TPS24711DGSR【PDF 23/34 页】IC CTRLR HOT SWAP 2.5-18V 10MSOP

W

=

PROG

LIM

SENSE

PROG

3125

R

=

P + R

herefore,

3125

R

=

= 53.15 k

(

)

( )

(

)

(

)

2

J(MAX)2

MAX

DS(on)

CA

A(MAX)

LIM

JC

2

LIM

T

I

r

R

T

0.8

,

R

herefore,

130 C

12 A

0.002

51 C/W 1.8 C/W

50 C

0.8

29.3 W

q

?/DIV>

-

?/DIV>

+

?/DIV>

?nbsp -

?/DIV>

W?nbsp ?/DIV>

- ?/DIV>

+ ?/DIV>

?/DIV>

=

?/DIV>

J(MAX)

A(MAX)

DS(on)(MAX)

2

MAX

JA

DS(on)(MAX)

2

T

,

I

R

herefore,

150 C 50 C

13.6 m

12 A

51 C/W

q

-

=

?/DIV>

?nbsp - ?/DIV>

=

W

?nbsp ?/DIV>

TPS24710,TPS24711

TPS24712,TPS24713

www.ti.com

SLVSAL2E JANUARY 2011REVISED NOVEMBER 2013

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 m& is required. Also consider the effect of r

DS(on)

upon the maximum operating

temperature T

J(MAX)

of the MOSFET. Equation 6 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癈 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 6.

(6)

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 m?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癈 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 Power-Limit Value, P

LIM

, and R

PROG

MOSFET M

1

dissipates large amounts of power during inrush. The power limit P

LIM

of the TPS24710/11/12/13

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 130癈 while still leaving ample margin to the usual manufacturers

rating of 150癈. Equation 7 is an expression for calculating P

LIM

,

(7)

where R

窲C

is the junction-to-case thermal resistance of the MOSFET, r

DS(on)

is the resistance at the maximum

operating temperature, and the factor of 0.8 represents the tolerance of the constant-power engine. For an

ambient temperature of 50癈, the calculated maximum P

LIM

is 29.3 W. From Equation 1, a 53.6-k&, 1% resistor

is selected for R

PROG

(see Equation 8).

(8)

V

SNS-PL_MIN

is the minimum sense voltage during power limit operation. Due to offsets of internal amplifiers,

programmed power limit (P

LIM

) accuracy degrades at low V

SNS-PL_MIN

and could cause start-up issues. To ensure

reliable operation, verify that V

SNS,PL,MIN

> 3 mV using Equation 9.

(9)

Copyright ?20112013, Texas Instruments Incorporated

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