11
LTC1643AL
LTC1643AL-1/LTC1643AH
1643afb
APPLICATIONS INFORMATION
WUU
U
Timer
During a power-up sequence, a 22µA current source is
connected to the TIMER pin (Pin 4) and current limit faults
are ignored until the voltage ramps to within 0.9V of 12V
IN
(Pin 1). This feature allows the chip to power up a PCI slot
that can accept boards with widely varying capacitive
loads on the supplies. The power-up time will be:
t
CV
II
ON
SUPPLY SUPPLY
LIMIT LOAD
≅
−
2
•
For C
SUPPLY
= 2000µF, V
SUPPLY
= 5V, I
LIMIT
= 7A, I
LOAD
= 5A, the turn-on time will be ~10ms. The timer period
should be set longer than the maximum supply turn-on
time but short enough to not exceed the maximum safe
operating area of the pass transistor during a short circuit.
The timer period will be:
t
C
V
A
TIMER
TIMER
=
•.11
1
22µ
For C
TIMER
= 0.1µF, the timer period will be ~ 50ms. The
TIMER pin (Pin 4) is immediately pulled low when ON
(Pin 5) goes low.
Thermal Shutdown
The internal switches for the 12V and –12V supplies are
protected by an internal current limit and thermal shut-
down circuit. When the temperature of chip reaches
150°C, all switches will be latched off and the FAULT pin
(Pin 6) will be pulled low.
Short-Circuit Protection
During a normal power-up sequence, if the TIMER (Pin 4)
is done ramping and a supply is still in current limit, all of
the pass transistors will be immediately turned off and the
FAULT pin (Pin 6) will be pulled low as shown in Figure 4.
If a short circuit occurs after the supplies are powered up,
the shorted supply’s current will drop immediately to the
limit value (Figure 5).
If the supply remains in current limit for more than 14.6µs,
all of the supplies will be latched off. The 14.6µs delay
prevents quick current spikes—for example, from a fan
turning on—from causing false trips of the circuit breaker.
The chip will stay in the latched-off state until ON pin
(Pin 5) is cycled low then high, or the 12V
IN
pin (Pin 1)
power supply is cycled.
To prevent excessive power dissipation in the pass tran-
sistors and to prevent voltage spikes on the supplies
during short-circuit conditions, the current limit on each
supply is designed to be a function of the output voltage.
As the output voltage drops, the current limit decreases.
Unlike a traditional circuit breaker function where huge
currents can flow before the breaker trips, the current
foldback feature assures that the supply current will be
kept at a safe level and prevent voltage glitches when
powering up into a short.
The current limit for the 5V and 3.3V supplies is set by
placing a sense resistor between 5V
IN
(Pin 13) and 5V
SENSE
(Pin 12) and between 3V
IN
(Pin 9) and 3V
SENSE
(Pin 10).
The current limit will be set by:
I
LIMIT
= 53mV/R
SENSE
For a 0.005Ω resistor, the current limit will be set at 10.6A
and fold back to 1.5A when the output is shorted. For a
0.007Ω resistor, the current limit will be set at 7.6A and
fold back to 1.1A when the output is shorted.
The current limit for the internal 12V switch is set at
850mA folding back to 250mA and the –12V switch at
450mA folding back to 160mA.
In systems where it is possible to exceed the current limit
for a short amount of time, it might be necessary to
prevent the analog current loop from responding quickly
so the output voltage does not droop. This can be accom-
plished by adding an RC filter across the sense resistor as
shown in Figure 6. R4 should be 20Ω or less to prevent
offset errors. A 0.1µF capacitor gives a delay of about
1.5µs and a 1µF capacitor gives a delay of about 15µs.
CompactPCI Application
The LTC1643AL is designed for hot swapping CompactPCI
boards. The typical application is shown in Figure 7. The
3.3V, 5V, 12V and –12V inputs to the LTC1643AL come
from the medium length power pins. The long 3.3V, 5V and
V(I/O) pins power up the pull-up resistors, bus precharge
