lt4351 Linear Technology Corporation, lt4351 Datasheet - Page 11

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

APPLICATIO S I FOR ATIO
For V
0.2 .
Note that V
A first order approximation of the input current is:
Under normal operation, the V
and the boost regulator operates in Burst Mode operation.
If any additional load is added, you must ensure that the
regulator is capable of supplying that load. As the load is
increased, the boost regulator will switch into continuous
mode operation. Further increases in load will collapse the
boost regulator voltage.
Operating the regulator with increased load will cause
increased IC power dissipation and temperature, which
must be taken into consideration.
A 100ns delay from detecting the switch current limit to
turning off the power switch produces an overshoot of the
inductor current from the 0.45A switch limit. The amount
of overshoot depends on the boost regulator inductance.
Choosing an inductor that can handle 0.75A peak current
will be sufficient for the recommended inductors.
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, are the best match for the LT4351
boost regulator. Select a diode that can handle 0.75A peak
current and a reverse breakdown of 15V greater than the
maximum V
I
VINVDD
IN
200mV
less than 2V, choose a DC resistance less than
V
Figure 7a. Example of Input Voltage Ringing
with Low C
IN
DD
IN
current referred to the input supply is higher.
.
1
10 6
IN
V
U
IN
Capacitor at MOSFET Turn Off
.
•
10 s/DIV
U
I
80
VDD
%
DD
current is under 10mA
W
4351 F07a
U
V
Low ESR (Equivalent Series Resistance) capacitors should
be used on V
Multilayer ceramic capacitors are the best choice, as they
have a very low ESR and are available in very small
packages. Always use a capacitor with a voltage rating at
least 12V greater than V
Capacitors
Two types of input capacitors are generally needed for the
LT4351. The first is a large bulk capacitor that takes care
of ringing associated with inductance of the input supply
lines and provides charge for the load when switching the
MOSFET. The input parasitic inductance in conjunction with
C
be stimulated by the boost regulator switch current or load
current transients when the MOSFETs are on. To reduce
ringing associated with input inductance, C
where C
ESR and L
While damped ringing is not necessarily bad, it may
produce unexpected results as the LT4351 ideal diode
reacts to the varying V
electrolytic or tantalum low ESR capacitor would be used.
Figure 7a illustrates V
shows it with a correctly sized value.
DD
B
C
and its ESR create an LCR network. The input LCR can
B
Capacitor Selection
200mV
B
R
4
V
Figure 7b. Example of Input Voltage with
Sufficient C
IN
ESR
is the capacitor value, R
•
IN
L
IN
is the inductance of the input lines.
2
DD
to minimize the output ripple voltage.
IN
IN
Capacitor at MOSFET Turn Off
for a low value of C
IN
IN
10 s/DIV
.
to OUT voltage. Typically an
ESR
is the capacitor’s
4351 F07b
B
LT4351
and Figure 7b
B
should be:
sn4351 4351fs
11