LTC4225CGN-1#TRPBF Linear Technology, LTC4225CGN-1#TRPBF Datasheet - Page 16

• Manufacturer Part Number: LTC4225CGN-1#TRPBF
• Manufacturer: Linear Technology

Specifications of LTC4225CGN-1#TRPBF

• Lead Free Status / RoHS Status: Compliant

LTC4225-1/LTC4225-2
applicaTions inForMaTion
Next, verify that the thermal ratings of the selected
MOSFET, Si7336ADP , are not exceeded during power-up
or an output short.
Assuming the MOSFET dissipates power due to inrush
current charging the load capacitor, C
energy dissipated in the MOSFET is the same as the energy
stored in the load capacitor, and is given by:
For C
calculated as:
The inrush current is set to 1A by adding capacitance,
C
Choose a practical value of 15nF for C
The average power dissipated in the MOSFET is calculated
as:
The MOSFET selected must be able to tolerate 6W for
19ms during power-up. The SOA curves of the Si7336ADP
provide for 1.5A at 30V (45W) for 100ms. This is suffi-
cient to satisfy the requirement. The increase in junction
temperature due to the power dissipated in the MOSFET
is ∆T = P
thermal impedance. Under this condition, the Si7336ADP
data sheet indicates that the junction temperature will
increase by 4.8°C using Zth
The duration and magnitude of the power pulse during an
output short is a function of the TMR capacitance, C
the LTC4225’s active current limit. The short-circuit dura-
tion is given as C
16
HG
E
P
C
t
CHARGE
, at the gate of the Hot Swap MOSFET.
CL
AVG
HG
L
=
=
= 1600µF , the time it takes to charge up C
=
2
AVG
1
C
t
L
• C
CHARGE
=
E
•I
I
• Zth
INRUSH
L
C
I
CL
INRUSH
HGATE(UP)
L
• V
T
• V
IN
• 12[ms/µF] = 0.56ms for C
JC
IN
=
2
where Zth
2
1
=
•
1600µF • 12V
=
1600µF • 12V
1600µF • 10µA
JC
1A
= 0.8°C/W (single pulse).
19ms
JC
1A
is the junction-to-case
(
L
HG
= 19ms
, at power-up, the
)
.
2
= 16nF
= 6W
T
= 0.047µF .
T
, and
L
is
The maximum short-circuit current is calculated using the
maximum active current limit threshold ∆V
and minimum R
So, the maximum power dissipated in the MOSFET is
18.9A • 12V = 227W for 0.56ms. The Si7336ADP data
sheet indicates that the worst-case increase in junction
temperature during this short-circuit condition is 22.7°C
using Zth
0.047µF will not cause the maximum junction temperature
of the MOSFET to be exceeded. The SOA curves of the
Si7336ADP provide for 15A at 30V (450W) for 1ms. This
also satisfies the requirement.
Next, select the resistive divider at the ON1 and ON2 pins
to provide an undervoltage threshold of 9.6V for the 12V
supply. First, choose the bottom resistors, R1 and R3, to be
20k. Then, calculate the top resistor value for R2 and R4:
Choose the nearest 1% resistor value of 137k for R2
and R4. In addition, there is a 0.1µF bypass (C1) at the
INTV
to prevent the supply glitches from turning off the Hot
Swap MOSFET.
PCB Layout Considerations
For proper operation of the LTC4225’s circuit breaker, Kelvin
connection to the sense resistor is strongly recommended.
The PCB layout should be balanced and symmetrical to
minimize wiring errors. In addition, the PCB layout for the
sense resistor and the power MOSFET should include good
thermal management techniques for optimal device power
dissipation. A recommended PCB layout is illustrated in
Figure 7.
I
SHORT(MAX)
R
R
TOP
TOP
CC
pin and a 10nF filter capacitor (C
=
=
JC




 
1.235V
V
= 0.1°C/W (single pulse). Choosing C
V
IN(UVTH)
9.6V
=
ON(TH)
S
∆V
value.
SENSE(ACL)(MAX)
– 1
R
– 1

 
S(MIN)

 •R

• 20k = 135k
BOTTOM
=
3.96mΩ
75mV
F
SENSE(ACL)(MAX)
) at the ON pin
= 18.9A
422512f
T
=