LTC4241

10

sn4241 4241f

Hot Circuit Insertion

When a circuit board is inserted into a live PCI slot, the

supply bypass capacitors on the board can draw huge

transient currents from the PCI power bus as they charge

up. The transient currents can cause permanent damage

to the connector pins and glitches the power bus, causing

other boards in the system to reset.

The LTC4241 is designed to turn a board’s supply voltages

on and off in a controlled manner, allowing the board to be

safely inserted or removed from a live PCI slot without

glitching the system power supplies. The chip also pro-

tects the PCI supplies from shorts and monitors the

supply voltages.

The LTC4241 is designed for motherboard applications

and includes an additional independent controller for the

3.3V auxiliary supply.

LTC4241 Feature Summary

1. Allows safe board insertion and removal from a

motherboard.

2. Primary controller to control the four PCI supplies:

3.3V, 5V, –12V, 12V and an independent auxiliary control-

ler to control the 3.3V auxiliary supply.

3. Adjustable foldback current limit for PCI supplies: an

adjustable analog current limit with a value that depends

on the output voltage. If the output is shorted to ground,

the current limit drops to keep power dissipation and

supply glitches to a minimum.

4. Electronic circuit breaker for all supplies: if a supply

remains in current limit for too long, the circuit breaker will

trip, the supplies will be turned off and the FAULT pin

pulled low.

5. Current limit power-up: the four PCI supplies are

allowed to power up in current limit. This allows the chip

to power up boards with a wide range of capacitive loads

without tripping the circuit breaker. The maximum allow-

able power-up time is programmable using the TIMER pin.

6. On-Chip –12V and 12V power switches

7. Power good output: monitors the voltage status of the

four PCI supply voltages. The 3.3V auxiliary supply is not

monitored.

8. Fault control: the current limit fault detected by either

the primary or auxiliary controller will not cause the other

controller to latch off. Both controllers use the FAULT

output to indicate a fault condition.

9. Space saving 20-pin narrow SSOP package.

PCI Power Requirements

PCI systems usually require four power rails: 5V, 3.3V,

–12V and 12V. Systems implementing the 3.3V signaling

environment are usually required to provide all four rails in

every system.

A 3.3V auxiliary supply is added in the PCI system to power

PCI logic functions that need to remain active when the

rest of the system is unpowered.

The tolerance of the supplies as measured at the compo-

nents is summarized in Table 1.

Table 1. PCI Power Supply Requirements

CAPACITIVE

SUPPLY

TOLERANCE

LOAD

5V

5V

± 5%

<3000

µF

3.3V

3.3V

± 0.3V

<3000

µF

12V

12V

± 5%

<500

µF

–12V

–12V

± 10%

<120

µF

3.3V

± 0.3V

<500

µF

Power-Up Sequence for PCI Power Supplies

The PCI power supplies are controlled by placing external

N-channel pass transistors in the 3.3V and 5V power

paths, and internal pass transistors for the 12V and –12V

power paths (Figure 1).

Resistors R1 and R2 provide a current signal for fault

detection and R7 and C1 provide current control loop

compensation. Resistors R4 and R5 prevent high fre-

quency oscillations in Q1 and Q2.

When the ON pin is pulled high, the GATE pin is pulled high

by an internal 60

µA current source and the pass transis-

tors are allowed to turn on. The internal 12V and –12V

switches are also turned on and a 22

µA current source is

connected to the TIMER pin (Figure 2).

APPLICATIO S I FOR ATIO