TPS2346PWRG4【PDF 24/30 页】IC HOT SWAP POWER MGR 24-TSSOP

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TPS2346

SLUS529 MAY 2002

www.ti.com

APPLICATION INFORMATION

To complete the design of the power interface for the plugin shown in the typical application diagram, assume

that a maximum slew rate of 1.5 A/ms is specified for each of the three positive-voltage loads. A value for

capacitor C2 must still be determined. Inspection of equation (4) indicates that, given the same di/dt requirement

on all supplies, the supply channel(s) with the lowest-value sense resistor dictates the minimum value for the

IRAMP capacitor. Therefore, the 5-V and 3.3-V supplies are suggested in this case for obtaining an initial

estimate for C

IRAMP

. Using the associated values, equation (4) produces the result shown in equation (5).

IRAMP2,3

67.5

(

0.002 W

)

1500 As

^ 0.336 mF

A value of 0.33 礔 can be used, or the next available standard value of 0.39 礔 provides some margin for

capacitor and sense-resistor tolerances. In either case, equation (4) can be rewritten as equation (6), which is

used here to verify that the 5.15-V slew rate is still within specification.

didt

67.5 R

SNSx

IRAMPx

where:

SNSx

is in ohms,

IRAMPx

is the value suggested by equation (5) in microfarads, and

(di/dt)

is given in amps/second.

For a C

IRAMP

of 0.39 礔, the maximum di/dt for the 5.15-V supply is approximately 860 mA/ms which is well

within the example requirement.

protection against faulted loads

The TPS2346 allows the time period of one IRAMP pulse for each back-end planes voltage to ramp-up to its

minimum level. After this delay period, the device latches off if an undervoltage fault is subsequently detected.

This nominal delay time, t

TIMER

, is set by the constant-current charging and subsequent discharging of C

IRAMP

and is therefore determined from equation (7).

TIMER

+ C

IRAMP

1.4

)

1.8

where:

IRAMP

is in microfarads.

The resultant fault timer period should be sufficient for most applications; however, it is good design practice

to verify that the delay is long enough for each load.

(5)

(6)

(7)

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