TPS2491DGSG4【PDF 14/25 页】IC POS HV HOT-SWAP CTRLR 10-MSOP

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0.05

1.2 I

GATE10V/div

OUT10V/div

TIMER1V/div

.5A/div

TPS2490

TPS2491

SLVS503D NOVEMBER 2003REVISED JULY 2012

www.ti.com

Automatic Restart (Figure 17)

The TPS2491 automatically initiates a restart after a fault has caused it to turn off M1. Internal control circuits use

to count 16 cycles before re-enabling M1. This sequence repeats if the fault persists. The TIMER has a 1:10

charge-to-discharge current ratio, and uses a 1-V lower threshold. The fault-retry duty cycle specification

quantifies this behavior. This small duty cycle often reduces the average short-circuit power dissipation to levels

associated with normal operation and eliminates special thermal considerations for surviving a prolonged output

short.

Figure 17. TPS2491 Restart Cycle Timing

DESIGN PROCEDURE

This design procedure seeks to control the junction temperature of M1 under both static and transient conditions

by selecting the devices package, cooling, R

DS(on)

, current limit, fault timeout, and power limit. The following

procedure assumes that a unit running at full load and maximum ambient temperature experiences a brief input

power interruption sufficient to discharge C

, but short enough to keep M1 from cooling. A full C

recharge then

takes place. Adjust this procedure to fit your application and design criteria. See SLVC033 for a calculation tool

to help with this process.

This procedure assumes that C

is the only load during inrush. Only simple first-order thermal models, natural

convection and a large PCB pad for M1 are assumed. The assumptions build generous safety margins into the

design to allow for the inherent inaccuracies of the models and variations of real-world conditions.

Other tools and applications information are available on the TI website that supplement the following procedure.

STEP 1. Choose R

Given the maximum operating current, I

MAX

, compute the current sense resistance, R

(4)

This equation allows for minimum current limit, a sense resistor tolerance of 5%, and 5% margin. Round the

result down to the nearest available standard value.

STEP 2. Choose M1

First select a V

rating that allows for the maximum input voltage and transients. Next select an operating

DS(on)

, package, and cooling to control operating temperature. The following equation computes the value of

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 the equation:

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