LTC4228-1/LTC4228-2
16
422812f
The inrush current is set to 1A by adding capacitance,
C
HG
, at the gate of the Hot Swap MOSFET.
C
HG
=
C
L
•I
HGATE(UP)
I
INRUSH
=
1600µF • 10µA
1A
= 16nF
Choose a practical value of 15nF for C
HG
.
The average power dissipated in the MOSFET is calculated
as:
P
AVG
=
E
CL
t
CHARGE
=
1
2
•
1600µF • 12V
( )
2
19ms
= 6W
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
AVG
• Zth
JC
where Zth
JC
is the junction-to-case
thermal impedance. Under this condition, the Si7336ADP
data sheet indicates that the junction temperature will
increase by 4.8°C using Zth
JC
= 0.8°C/W (single pulse).
The duration and magnitude of the power pulse during an
output short is a function of the TMR capacitance, C
T
, and
the LTC4228’s active current limit. The short-circuit dura-
tion is given as C
T
• 12[ms/µF] = 0.56ms for C
T
= 0.047µF.
The maximum short-circuit current is calculated using the
maximum active current limit threshold ∆V
SENSE(ACL)(MAX)
and minimum R
S
value.
I
SHORT(MAX)
=
∆V
SENSE(ACL)(MAX)
R
S(MIN)
=
75mV
3.96mΩ
= 18.9A
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
JC
= 0.1°C/W (single pulse). Choosing C
T
=
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:
R
TOP
=
V
IN(UVTH)
V
ON(TH)
– 1
• R
BOTTOM
R
TOP
=
9.6V
1.235V
– 1
• 20k =135k
Choose the nearest 1% resistor value of 137k for R2 and
R4. In addition, there is a 0.1µF bypass (C1) at the INTV
CC
pin and a 10nF filter capacitor (C
F
) at the ON pin to prevent
the supply glitches from turning off the Hot Swap MOSFET.
PCB Layout Considerations
For proper operation of the LTC4228’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.
Connect the IN and OUT pin traces as close as possible to
the MOSFETs’ terminals. Keep the traces to the MOSFETs
wide and short to minimize resistive losses. The PCB traces
associated with the power path through the MOSFETs
should have low resistance. The suggested trace width for
1oz copper foil is 0.03" for each ampere of DC current to
keep PCB trace resistance, voltage drop and temperature
rise to a minimum. Note that the sheet resistance of 1oz
copper foil is approximately 0.5mΩ/square, and voltage
drops due to trace resistance add up quickly in high cur-
rent applications.
It is also important to place the bypass capacitor, C1, for
the INTV
CC
pin, as close as possible between INTV
CC
and
GND. Also place C
CP1
near the CPO1 and IN1 pins, and
C
CP2
near the CPO2 and IN2 pins. The transient voltage
suppressors, Z1 and Z2, when used, should be mounted
close to the LTC4228 using short lead lengths.
applicaTions inForMaTion
