LTC4223-1/LTC4223-2
19
422312f
P
CV
t
AVG
L OUT
CHARGE
=
1
2
12
2
•
•
The inrush current can be limited by using the GATE ca-
pacitance (C
G
) so that the power dissipated in the MOSFET
is well within its safe operating area (SOA). For I
GATE
=
10μA and C
L1
= 800μF, we choose C
G
= 15nF to set the
inrush current to 0.5A.
I
CI
C
A
t
CV
I
INRUSH
LGATE
G
CHARGE
L OUT
==
=
1
1
05
12
•
.
•
IINRUSH
ms= 19
This results in P
AVG
= 3W and the MOSFET selected must
be able to tolerate 3W for 19ms. The increase in steady
state junction temperature due to power dissipated in
the MOSFET is ΔT = P
AVG
• Z
th
where Z
th
is the thermal
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
thJC
= 0.8°C/W
(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
T
= 0.1μF. The maximum short-
circuit current is calculated using the maximum analog
current limit threshold, ΔV
SENSE(ACL)(MAX)
and minimum
R
SENSE
value.
I
V
R
SHORT MAX
SENSE ACL MAX
SENSE MIN
()
()( )
()
=
Δ
=
66
mmV
m
A
594
11
. Ω
=
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
thJC
= 0.1°C/W (single pulse).
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.
APPLICATIONS INFORMATION
I
V
R
TRIP MIN
SENSE CB MIN
SENSE MAX
()
()( )
()
.
=
Δ
=
47 5mmV
m
A
I
V
R
TRIP MAX
SENSE CB MAX
S
606
78
.
.
()
()( )
Ω
=
=
Δ
EENSE MIN
mV
m
A
()
.
.
.==
52 5
594
88
Ω
For proper operation, I
TRIP(MIN)
must exceed the maximum
load current with margin, so R
SENSE
= 6mΩ should suffi ce
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.
t
CV
I
µF
STUP AUX
LAUX
AUX ACL MIN
()
()()
•.
•
==
2
33
150
333
165
3
.V
mA
ms=
For a start-up time of 3ms with a 2x safety margin, the
TIMER capacitance is calculated as:
C
t
ms µF
ms
ms µF
T
STUP AUX
=
[]
=
[]
≅
2
123
6
123
00
•
//
.
()
55µF
Considering the tolerances for the TIMER charging rate
and capacitance, a value of 0.1μF (±10%) for C
T
should
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:
