Datasheet

LTC4228-1/LTC4228-2
11
422812f
applicaTions inForMaTion
monitors the load current for overcurrent detection. The
HGATE capacitor, C
HG
, controls the gate slew rate to limit
the inrush current. Resistor R
HG
with C
HG
compensates
the current control loop, while R
H
prevents high frequency
oscillations in the Hot Swap MOSFET.
During a normal power-up, the ideal diode MOSFET turns
on first. As soon as the internally generated supply, INTV
CC
,
rises above its 2.2V undervoltage lockout threshold, the
internal charge pump is allowed to charge up the CPO
pins. Because the Hot Swap MOSFET is turned off at
power-up, OUT remains low. As a result, the ideal diode
gate drive amplifier senses a large forward drop between
the IN and OUT pins, causing it to pull up DGATE to the
CPO pin voltage.
Before the Hot Swap MOSFET can be turned on, EN must
remain low and ON must remain high for a 100ms debounce
cycle to ensure that any contact bounces during the inser-
tion have ceased. At the end of the debounce cycle, the
internal fault latches are cleared. The Hot Swap MOSFET
is then allowed to turn on by charging up HGATE with a
10µA current source from the charge pump. The voltage
at the HGATE pin rises with a slope equal to 10µA/C
HG
and
the supply inrush current flowing into the load capacitor,
C
L
, is limited to:
I
INRUSH
=
C
L
C
HG
• 10µA
The OUT voltage follows the HGATE voltage when the
Hot Swap MOSFET turns on. If the voltage across the
current sense resistor, R
S
, becomes too high, the inrush
current will be limited by the internal current limiting
circuitry. Once the MOSFET gate overdrive exceeds 4.2V,
the corresponding PWRGD pin pulls low to indicate that
the power is good. Once OUT reaches the input supply
voltage, HGATE continues to ramp up. An internal 12V
clamp limits the HGATE voltage above OUT.
When both of the MOSFETs are turned on, the gate drive
amplifier controls the gate of the ideal diode MOSFET, to
servo its forward voltage drop across R
S
, M
D
and M
H
to
25mV. If the load current causes more than 25mV of drop,
the MOSFET gate is driven fully on and the voltage drop
across the MOSFET is equal to I
LOAD
• R
DS(ON)
.
Turn-Off Sequence
The external MOSFETs can be turned off by a variety of
conditions. A normal turn-off for the Hot Swap MOSFET is
initiated by pulling the ON pin below its 1.155V threshold
(80mV ON pin hysteresis), or pulling the EN pin above
its 1.235V threshold. Additionally, an overcurrent fault
of sufficient duration to trip the circuit breaker also turns
off the Hot Swap MOSFET. Normally, the LTC4228 turns
off the MOSFET by pulling the HGATE pin to ground with
a 300µA current sink.
All of the MOSFETs turn off when INTV
CC
falls below its
undervoltage lockout threshold (2.2V). The DGATE pin is
pulled down with a 100µA current to one diode voltage
below the IN pin, while the HGATE pin is pulled down to
the OUT pin by a 200mA current.
The gate drive amplifier controls the ideal diode MOSFET
to prevent reverse current when the input supply falls
below OUT. If the input supply collapses quickly, the gate
drive amplifier turns off the ideal diode MOSFET with a
fast pull-down circuit as soon as it detects that IN is 20mV
Figure 2. Ideal Diode Controller Start-Up Waveforms
Figure 3. Hot Swap Controller Power-Up Sequence
IN
10V/DIV
CPO
10V/DIV
DGATE
10V/DIV
OUT
10V/DIV
20ms/DIV
422812 F02
ON
5V/DIV
HGATE
10V/DIV
OUT
10V/DIV
PWRGD
10V/DIV
50ms/DIV
422812 F03