LTC4228-1/LTC4228-2
9
422812f
operaTion
The LTC4228 functions as an ideal diode with inrush cur-
rent limiting and overcurrent protection by controlling two
external N-channel MOSFETs (M
D
and M
H
) on a supply
path. This allows boards to be safely inserted and removed
in systems with a backplane powered by redundant sup-
plies, such as µTCA applications. The LTC4228 has two
separate ideal diode and Hot Swap controllers, each
providing independent control for the two input supplies.
When the LTC4228 is first powered up, the gates of the
external MOSFETs are held low, keeping them off. The gate
drive amplifier (GA1, GA2) monitors the voltage between the
IN and OUT pins and drives the DGATE pin. The amplifier
quickly pulls up the DGATE pin, turning on the MOSFET
for ideal diode control, when it senses a large forward
voltage drop. The stored charge in an external capacitor
connected between the CPO and IN pins provides the
charge needed to quickly turn on the ideal diode MOSFET.
An internal charge pump charges up this capacitor at device
power-up. The DGATE pin sources current from the CPO
pin and sinks current into the IN and GND pins. When the
DGATE to IN voltage exceeds 0.7V, the STATUS pin pulls
low to indicate that the ideal diode MOSFET is turned on.
Pulling the ON pin high and the EN pin low initiates a
100ms debounce timing cycle. After this timing cycle,
a 10µA current source from the charge pump ramps up
the HGATE pin. When the Hot Swap MOSFET turns on,
the inrush current is limited at a level set by an external
sense resistor (R
S
) connected between the SENSE
+
and
SENSE
–
pins. An active current limit amplifier (A1, A2)
servos the gate of the MOSFET to 65mV across the current
sense resistor. Inrush current can be further reduced, if
desired, by adding a capacitor from HGATE to GND. When
the MOSFET ’s gate overdrive (HGATE to OUT voltage)
exceeds 4.2V, the PWRGD pin pulls low.
When both of the MOSFETs are turned on, the gate drive
amplifier controls DGATE to servo the forward voltage
drop (V
IN
– V
OUT
) across the sense resistor and the two
MOSFETs to 25mV. If the load current causes more than
25mV of voltage drop, the DGATE voltage rises to enhance
the MOSFET used for ideal diode control. For large output
currents, the ideal diode MOSFET is driven fully on and
the voltage drop across the MOSFETs is equal to the sum
of the I
LOAD
• R
DS(ON)
of the two MOSFETs in series.
In the case of an input supply short circuit when the
MOSFETs are conducting, a large reverse current starts
flowing from the load towards the input. The gate drive
amplifier detects this failure condition as soon as it ap-
pears and turns off the ideal diode MOSFET by pulling
down the DGATE pin.
In the case where an overcurrent fault occurs on the sup-
ply output, the current is limited to 65mV/R
S
. After a fault
filter delay set by 100µA charging the TMR pin capacitor,
the circuit breaker trips and pulls the HGATE pin low, turn-
ing off the Hot Swap MOSFET. Only the supply at fault is
affected, with the corresponding FAULT pin latched low.
At this point, the DGATE pin continues to pull high and
keeps the ideal diode MOSFET on.
Internal clamps limit both the DGATE to IN and CPO to IN
voltages to 12V. The same clamp also limits the CPO and
DGATE pins to a diode voltage below the IN pin. Another
internal clamp limits the HGATE to OUT voltage to 12V
and also clamps the HGATE pin to a diode voltage below
the OUT pin.
Power to the LTC4228 is supplied from either the IN or
OUT pins, through an internal diode-OR circuit to a low
dropout regulator (LDO). That LDO generates a 5V supply
at the INTV
CC
pin and powers the LTC4228’s internal low
voltage circuitry.
