Datasheet

LTC4364-1/LTC4364-2
18
436412f
Choosing 250μA for the resistive divider:
R8 =
1.25V
250µA
= 5k
Select 4.99k for R8.
R7 =
27V –1.25V
( )
• R8
1.25V
= 102.8k
The closest standard value for R7 is 102k.
Now, calculate the sense resistor, R
SNS
, value:
R
SNS
=
∆V
SNS(MIN)
I
LIM
=
45mV
4A
= 11mΩ
Choose 10mΩ for R
SNS
.
C
TMR
is then chosen for 1ms of early warning time:
C
TMR
=
1ms• 5µA
100mV
= 50nF
The closest standard value for C
TMR
is 47nF.
Finally, calculate R1, R2 and R3 for 6V low battery detec-
tion and 60V input overvoltage level:
6V
R1+R2+R3
=
1.25V
R2+ R3
60V
R1+R2+R3
=
1.25V
R3
Simplify the equations and choose 10k for R3 to get:
R2=
60V
6V
–1






•R3 = 9 •R3= 90k
R1=
6V
1.25V
–1






• R2+R3
( )
= 3.8 • R1+R2
( )
= 380k
Select 90.9kΩ for R2 and 383kΩ for R1.
The pass device, M1, should be chosen to withstand an
output short condition with V
CC
= 14V. In the case of a
severe output short where V
OUT
= 0V, I
TMR(UP)
= 55μA and
the total overcurrent fault time is:
t
OC
=
C
TMR
• V
TMR(G)
I
TRM(UP)
=
47nF • 1.35V
55µA
= 1.15ms
The maximum power dissipation in M1 is:
P =
∆V
DS(M1)
• ∆V
SNS(MAX)
R
SNS
=
14V • 32mV
10mΩ
= 45W
The corresponding P
2
t is 2.3W
2
s.
During an output overload or soft short, the voltage at the
OUT pin could stay at 2V or higher. The total overcurrent
fault time when V
OUT
= 2V is:
t
OC
=
47nF •1.35V
49µA
= 1.3ms
The maximum power dissipation in M1 is:
P =
14V – 2V
( )
• 55mV
10m
Ω
= 66W
The corresponding P
2
t is 5.7W
2
s. Both of the above condi-
tions are well within the safe operating area of FDB33N25.
To select the pass device, M2, first calculate R
DS(ON)
to
achieve the desired forward drop V
FW
at maximum load
current (5.5A). If V
FW
= 0.25V:
R
DS(ON)
≤
V
FW
I
LOAD(MAX)
=
0.25V
5.5A
= 45.5mΩ
The FDB3682 offers a maximum R
DS(ON)
of 36mΩ at
V
GS
= 10V so is a good fit. Its minimum BV
DSS
of 100V
is also sufficient to handle V
OUT
transients up to 100V
during an input short-circuit event.
Layout Considerations
To achieve accurate current sensing, use Kelvin connections
to the current sense resistor, R
SNS
. Limit the resistance
from the SOURCE pin to the sources of the MOSFETs to
below 10Ω. The minimum trace width for 1oz copper foil
is 0.02" per amp to ensure the trace stays at a reason-
able temperature. Note that 1oz copper exhibits a sheet
resistance of about 530μΩ/square. Small resistances can
cause large errors in high current applications. Noise im-
munity will be improved significantly by locating resistive
dividers close to the pins with short V
CC
and GND traces.
APPLICATIONS INFORMATION