LTC4210 LINER [Linear Technology], LTC4210 Datasheet - Page 19

• Manufacturer Part Number: LTC4210
• Description: Dual Supply Hot Swap Controller for Advanced Mezzanine Card
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
For proper operation, I
load current with margin, so R
for the 12V supply.
The second step is to determine the TIMER capacitance
based on the time required to charge up completely the
output load capacitor on auxiliary supply in active current
limit without exceeding the fault fi lter delay. The worst-
case start-up time is calculated using the minimum active
current limit value for the auxiliary supply.
For a start-up time of 3ms with a 2x safety margin, the
TIMER capacitance is calculated as:
Considering the tolerances for the TIMER charging rate
and capacitance, a value of 0.1μF (±10%) for C
suffi ce.
Since the TIMER charging rate during fault time-out is
20 times faster for the 12V supply as compared to the
auxiliary supply during start-up, this scheme ensures that
the external MOSFET will not overheat under any output-
short condition. The fault fi lter delay for the 12V supply
is given by 0.1μF • 6[ms/μF] = 600μs versus 12ms for
the auxiliary supply.
The next step is to verify that the thermal ratings of the
selected external MOSFET for the 12V supply aren’t ex-
ceeded during power-up or an output-short.
Assuming the MOSFET dissipates power only due to inrush
current charging the load capacitor, the energy dissipated
in the MOSFET during power-up is the same as that stored
into the load capacitor. The average power dissipated in
the MOSFET is given by:
I
I
C
t
TRIP MIN
TRIP MAX
STUP AUX
T
=
(
(
2
(
123
•
t
)
STUP AUX
)
[
=
)
=
ms µF
=
Δ
Δ
I
C
V
(
AUX ACL MIN
R
/
V
SENSE CB MIN
L
R
SENSE CB MAX
SENSE MAX
2
S
• .
(
]
E E NSE MIN
)
TRIP(MIN)
3 3
=
(
(
(
123
)(
V
(
AUX
)(
)(
[
)
6
SENSE
)
)
ms µF
ms
must exceed the maximum
=
)
)
=
150
/
=
6 06
47 5m m V
= 6mΩ should suffi ce
165
.
5 94
52 5
µF
]
.
.
≅
.
m
mA
•
0 0
m
mV
3 3 3
Ω
.
Ω
. V
5 5 µF
=
=
7 8
=
8 8
.
T
.
3
A
should
ms
A
The inrush current can be limited by using the GATE ca-
pacitance (C
is well within its safe operating area (SOA). For I
10μA and C
inrush current to 0.5A.
This results in P
be able to tolerate 3W for 19ms. The increase in steady
state junction temperature due to power dissipated in
the MOSFET is ΔT = P
impedance.
Under this condition, the Si7336ADP datasheet’s Tran-
sient Thermal Impedance plot indicates that the junction
temperature will increase by 2.4°C using Z
(single pulse).
The duration and magnitude of the power pulse that results
during a short-circuit condition on the 12V output are a
function of the TIMER capacitance and LTC4223’s analog
current limit. The short-circuit duration is given as 0.1μF
• 6[ms/μF] = 600μs for C
circuit current is calculated using the maximum analog
current limit threshold, ΔV
R
So the maximum power dissipated in the MOSFET is 11A •
12V or 132W for 600μs. The Si7336ADP datasheet’s Tran-
sient Thermal Impedance plot indicates that the worse-case
increase in junction temperature during the short-circuit
condition is 13.2°C using Z
This will not cause the maximum junction temperature to
be exceeded. The SOA curves of the Si7336ADP are also
checked to be safe under this condition.
SENSE
P
I
I
t
INRUSH
SHORT MAX
CHARGE
AVG
value.
=
(
C
=
=
L
2
L1
G
LTC4223-1/LTC4223-2
C
1
•
) so that the power dissipated in the MOSFET
C
L
)
•
= 800μF, we choose C
t
L
1
12
CHARGE
=
AVG
1
C
I
•
I I NRUSH
•
I
Δ
G
V
GATE
12
OUT
V
= 3W and the MOSFET selected must
SENSE ACL MAX
R
V
SENSE MIN
OUT
AVG
2
=
T
0 5
(
SENSE(ACL)(MAX)
.
= 0.1μF. The maximum short-
• Z
= 19
thJC
(
A
th
)(
= 0.1°C/W (single pulse).
ms
where Z
)
)
G
=
= 15nF to set the
5 94
th
66m m V
.
thJC
is the thermal
and minimum
m
Ω
= 0.8°C/W
=
19
GATE
11
422312f
A
=