LTC4245
13
4245fa
OPERATIO
U
Start-Up
The LTC4245 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 backplane slot. When
a supply turn-on command is received, current sources
start pulling up the TIMER and SS pins. The 100μA I
TMR
current and the external TIMER capacitor determine the
time a supply can be in current limit during start-up. The
gate of a supply’s external N-channel MOSFET is servoed
by an amplifer (ACL
n
) so that the current, as indicated by
the sense resistor voltage drop, never exceeds an internal
current limit. This current limit rises at a rate determined
by I
SS
and the capacitor at the SS pin. A foldback circuit
determines the maximum value of the current limit and
reduces it to 30% of the maximum when a supply’s output
is shorted to ground. When the TIMER pin crosses 2.56V
it is reset to ground and the start-up timing cycle ends. If
a supply is still in current limit all gates are turned off, an
overcurrent fault is logged and the TIMER goes through
a cool-down timing cycle using 2μA for I
TMR
. Otherwise,
its circuit breaker (ECB
n
) is armed and the current limit
is raised to 3 times the circuit breaker threshold. The SS
pin is then reset by switch M2.
Any combination of the four supplies can be turned on
together or one after another. Whenever a supply is ramp-
ing up, its output voltage will affect, through the foldback
circuit, where the internal current limit ramp stops. The
default confi guration turns on all supplies together. If
sequence control bit C6 (Table 9) is set, the supplies turn
on in a 12V, 5V, 3.3V, –12V sequence. With this bit set,
the end of a supply ramp-up triggers the start of the next
one in the sequence. The I
2
C interface allows independent
on and off control for each supply through its On control
bit. Turn-off is simultaneous under fault conditions and
when using the ON or BD_SEL# pins.
At the end of the last start-up timing cycle, HEALTHY#
is pulled low by M3 if all supply outputs are above their
power bad thresholds. LOCAL_PCI_RST# which was held
low (M4), now follows PCI_RST#. The TIMER pin goes
through a PGI timeout cycle using 10μA for I
TMR
. The PGI
pin is sampled at the end of the cycle. If it is low, then
all external MOSFETs are shut-off, a PGI fault is logged
and TIMER goes through a cool-down cycle using 2μA
for I
TMR
. If PGI is high, the part enters the normal mode
of operation.
Normal Operation
During normal operation, the gates of the MOSFETs are
clamped about 6.2V above their sources. The 12V gate
driver uses a charge pump, the 5V and 3.3V gate drive is
derived from 12V
IN
and the –12V gate drive from INTV
CC
.
Each supply is continuously monitored for undervoltage,
overcurrent and power bad conditions. Overcurrent moni-
toring consists of an electronic circuit breaker comparator
(ECB
n
) and an active current limit circuit (ACL
n
) set at 3x the
ECB threshold. Undervoltage and overcurrent faults cause
all MOSFETs to be shut off. A power bad condition causes
HEALTHY# to go high impedance and LOCAL_PCI_RST#
to pull low, without shutting off the MOSFETs. If the PGI
pin is not disabled (register bit C3 not set), then PGI pin
going low will also shut off all MOSFETs.
ADC
Included in the LTC4245 is an 8-bit A/D converter. The
converter has a 13-input multiplexer to select between
input, output and current sense voltage of each supply,
and the GPIO channel. The ADC can either cycle through
all channels or measure a channel on-demand.
Serial Interface
An I
2
C interface is provided to read from or write to the
status, control and A/D registers. It allows the host to poll
the device and determine if faults have occurred. If the
ALERT# line is used as an interrupt, the host can respond
to a fault in real time. The LTC4245 I
2
C interface slave ad-
dress is decoded using the ADR0 to ADR3 pins.
Confi guration, GPIO and Precharge
The three-state CFG pin can be used to disable the V
EE
undervoltage lockout, power bad and foldback functions.
It can also convert the 5V undervoltage, power bad and
ADC levels to 3.3V levels. The GPIO1 to GPIO3 pins can
be used as general purpose inputs or outputs (M5 to
M7). One of the pins can also be multiplexed to the GPIO
channel of the ADC. A 1V reference voltage derived from
3V
IN
is provided at the PRECHARGE pin. This can be used
to pre-charge I/O lines on the board so as not to corrupt
the backplane bus.
