Also consider the effect of r
DS(on)
upon the maximum operating
temperature T
J(MAX)
of the MOSFET. Equation 4 computes the value of r
DS(on)(MAX)
at a junction temperature of
T
J(MAX)
. Most manufacturers list r
DS(on)(MAX)
at 25°C and provide a derating curve from which values at other
temperatures can be derived. Compute the maximum allowable on-resistance, r
DS(on)(MAX)
, using Equation 4.
(4)
Taking these factors into consideration, the TI CSD16403Q5 was selected for this example. This transistor has a
V
GS(MAX)
rating of 16 V, a V
DS(MAX)
rating of 25 V, and a maximum r
DS(on)
of 2.8 mΩ at room temperature. During
normal circuit operation, the MOSFET can have up to 10 A flowing through it. The power dissipation of the
MOSFET equates to 0.24 W and a 9.6°C rise in junction temperature. This is well within the data sheet limits for
the MOSFET. The power dissipated during a fault (e.g., output short) is far larger than the steady-state power.
The power handling capability of the MOSFET must be checked during fault conditions.
STEP 3. 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 5 defines t
ON
, where V
CC(MAX)
is the maximum
input power bus voltage value and I
LIM
is the current limit value.
(5)
The next step is to determine the minimum fault-timer period. In Equation 5, the output rise time, t
ON
, 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 TPS24700/1 is still in inrush
limit. The fault timer continues to run until V
GATE
rises 6 V above the input voltage (for V
VCC
= 12 V). Some
additional time must be added to the total time to account for this additional gate voltage rising. The minimum
fault time can be calculated using Equation 6,
(6)
where C
ISS
is the MOSFET input capacitance and I
GATE
is the minimum gate sourcing current of TPS24700, or
20 μA. Using the example parameters in Equation 6 and the CSD16403Q5 data sheet leads to a minimum fault
time of 1.08 ms. This time is derived considering the tolerances of C
OUT
, C
ISS
, I
LIM
, I
GATE
, C
OUT
, and V
VCC
. The
fault timer must be set to a value higher than 1.08 ms to avoid turning off during start-up, but lower than any
maximum fault time limit determined by the device SOA curve.
There is a maximum time limit set by the SOA curve of the MOSFET. Referring to Figure 31, which shows the
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