LTC4240
11
4240f
The LTC4240 is a Hot Swap controller that allows a board
to be safely inserted and removed from a CompactPCI bus
slot. The LTC4240 has built-in 2-wire I
2
C compatible
interface hardware to allow software control and monitor-
ing of device function and power supply status.
Hot Circuit Insertion
When a circuit board is inserted into a live CompactPCI
(CPCI) backplane slot, supply bypass capacitors on the
board can draw huge supply transient currents from the
CPCI backplane power bus. The transient currents can
cause glitches on the power bus, thus causing other
boards in the system to reset.
The LTC4240 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 CPCI slot without
disturbing the system power supplies. The device also
protects the supplies from shorts, precharges the bus I/O
pins during insertion and extraction and monitors the
supply voltages. The LTC4240 includes an I
2
C compatible
interface, which allows software control of device func-
tions.
The LTC4240 is specifically designed for CPCI applica-
tions where it resides on the plug-in board. For best
results, a well bypassed backplane is recommended.
LTC4240 Feature Summary
• Allows safe board insertion and removal from a CPCI
backplane. Status LED visually identifies when a board
is ready for removal.
• Controls all four CPCI supplies: –12V, 12V, 3.3V and
5V.
• Foldback current limit: An 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.
• 12V and –12V circuit breakers: if either supply remains
in current limit for more than 35µs, the circuit breaker
will trip, the supplies will turn off and the FAULT pin
pulls low.
• Adjustable 5V and 3.3V circuit breakers: if either supply
exceeds current limit for more than 35µs, the circuit
breaker will trip, the supplies will be turned off and the
FAULT pin will be pulled low. In addition, an analog loop
will servo the GATE pin to limit the current to three times
circuit breaker limit during transient conditions.
•I
2
C interface: software control allows user to both write
to and read from the device. The user can turn the
device off and on, set the status LED, set RESETOUT
and disable faults on 12V
IN
and V
EEIN
. The user can also
read the device status: FAULT, RESETIN, RESETOUT
PWRGD, PRSNT1#, PRSNT2#, FAULTCODE0 and
FAULTCODE1. If a fault occurs, the FAULTCODE bits
identify which supply generated the fault.
• Current limit during power-up: the supplies are allowed
to power-up in current limit. This allows the LTC4240 to
power-up boards with widely varying capacitive loads
without tripping the circuit breaker. The maximum
allowable power-up time is programmable using an
external capacitor connected to the TIMER pin. See
TIMER section
• Internal 12V and –12V power switches.
• PWRGD output: indicates the voltage status of the four
supply voltages.
• PCI_RST# is combined with HEALTHY# and
with the I
2
C RESETIN latch output to create
LOCAL_PCI_RST# output. If HEALTHY# asserts,
LOCAL_PCI_RST# is asserted independent of the other
two inputs.
• Precharge output: an internal reference and amplifier
provide 1V for biasing bus I/O connector pins during
CPCI card insertion and extraction.
• Space saving 28-pin SSOP package.
I
2
C Interface
The LTC4240 incorporates an I
2
C compatible 2-wire (clock
and data) interface that allows the user to easily query and
control the status of the LTC4240. A single analog input
pin selects 1 of 32 allowed addresses. The I
2
C bus can be
APPLICATIO S I FOR ATIO
WUUU
