TPS2491DGS datasheet(15/20 Pages) TI

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DSON(MAX) v

J(MAX) * TA(MAX)

MAX

LIM v 0.7

J(MAX)2 *

MAX

DSON

CA ) TA(MAX)

PROG +

LIM

where I

LIM +

0.05

R3 ) R4

PROG

REF

LIM

)

VCC(MAX)

LIM

if P

LIM t ILIMVVCC(MAX)

ON +

VCC(MAX)

LIM

if P

LIM w ILIM VVCC(MAX)

T + 8.5

10*6

1 ) COUT_TOL ) CT_TOL

ON +

1.35 V

R1)R2

OFF +

1.25 V

R1)R2

TPS2490

TPS2491

SLVS503 – NOVEMBER 2003

APPLICATION INFORMATION (continued)

where TJ(MAX) is the desired maximum steady-state junction temperature (typically 125°C), and TA(MAX) is the

maximum ambient temperature. RθJA, the junction-to-ambient thermal resistance, depends upon the package

style chosen and the details of heat-sinking and cooling. Note the RθJC and RθJA for use below.

STEP 3. Choose PLIM, R3, R4

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

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

TJ(MAX)2. The short-term TJ(MAX)2 could be set as high as 150°C while still leaving ample margin to the usual

manufacturer’s rating of 175

°C. An expression for calculating P

LIM is:

where RθJC is M1 junction-to-case thermal resistance, RDSON 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 VPROG

and the divider resistors R3 and R4. R3 must be greater than 4 k

Ω, but it is recommended that 10 kΩ or greater

be used.

STEP 4. Choose tON, CT

The on-time, tON, set by capacitor CT must suffice to fully charge the load capacitance CO without triggering the

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

Using this value of tON, CTis computed as:

where CT_TOL and COUT_TOL are the tolerances associated with each capacitor. Assuming CO is a 20% tolerance

part, COUT_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 & R2

Assuming that EN is used as an analog input, the turn-on voltage, VON and turn-off voltage, VOFF are defined as:

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

threshold voltages.

TPS2491DGS Datasheet(PDF) 15 Page - Texas Instruments

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