lt4351 Linear Technology Corporation, lt4351 Datasheet - Page 12

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

As an example, for 500nH of inductance and R
100m , then:
Check vendor data for ESR and iterate to get the best
value. Additional C
concerns.
If the boost regulator is being used, place a 10 F low ESR
ceramic capacitor from V
0.1 F ceramic capacitor close to V
capacitors should have low ESR (less than 10m for the
10 F and 40m for the 0.1 F). These capacitors help to
eliminate problems associated with noise produced by the
boost regulator. They are decoupled from the V
by a small 1 resistor as shown in Figure 8. The LT4351
will perform better with a small ceramic capacitor (10 F)
on OUT to GND.
APPLICATIO S I FOR ATIO
LT4351
External Boost Supply
The V
case, simply omit the boost regulator inductor and diode
and leave the SW pin open. Suitable V
(minimum of a 1 F ceramic) should remain due to the cur-
rent pulses required for the gate driver.
The V
current required to charge the MOSFET’s gate which is de-
pendent on the gate charge required and frequency of
switching. Typically the average current will be under 10mA.
MOSFET Selection
The LT4351 uses either a single N-channel MOSFET or
back-to-back N-channel MOSFETs as the pass element.
12
C
V
DD
IN
DD
4 500
pin may be powered by an external supply. In this
current consists of 3.5mA of DC current with the
•
PARASITIC
0 1
.
L
IN
2
nF
Figure 8. V
B
C
U
200
capacitance may be required for load
B
C
10 F
V3
F
IN
U
IN
to GND. Place a 10 F and a
1
Capacitors
C
10 F
V1
W
C
0.1 F
IN
V2
and GND. These
DD
V
GND
IN
capacitance
LT4351
ESR
GATE
U
IN
4351 F08
of about
supply
Back-to-back MOSFETs prevent the MOSFET body diode
from passing current.
Use a single MOSFET if current flow is allowable from
input to output when the input supply is above the output
(limited overvoltage protection). In this case the MOSFET
should have a source on the input side so the body diode
conducts current to the load. Back-to-back MOSFETs are
normally connected with their sources tied together to
provide added protection against exceeding maximum
gate to source voltage.
Selection of MOSFETs should be based on R
and BV
breakdown when V
R
power rating at the maximum load current (I
BV
GATE to 7.5V above the lesser of V
back MOSFETs where sources are tied together, this
allows the use of MOSFETs with a VGS max rating of 8V or
more. If a single MOSFET is used, care must be taken to
ensure the VGS max rating is not exceeded. When the
MOSFET is turned off, the GATE voltage is near ground, the
source at V
than V
If a single MOSFET is used with source to V
should be greater than the maximum V
gate is at 0.2V when off.
The gate drive amplifier will attempt to regulate the
voltage across the MOSFETs to 15mV. Regulation will be
achieved if:
This requires very low R
by paralleling MOSFETs, but be careful to keep intercon-
nection trace resistance low. In the event that regulation
cannot be achieved, the gate drive amplifier will drive
GATE to its clamp and achieve the best R
level.
DS(ON)
GSS
R
R
DS
DS
IN(MAX)
should be at least 8V. The LT4351 will clamp the
GSS
should be selected to keep within the MOSFET
15
2 •
I
LOAD
. BV
15
IN
mV
I
. Thus, MOSFET VGS max must be greater
.
LOAD
mV
DSS
for a single MOSFET
IN
for two MOSFETs and
should be high enough to prevent
or OUT are at their maximum value.
DS
values. This may be achieved
IN
or OUT. For back-to-
IN
since the MOSFET
DS
possible at that
IN
DS(ON)
, then BV
2
• R
sn4351 4351fs
, BV
DS(ON)
DSS
GSS
)