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% <3000µF
3.3V 3.3V ± 0.3V <3000µF
12V 12V ± 5% <500µF
–12V –12V ± 10% <120µF
3.3V
AUX
3.3V ± 0.3V <500µF
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 60µA current source and the pass transis-
tors are allowed to turn on. The internal 12V and –12V
switches are also turned on and a 22µA current source is
connected to the TIMER pin (Figure 2).
APPLICATIO S I FOR ATIO
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