Datasheet

LTC4227
11
422712fa
For more information www.linear.com/LTC4227
applicaTions inForMaTion
During a normal power-up, the ideal diode MOSFETs turn
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 ideal diode MOSFETs are connected in
parallel as a diode-OR, the SENSE
+
pin voltage approaches
the highest of the supplies at the IN1 and IN2 pins. The
MOSFET associated with the lower input supply voltage
will be turned off by the corresponding gate drive amplifier.
Before the Hot Swap MOSFET can be turned on, EN must
remain low and ON must remain high for a t
D(HGATE)
de-
bounce timing cycle to ensure that any contact bounces
during the insertion 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 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 the ideal diode MOSFET is turned on, the gate
drive amplifier controls the gate of the MOSFET to servo
the forward voltage drop across the MOSFET to 25mV.
If the load current causes more than 25mV of drop, the
MOSFET gate is driven fully on and the voltage drop 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 LTC4227 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. When D2ON is pulled
high above 1.235V, the ideal diode MOSFET in the IN2
supply path is turned off with DGATE2 pulled low by a
100µA current.
The gate drive amplifier controls the ideal diode MOSFET
to prevent reverse current when the input supply falls
below SENSE
+
. If the input supply collapses quickly, the
gate drive amplifier turns off the MOSFET with a fast pull-
down circuit as soon as it detects that IN is 25mV below
SENSE
+
. If the input supply falls at a more modest rate,
the gate drive amplifier controls the MOSFET to maintain
SENSE
+
at 25mV below IN.
Figure 2. Ideal Diode Controller Start-Up Waveforms
Figure 3. LTC4227-1/LTC4227-2 Hot Swap Controller
Power-Up Sequence
VOLTAGE
10V/DIV
5ms/DIV
4227 F02
IN1
SENSE
+
DGATE1
DGATE2
CP01
CP02
ON
5V/DIV
HGATE
10V/DIV
OUT
10V/DIV
PWRGD
10V/DIV
50ms/DIV
4227 F03