TPS2346PWRG4【PDF 25/30 页】IC HOT SWAP POWER MGR 24-TSSOP

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TPS2346

SLUS529 MAY 2002

www.ti.com

APPLICATION INFORMATION

Due to the potential three-phase nature of the load current ramp (refer to Figure 20), the increasing load voltage

may also have three distinct periods. The first phase is during the slow turn-on period at the start of each IRAMP

cycle. Depending on a number of factors, significant voltage ramping may or may not occur during this time.

For verification of the fault timeout delay, the worst case situation is no appreciable load charging (i.e., a longer

overall charge time); therefore it is assumed that no voltage ramp occurs here.

The next phase is the linear load current ramp period. For any device channel x, if the IMAX level is not reached

while charging a given load capacitor of C

, then the time to reach the input dc level, V

INx

, is estimated by

equation (8).

SSx

2 C

IRAMP

SNSx

INx

58 mA

where:

= 67.5 for Channels 1, 2, and 3, and

= 7.5 for Channel 4.

For example, assuming the 5-V back-end plane in this application has 220-礔 bulk capacitance, the anticipated

typical ramp-up time is about 1.41 ms.

During inrush slewing, the load current ramp tracks the voltage ramp on the IRAMP capacitor, up to a voltage

of about 1.35 V on the IRAMP pin. Therefore, the time duration of this ramp activity, t

IRAMP

, is given by

equation (9).

IRAMP

(

1.25 V

)

where:

IRAMP

is in microfarads.

If, for any channel, the time for soft-start charging of the load voltage is greater than the current ramp period,

SSx

> t

IRAMP

), back-end plane ramp-up completes at the dc IMAX level. In this case, the following procedure

can be used to estimate load ramp-up time.

First, equation (10) is used to determine the load voltage level, v

(t), attained during the current ramp period.

IRAMP

58 mA

2 C

IRAMP

SNSx

IRAMP

Once this voltage level is known, it can be used to estimate the additional charging time required at constant

current to reach the input dc potential, t

CCx

. This time is calculated from equation (11).

CCx

INx

* V

IRAMP

IMAXx

The sum of t

IRAMP

and t

CCx

can then be used to estimate the total load ramp-up time for Channel x.

(8)

(9)

(10)

(11)

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