LTC4222 Linear Technology, LTC4222 Datasheet - Page 17

• Manufacturer Part Number: LTC4222
• Description: Dual Hot Swap Controller
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIONS INFORMATION
start up and turn off independently. When both ON signals
are brought high sequentially, the channel turned on fi rst
immediately begins to start up and the second channel has
a 200ns window to assert it’s ON signal in order to start
up in the same timer period. If the second ON signal is
asserted after the 200ns window but before the end of the
fi rst channel’s startup time, the second channel startup is
delayed. The second channel will then start 100ms after the
fi rst channel’s start up timer has expired and the TIMER
pin, if used, reaches its 200mV low threshold.
When an external TIMER capacitor is used, the TIMER
capacitor voltage ramps up with a 100μA current. Once the
TIMER pin reaches its 1.235V threshold the TIMER begins
to discharge. While the TIMER capacitor is discharging, the
ON signal for the second channel should not be asserted
for 2ms/μF of TIMER capacitance. This allows the TIMER
capacitor to return to its low state and ensures that the next
channel to start receives a full timer cycle. This wait time
is unnecessary when using the internal 100ms timer.
Board Present Change of State
The EN pins may be used to detect the presence of one or
two downstream cards. Whenever an EN pin toggles, FAULT
bit 4 is set to indicate a change of state. When the EN pin
goes high, indicating board removal, the corresponding
GATE turns off immediately (with a 1mA current to ground)
and the board present STATUS bit 4, is cleared. If the EN
pin is pulled low, indicating a board insertion, all fault bits
for that channel except FAULT bit 4 are cleared and enable
STATUS bit 4, is set. If the EN pin remains low for 100ms
the state of the ON pin is captured in ‘FET On’ CONTROL
bit 3. This turns the switch on if the ON pin is tied high.
There is an internal 10μA pull-up current source on the
EN pin. If the CONFIG pin is tied low, both EN pins must
be low for 100ms for the two channels to be enabled and
if either EN pin goes high both channels will turn off.
If a channel shuts down due to a fault, it may be desirable
to restart that channel simply by removing and reinserting
the related load card. In cases where the LTC4222 and the
switch reside on a backplane or midplane and the load
resides on a plug-in card, the EN pin detects when the
plug-in card is removed. Figure 4 shows an example where
the EN pin is used to detect insertion. Once the plug-in card
is reinserted the fault register is cleared except for FAULT
bit 4. After 100ms the state of the ON pin is latched into
bit 3 of the CONTROL register. At this point the channel
starts up again.
If a connection sense on the plug-in card is driving an EN
pin, insertion or removal of the card may cause the pin
voltage to bounce. This results in clearing the fault register
when the card is removed. The pin may be debounced
using a fi lter capacitor, C
Figure 4. The fi lter time is given by:
FET Short Fault
A FET short fault is reported if the data converter measures
a current sense voltage greater than or equal to 2mV while
the corresponding GATE is turned off. This condition sets
FET short bit, Fault bit 5.
Power Bad Fault
A power bad fault is reported if a FB pin voltage drops
below its 1.235V threshold for more than 2μs when the
corresponding GATE is above the 4.3V gate to source
threshold. This pulls the GPIO pin low immediately in the
default power good confi guration, and sets power-bad
present bit, STATUS bit 3, and power bad bit, FAULT bit 3.
A circuit prevents power-bad faults if the GATE-to-SOURCE
voltage is low, eliminating false power-bad faults during
power-up or power-down. If the FB pin voltage subsequently
rises back above the threshold, a power-good confi gured
GPIO pin returns to a high impedance state and STATUS
bit 3 is reset.
t
FILTER
LTC4222
= C
Figure 4. Plug-In Card Insertion/Removal
MOTHERBOARD
EN
OUT
SOURCE
+
–
GND
• 123 (ms/μF)
1.235V
10μA
EN
EN
, on the EN pin as shown in
C
EN
CONNECTOR
LTC4222
4222 F04
PLUG-IN
CARD
LOAD
17
4222f