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LTC4223-1/LTC4223-2

422312f

AVG

OUT

CHARGE

= 1

•

The inrush current can be limited by using the GATE ca-

pacitance (CG) so that the power dissipated in the MOSFET

is well within its safe operating area (SOA). For IGATE =

10μA and CL1 = 800μF, we choose CG = 15nF to set the

inrush current to 0.5A.

INRUSH

LGATE

CHARGE

OUT

•

IINRUSH

= 19

This results in PAVG = 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 = PAVG • Zth where Zth 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 ZthJC = 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 CT = 0.1μF. The maximum short-

circuit current is calculated using the maximum analog

current limit threshold, ΔVSENSE(ACL)(MAX) and minimum

RSENSE value.

SHORT MAX

SENSE ACL MAX

SENSE MIN

()

()(

)

()

66m

594

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 ZthJC = 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

TRIP MIN

SENSE CB MIN

SENSE MAX

()

()(

)

()

47 5m

TRIP MAX

SENSE CB MAX

606

()

()(

)

ENSE MIN

()

52 5

594

For proper operation, ITRIP(MIN) must exceed the maximum

load current with margin, so RSENSE = 6mΩ should sufﬁce

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 ﬁlter delay. The worst-

case start-up time is calculated using the minimum active

current limit value for the auxiliary supply.

µF

STUP AUX

LAUX

AUX ACL MIN

()

()(

)

•.

•

2 33

150

165

For a start-up time of 3ms with a 2x safety margin, the

TIMER capacitance is calculated as:

ms µF

STUP AUX

[]

≅

123

•

()

5µF

Considering the tolerances for the TIMER charging rate

and capacitance, a value of 0.1μF (±10%) for CT should

sufﬁce.

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 ﬁlter 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:

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