TPS2343

SLUS644B − FEBRUARY 2005 − MAY 2005

18

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APPLICATION INFORMATION

3VAUX Supply Control

The TPS2343 3VAUX supply is completely independent of the main supply. Supply status and faults on main

supplies have no effect on 3VAUX and faults on 3VAUX have no effect on main supply operation.

The TPS2343 uses internal power MOSFETs for the 3VAUX supply and voltage multiplying charge pumps to

drive the gates of the power MOSFETs to 8 V. Inrush current for the 3VAUX supply is controlled because the

slew rate of the 3VAUX supply is limited. This slew rate is set by the capacitor from 3VAUXGx to AGND. Slew

rate can be estimated as:

dV

dt

+

I

GATE

C

3VAUXGx

Inrush current caused by this slewing and any adapter card load capacitance can be estimated as:

PCI specifications allow for 3.3VAUX supply adapter card bulk capacitance of up to 150

µF. This load

capacitance causes additional inrush current of:

I

dV

dt

+ C

LOAD

5

mA

C

3VAUXGx

current can be estamated as:

I

5

mA

0.022

mF +

0.034 A

The 3VAUXx current-sense threshold is internally set and can not be adjusted.

When main power is applied to the TPS2343, all gates are actively held low. When main power is removed,

leakage current can potentially raise gate voltage, but because main power is not applied, no malfunction

occurs. This is noted here as floating gates may be observed during bench testing, but ths is not an application

problem.