TPS2343

SLUS644B − FEBRUARY 2005 − MAY 2005

14

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APPLICATION INFORMATION

Turn-On Sequence

Main power to the slot turns on when all input supplies are active and power is commanded, by asserting

PWRENx. The charge pump combined with the P12VGx capacitor produces a linear voltage ramp on P12VGx,

which produces a linear ramping of the 12-V output and the −12-V output. At the same time, a current source

on 5VG combined with the 5VG capacitor produces a linear voltage ramp on 5VG and a current source on 3VG

combined with the 3VG capacitor produces a linear voltage ramp on 3VG, which produces a linear ramping of

the 3.3-V and 5-V main outputs.

During this time, if any main slot current exceeds the appropriate over-current threshold for more than the

over-current sensitivity time, the slot latches off and remains off until the logic command is turned off and on

again.

When P12VGx exceeds the 12-V gate good threshold, 5VG exceeds the 5-V good threshold, and 3VG exceeds

the 3-V gate good threshold, outputs should be fully ramped and the power MOSFETs should be fully enhanced.

+12-V Supply Control

The TPS2343 integrates an N-channel power MOSFET for the 12-V supply and a voltage multiplying charge

pump to drive the gate of the power MOSFET to 20 V. Inrush current for the 12-V supply is controlled because

the slew rate of the 12-V supply is limited. The slew rate for the 12-V supply is set by the capacitor from P12VG

to AGND.

Slew rate can be estimated as:

dV

dt

+

I

GATE

C

P12VGx

PCI specifications allow for 12-V supply adapter card bulk capacitance of up to 300

µF. This load capacitance

causes additional inrush current of:

I

dV

dt

+ 300 mF

I

GATE

C

P12VGx

current can be estimated as:

I

10

mA

0.022

mF +

0.136 A

An internal current−sense circuit monitors the 12-V supply. The over-current threshold for the 12-V supply is

directly proportional to the resistor from MISET to AGND. Raising the MISET resistor simultaneously raises the

current limit threshold for the 12-V, 5-V, 3.3-V and −12-V supplies. For example, to raise the nominal output

current from the 12-V supply by 20%, increase the MISET resistor 20%. This resistor can be as high as 12 k

Ω

if necessary.

Osolete Device