lt4351 Linear Technology Corporation, lt4351 Datasheet - Page 10

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

UV
External Shutdown
To externally turn off the MOSFETs, such as to disable the
supply, use an open-collector transistor pulling down on
the UV pin. Note this will not turn off the boost regulator
which will continue to operate.
APPLICATIO S I FOR ATIO
LT4351
In that case the resistor values are set by:
Hysteresis helps prevent erratic behavior due to the noise
on V
dipping when the MOSFETs first turn on and draw down the
voltage on the V
turning on and drawing current from the V
Use low ESR capacitors for V
Note that because the UV pin uses current hysteresis,
placing a capacitor on UV to ground to filter noise will
reduce the effective hysteresis. Filtering can be achieved
by splitting the R2 resistor as shown in Figure 4.
To defeat undervoltage fault detection, the UV pin should
be tied higher than 0.33V. UV can be tied to V
V
grounding the OV pin. Do not exceed V
10
Figure 5. Graphical Representation of the UV and OV Functions
IN
FAULT
R
R
R
< 9V. Overvoltage fault detection can be defeated by
3
2
1
IN
REFERRED
+ UV
OV
UV
. Two of the most common noise sources are: V
INPUT
FAULT
FAULT
OV
HYST
I
UV
V
UVHYST
UV
FAULT
HYST
UV
–
FAULT
V
IN
UV
OV
OVERVOLTAGE FILTERED FAULT
OV
UNDERVOLTAGE HYSTERESIS
capacitors, and the boost regulator switch
•
UNDERVOLTAGE FAULT:
FAULT
FAULT
OVERVOLTAGE FAULT:
•
U
GATE CONTROLLED
UV
UV
–
BY V
FAULT
GATE LOW
GATE LOW
V
FAULT
UV
IN
•
U
– V
V
OV
OUT
–
IN
V
•
UV
and OUT filtering.
R
3
W
•
R
4351 F05
3
IN
OV
REFERRED
V
V
.
OV
OV
IN
> 0.3V
= 0.3V
IN
capacitors.
U
provided
UV
REFERRED
V
V
V
UV
UV
UV
= 0.33V
= 0.3V
< 0.3V
IN
Boost Regulator
The boost regulator will start working as soon as V
greater than 0.85V. The regulator will supply all the current
for the gate drive amplifier. While the amplifier itself
requires only about 3mA, larger current pulses are re-
quired when charging the MOSFET gate. The reservoir
capacitor on V
The regulator performance is relatively insensitive to the
inductor value. The inductor value does control the fre-
quency of operation. A 4.7 H inductor is recommended
for V
greater than 10V. Several inductors that work well with the
LT4351 are listed in Table 1. Many different sizes and
shapes are available. Consult each manufacturer for more
detailed information and for their entire selection of related
parts. The switching frequency for the boost regulator is
around 1MHz so ferrite core inductors should be used to
obtain the best efficiency. The inductor must handle a peak
current of 0.7A minimum and have a DC resistance of
0.5
reduce the noise due to inductive switching.
Table 1. Recommended Inductors
PART NUMBER
DS1608C-472
DS1608C-103
CDRH4D18-4R7
CDRH4D28-100
LMNP045B4R7N
LMNP045B100M
IN
or less. Shielded inductors are recommended to
voltages less than 10V and 10 H for V
DD
QSW
LT4351
will provide this current (Figure 6).
GND
SW
( H)
IND
4.7
4.7
4.7
10
10
10
Figure 6
V
IN
L1
(m )
MAX
DCR
160
125
90
95
50
52
D2
D1
4351 F06
C
V
VENDOR
Coilcraft
[phone]
www.coilcraft.com
Sumida
[phone]
www.sumida.com
Taiyo Yuden
[phone]
www.t-yuden.com
DD
DD
IN
sn4351 4351fs
voltages
IN
is