lt4351 Linear Technology Corporation, lt4351 Datasheet - Page 15

• Manufacturer Part Number: lt4351
• Description: Mosfet Diode-or Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The die junction temperature is then computed as:
where T
temperature,
(120 C/W) and P
Therefore, a 0.1W power dissipation causes a 12 degree
temperature rise above ambient.
Design Example
The following demonstrates the calculations involved for
setting design components for a 5V system that requires
5A. Two supplies are used to do this. The V
be deemed in spec when it is within 5% of nominal. Allow
5% of hysteresis for UV.
So,
Two separate resistive dividers are used.
For the UV divider:
T
UV
OV
R1 = 1.68k. The closest 1% value is 1.69k
R
R
J
2
1
= T
FAULT
FAULT
J
UV
I
0.004
0.002
0.001
A
UV
0.003
is the die junction temperature, T
UVHYST
Figure 13. P
+
FAULT
R
0
= 4.75V, UV
= 5.5
HYST
0
2
f
Q
JA
GATE
G
•
JA
= 50nC
V
• P
UV
–
= 1kHz
TOTAL
is the thermal resistance of the part
V
U
TOTAL
0 25
5
10
UV
GATE
.
HYST
A
V
is ascertained from the above.
vs V
U
V
4 75
IN
24 9
10
.
(V)
IN
= 0.25V
25
.
(V
V
k Use
k
DD
• .
– .
15
0 3
W
= V
0 3
IN
V
V
24 9
4351 F13
+ 10.7)
A
.
20
is the ambient
IN
k
supply will
U
The OV resistors are set as a straight resistive divider.
If the current in the RA, RB divider is 200 A, then:
For regulation, the MOSFETs must have:
This very low value cannot be accomplished with a single
set of MOSFETs so a decision must be made whether to
use multiple MOSFETs or to live with an unregulated
offset. Since low m
using a single FET would still be acceptable. For R
4m
schematic is shown in Figure 15.
then
R
R
R
R
DS
B
B
A
the drop is 2 • 5A • 4m
25.48k use 25.5k
200 A
0.16
0.14
0.12
0.10
0.08
0.06
15
2 5
0.3V
OV
•
V
mV
0
FAULT
Figure 14. Total Power (Typical)
OV
V
0.5mA GATE CURRENT
A
DD
L = 10 H
= V
IN
1 5
1.5k use 1.47k (1%)
–
5
.
+ 10
L = 4.7 H
1
m
R
R
DS(ON)
V
A
IN
10
(V)
5 5
0 3
is available, the IR drop
.
.
= 40mV. The finished
15
–
1 1 47
4351 F14
.
20
LT4351
k
sn4351 4351fs
DS(ON)
15
=