Datasheet
LT4351
10
4351fd
In that case, the resistor values are set by:
R3 =
UV
HYST
I
UVHYST
R2 =
V
UV
–
UV
FAULT
OV
FAULT
• V
OV
UV
FAULT
– V
UV
• R3
R1=
V
OV
• UV
FAULT
OV
FAULT
• UV
FAULT
– V
UV
( )
• R3
Hysteresis helps prevent erratic behavior due to the noise
on V
IN
. Two of the most common noise sources are: V
IN
dipping when the MOSFETs first turn on and draw down
the voltage on the V
IN
capacitors, and the boost regulator
switch turning on and drawing current from the V
IN
ca-
pacitors. Use low ESR capacitors for V
IN
and OUT filtering.
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
IN
provided
V
IN
< 9V. Overvoltage fault detection can be defeated by
grounding the OV pin. Do not exceed V
IN
.
APPLICATIONS INFORMATION
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.
Boost Regulator
The boost regulator will start working as soon as V
IN
is
greater than 0.85V. The regulator will supply all the cur-
rent for the gate drive amplifier. While the amplifier itself
requires only about 3mA, larger current pulses are required
when charging the MOSFET gate. The reservoir capacitor
on V
DD
will provide this current (Figure 6).
The regulator performance is relatively insensitive to the
inductor value. The inductor value does control the fre-
quency of operation. A 4.7µH inductor is recommended
for V
IN
voltages less than 10V and 10µH for V
IN
voltages
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 0.5Ω or less. Shielded inductors are recommended to
reduce the noise due to inductive switching.
Table 1. Recommended Inductors
PART NUMBER IND (µH) DCR (mΩ) VENDOR
LPS3314-472ML
LPS4012-103ML
4.7
10
175
350
Coilcraft
847-639-6400
www.coilcraft.com
744029004
744042100
4.7
10
200
150
Würth Elektronik
www.we-online.com
SD3112-4R7-R
SD3118-100-R
4.7
10
246
295
Coiltronics
www.coiltronics.com
L1
D1
D2
QSW
GND
SW
V
DD
C
DD
4351 F06
V
IN
LT4351
Figure 5. Graphical Representation of the UV and OV Functions
OVERVOLTAGE FILTERED FAULT
INPUT
REFERRED
OV
REFERRED
UV
REFERRED
V
UV
= 0.33V
V
UV
= 0.3V
V
UV
< 0.3V
V
OV
> 0.3V
V
OV
= 0.3V
4351 F05
OV
FAULT
UV
FAULT
+ UV
HYST
UV
FAULT
UNDERVOLTAGE HYSTERESIS
OVERVOLTAGE FAULT:
GATE LOW
UNDERVOLTAGE FAULT:
GATE LOW
GATE CONTROLLED
BY V
IN
– V
OUT
Figure 6