LTC4222 Linear Technology, LTC4222 Datasheet - Page 14

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

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LTC4222
APPLICATIONS INFORMATION
A MOSFET is turned on by charging up the GATE with a
12μA current source. When the GATE voltage reaches the
MOSFET threshold voltage, the MOSFET begins to turn on
and the SOURCE voltage then follows the GATE voltage
as it increases.
While the MOSFET is turning on, the inrush current in-
creases linearly at a dI/dt rate selected by capacitor C
This is accomplished using the current limit amplifi er
controlling the GATE pin voltage. Once the inrush current
reaches the limit set by the FB pin, the dI/dt ramp stops and
the inrush current follows the foldback profi le as shown
in Figure 2. When both channels turn on simultaneously,
the foldback current limit is set by the lower of the two
FB pins.
A start-up timer is used to prevent damaging the MOSFET
when starting up into a short-circuit. The TIMER capacitor
integrates at 100μA during start-up and once the TIMER
14
ALERT
3.3V
V
GND
V
SDA
12V
SCL
IN1
IN2
ON
BACKPLANE
PLUG-IN
CARD
0.1μF
0.1μF
C
C
F1
F2
Z1
SA14A
13.7k
Z2
SA14A
R31
C3
0.1μF
Figure 1. Typical Application
R11
140k
R21
4.53k
R32
13.7k
R22
3.32k
R12
23.7k
NC
SS
OV1
ON
SDA
SCL
ALERT
CONFIG
ADR0
ADR1
ADR2
INTV
GND
OV2
UV1 V
UV2 V
.
CC
DD1
DD2
pin reaches threshold of 1.235V, the part checks to see if
it is in current limit. If this is the case, the overcurrent fault
bit, FAULT bit 2 in Table 6, is set and the part turns off. If
the part is not in current limit, the 50mV circuit breaker is
armed and the current limit is switched to 150mV. Alter-
nately an internal 100ms start-up timer may be selected
by tying the TIMER pin to INTV
As the SOURCE voltage rises, the FB pin voltage follows as
set by R7 and R8. Once FB crosses its 1.235V threshold,
and the start-up timer has expired, the corresponding GPIO
pin, in the default power good confi guration, ceases to
pull low and indicates that power is now good. Alternately
STATUS bit 3 can be read to check power-good status,
where a zero indicates that power is good.
If a series resistor and capacitor from GATE to GROUND
(R6 and C1) are employed to provide a constant inrush
current during start-up, which provides a constant dV/dt at
0.01Ω
0.01Ω
R
R
S1
S2
SENSE1
SENSE2
LTC4222
R5-1
R5-2
10Ω
–
–
10Ω
FDD3706
FDD3706
GATE1
GATE2
Q1
Q2
3.9nF
3.9nF
R
R
15k
15k
C
C
G1
G2
G1
G2
SOURCE1
SOURCE2
TIMER
GPIO1
ADIN1
ADIN2
GPIO2
EN1
EN2
FB1
FB2
SS
68nF
C
SS
R71
10.2k
R81
3.57k
R82
3.57k
R72
4.99k
CC
.
4222 TA01a
C
0.68μF
TIMER
R41
100k
R42
100k
4222f