Datasheet

LT4363
17
4363fa
APPLICATIONS INFORMATION
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 a transient of
150V and decay time constant (τ) of 400ms, V
OUT
≤ 27V,
current limit (I
LIM
) at 5A, low battery detection of 6V, input
overvoltage level at 60V, and 1ms of overvoltage early
warning (Figure 8).
Selection of SMAJ58A for D1 will limit the voltage at the
V
CC
pin to less than 71V during 150V surge. The minimum
required voltage at the V
CC
pin is 4V when V
IN
is at 8V;
the supply current for LT4363 is 1.5mA. The maximum
value for R7 to ensure proper operation is:
R7 =
8V – 4V
1.5mA
= 2.67kΩ
Select 1kΩ for R7 to accommodate all conditions.
The maximum current through R7 into D1 is then calcu-
lated as:
I
D1
=
150V – 64V
1kΩ
= 86mA
which is easily handled by the SMAJ58A for more than
500ms.
With 0.1µF of bypass capacitance, C1, along with 1k of
R7, high voltage transients up to 200V with a pulse width
less than 10µs are filtered out at the V
CC
pin.
Next, calculate the resistive divider value to limit V
OUT
to
27V during an overvoltage event:
V
REG
=
1.275V • R1+ R2
( )
R2
= 27V
Set the current through R1 and R2 during the overvoltage
condition to 250µA.
R2=
1.275V
250µA
= 5kΩ
Choose 4.99kΩ for R2.
R1=
27V – 1.275V
( )
• R2
1.275V
= 100.7kΩ
The nearest standard value for R1 is 100kΩ.
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 • 6µA
100mV
= 60nF
The nearest standard value for C
TMR
is 47nF.
Finally, calculate R4, R5, and R6 for 6V low battery detec-
tion and 60V input overvoltage level:
6V •
R5+ R6
R4
+
R5
+
R6
= 1.275V
60V •
R6
R4 + R5+ R6
= 1.275V
Choose 10kΩ for R6.
R4 + R5=
60V • 10kΩ
1.275V
– 10kΩ = 460.6kΩ
R5=1.275V •
460.6kΩ + 10kΩ
6V
– 10kΩ = 90kΩ
R4 = 460.6kΩ – 90kΩ = 370.6kΩ
Select 90.9kΩ for R5 and 374kΩ for R4.
The pass transistor, Q1, should be chosen to withstand a
short-circuit with V
CC
= 14V. In the case of a severe output
short where V
OUT
= 0V, the total overcurrent fault time is:
t
OC
=
47nF • 0.875V
45.5µA
= 0.904ms