LTC4219 LINER [Linear Technology], LTC4219 Datasheet - Page 9

• Manufacturer Part Number: LTC4219
• Description: 5A Integrated Hot Swap Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
The Functional Diagram displays the main circuits of the
device. The LTC4219 is designed to turn a board’s supply
voltage on and off in a controlled manner allowing the board
to be safely inserted and removed from a live backplane.
The LTC4219 includes a 25mΩ MOSFET and a 7.5mΩ cur-
rent sense resistor. During normal operation, the charge
pump and gate driver turn on the pass MOSFET’s gate to
provide power to the load. The inrush current control is
accomplished by the INRUSH circuit. This circuit limits
the GATE ramp rate to 0.3V/ms and hence controls the
voltage ramp rate of the output capacitor.
The current sense (CS) amplifi er monitors the load current
using the voltage sensed across the current sense resistor.
The CS amplifi er limits the current in the load by reduc-
ing the GATE-to-OUT voltage in an active control loop. It
is simple to adjust the current limit threshold using the
current setting (I
during other times such as start-up.
A short circuit on the output to ground causes signifi cant
power dissipation during active current limiting. To limit
this power, the foldback amplifi er reduces the current
limit value from 5.6A to 1.5A in a linear manner as the
FB pin drops below 0.6V (see the Typical Performance
Characteristics section).
If an overcurrent condition persists, the TIMER pin ramps
up with a 100μA current source until the pin voltage ex-
ceeds 1.2V (comparator TM2). This indicates to the logic
that it is time to turn off the pass MOSFET to prevent
overheating. At this point the TIMER pin ramps down us-
ing the 2μA current source until the voltage drops below
0.2V (Comparator TM1) which tells the logic to start an
SET
) pin. This allows a different threshold
internal 100ms timer. After this delay, the pass transistor
has cooled and it is safe to turn it on again. It is suitable
for many applications to use an internal 2ms overcurrent
timer with a 100ms cooldown period. Tying the TIMER
pin to INTV
The fi xed 5V and 12V versions, LTC4219-5 and LTC4219-12,
use an internal divider from OUT to drive the FB pin. This
divider also sets the foldback current limit profi le. The
output voltage is monitored using the FB pin and the PG
comparator to determine if the power is available for the
load. The power good condition is signaled by the PG pin
using an open-drain pull-down transistor.
The Functional Diagram also shows the monitoring blocks
of the LTC4219. The two comparators on the left side in-
clude the EN1 and EN2 comparators. These comparators
determine if the enable inputs are valid prior to turning
on the MOSFET. But fi rst the undervoltage lockout circuits
UVLO1 and UVLO2 must validate the input supply and
the internally generated 3.1V supply (INTV
ate the power up initialization to the logic circuits. If the
external conditions remain valid for 100ms the MOSFET
is allowed to turn on.
Other features include MOSFET current and temperature
monitoring. The current monitor (CM) outputs a current
proportional to the sense resistor current. This current can
drive an external resistor or other circuits for monitoring
purposes. A voltage proportional to the MOSFET tempera-
ture is output to the I
allows external circuits to predict failure and shutdown
the system.
CC
sets this default timing.
SET
pin. The MOSFET temperature
LTC4219
CC
) and gener-
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