Datasheet

LT4356-1/LT4356-2
17
4356fb
For more information www.linear.com/LT4356-1
Layout Considerations
To achieve accurate current sensing, Kelvin connection
to the current sense resistor (R
SNS
in Figure 9) is recom-
mended. The minimum trace width for 1oz copper foil is
0.02" per amp to ensure the trace stays at a reasonable
temperature. 0.03" per amp or wider is recommended.
Note that 1oz copper exhibits a sheet resistance of about
530µΩ/square. Small resistances can cause large errors in
high current applications. Noise immunity will be improved
significantly by locating resistive dividers close to the pins
with short V
CC
and GND traces.
Design Example
As a design example, take an application with the follow
-
ing specifications: V
CC
= 8V to 14V DC with transient up
to 80V, V
OUT
≤ 16V, current limit (I
LIM
) at 5A, low battery
detection at 6V, and 1ms of overvoltage early warning
(Figure 9).
First, calculate the resistive divider value to limit V
OUT
to
16V during an overvoltage event:
V
REG
=
1.25V • R1 + R2
( )
R2
=16V
Set the current through R1 and R2 during the overvoltage
condition to 250µA.
R2 =
1.25V
250µA
= 5k
Choose 4.99k for R2.
R1 =
16V – 1.25V
( )
• R2
1.25V
= 58.88k
The closest standard value for R1 is 59k.
Next calculate the sense resistor, R
SNS
, value:
R
SNS
=
50mV
I
LIM
=
50mV
5A
= 10mΩ
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 R4 and R5 for the 6V low battery threshold
detection:
6V =
1.25V • R4 + R5
( )
R5
Choose 100k for R5.
R4 =
6V – 1.25V
( )
• R5
1.25V
= 380k
Select 383k for R4.
The pass transistor, Q1, should be chosen to withstand
the output short condition with V
CC
= 14V.
The total overcurrent fault time is:
t
OC
=
47nF • 0.85V
45.5µA
= 0.878ms
The power dissipation on Q1 equals to:
P =
14V • 50mV
10mΩ
= 70W
These conditions are well within the Safe Operating Area
of IRLR2908.
ApplicAtions inForMAtion
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