TPS2491DGSG4【PDF 15/25 页】IC POS HV HOT-SWAP CTRLR 10-MSOP

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Sheet目录47

11页 12页 13页 14页 [15页]16页 17页 18页 19页

1.35 V

R1)R2

OFF

1.25 V

R1)R2

+ 8.5 10

1 ) C

OUT_TOL

) C

T_TOL

LIM

2 I

LIM

)

VCC(MAX)

2 P

LIM

if P

LIM

t I

LIM

VCC(MAX)

LIM

if P

LIM

w I

LIM

VCC(MAX)

PROG

LIM

10 I

LIM

where I

LIM

0.05

R3 ) R4

PROG

REF

LIM

v 0.7

J(MAX)2

MAX

DSON

qCA

) T

A(MAX)

DSON(MAX)

J(MAX)

* T

A(MAX)

qJA

MAX

TPS2490

TPS2491

www.ti.com

SLVS503D NOVEMBER 2003REVISED JULY 2012

(5)

where T

J(MAX)

is the desired maximum steady-state junction temperature (typically 125癈), and T

A(MAX)

is the

maximum ambient temperature. R

窲A

, the junction-to-ambient thermal resistance, depends upon the package

style chosen and the details of heat-sinking and cooling. Note the R

窲C

and R

窲A

for use below.

STEP 3. Choose P

LIM

, R3, R4

M1 dissipates large amounts of power during power-up or output short circuit. The power limit P

LIM

of the

TPS2940/91 should be set to prevent the die temperature from exceeding a short term maximum temperature,

J(MAX)2

. The short-term T

J(MAX)2

could be set as high as 150癈 while still leaving ample margin to the usual

manufacturers rating of 175癈. An expression for calculating P

LIM

is:

(6)

where R

窲C

is M1 junction-to-case thermal resistance, R

DS(on)

is the channel resistance at the maximum operating

temperature, and the factor of 0.7 represents the tolerance of the constant power engine. Next calculate V

PROG

and the divider resistors R3 and R4. R3 must be greater than 4 k&, but it is recommended that 10 k& or greater

be used.

(7)

STEP 4. Choose t

, C

The on-time, t

, set by capacitor C

must suffice to fully charge the load capacitance C

without triggering the

fault circuitry. Assuming that only the load capacitance draws current during startup:

(8)

Using this value of t

, C

is computed as:

(9)

where C

T_TOL

and C

OUT_TOL

are the tolerances associated with each capacitor. Assuming C

is a 20% tolerance

part, C

OUT_TOL

has a value of 0.2. This expression assures the worst case set of parts will always start.

STEP 5. Choose The Turn On Voltage, R1 and R2

Assuming that EN is used as an analog input, the turn-on voltage, V

and turn-off voltage, V

OFF

are defined as:

(10)

Use caution in selecting large values of R1 and R2 because the leakage current causes errors in the threshold

voltages.

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