LT4356MP-1_15 Datasheet, PDF (16/24 Pages) Linear Technology – Surge Stopper

LT4356MP-1/LT4356MP-2
APPLICATIONS INFORMATION
A total bulk capacitance of at least 22μF low ESR electro-
lytic is required close to the source pin of MOSFET Q1. In
addition, the bulk capacitance should be at least 10 times
larger than the total ceramic bypassing capacitor on the
input of the DC/DC converter.
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 = 5k
250μA
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} = 380k
1.25V
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} = 70W
10mΩ
These conditions are well within the Safe Operating Area
of IRLR2908.
Choose 4.99k for R2.
R1 = ^{(16V – 1.25V) • R2} = 58.88k
1.25V
The closest standard value for R1 is 59k.
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