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Dt +

C

O

V

VCC

I

CHARGE

C

I

GATE

Dt

V

VCC

TPS2490

TPS2491

SLVS503 – NOVEMBER 2003

APPLICATION INFORMATION (continued)

STEP 6. Choose R5, R6, & C1

R5 is intended to suppress high-frequency oscillations; a resistor of 10

Ω will serve for most applications but if M1

require R5. R6 is required only if the PG output drives a circuit that requires it. It is recommended that the sink

current be less than 2 mA. C1 is a bypass capacitor to help with control of transient voltages, unit emissions, and

local supply noise while in the disabled state. Where acceptable, a value in the range of 0.001

µF to 0.1 µF is

recommended.

STEP 7. Choose D1

Transient voltage suppressor D1 is required in applications where there will be enough energy in the distribution

inductance to cause a voltage surge above the TPS2490/91 rated maximum. Such transients can be caused by

card insertions or shorts on the input or output of the TPS2490/91.

ALTERNATIVE INRUSH DESIGNS

Gate Capacitor (dV/dt) Control

The TPS2490/91 can be used with applications that require constant turn-on currents. The current is controlled

by a single capacitor from the GATE terminal to ground with a series resistor. M1 appears to operate as a source

follower (following the gate voltage) in this implementation. Choose a time to charge,

∆t, based on the output

power limit feature is kept.

To select the gate capacitance:

can then be selected to be much smaller as the current and power limit is not active during initial power on. A

series resistor of about 1 k

Ω should be used in conjunction with C

PROG Inrush Control

A capacitor can be connected from the PROG pin to ground to reduce the initial current step seen in Figure 12

based on the typical application circuit on page 1. This method maintains a relatively fast turn-on time without the

drawbacks of a gate-to-ground capacitor that include increased short circuit response time and less predictable

gate clamping.

ADDITIONAL DESIGN CONSIDERATIONS

Use of PG

to fully charge before the converter starts if this is not done. An undesirable latchup condition can be created

between the TPS2490 output characteristic and the dc/dc converter input characteristic if the converter starts

Faults and Backplane Voltage Droop

A hard short at the output of the TPS2490/91 during normal operation could result in activation of the enable or

UVLO circuit instead of the current limit if the input voltage droops sufficiently. The lower GATE drive in this

condition will cause a prolonged, larger over-current spike. This can be eliminated by filtering EN, or distributing

capacitance on the bus itself. Capacitance from adjacent plugged-in units may help with this as well.

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