LT4351
11
4351fd
For V
IN
less than 2V, choose a DC resistance less than 0.2Ω.
Note that V
DD
current referred to the input supply is higher.
A first order approximation of the input current is:
I
VINVDD
= 1+
10.6
V
IN
•
I
VDD
80%
Under normal operation, the V
DD
current is under 10mA
and the boost regulator operates in Burst Mode
®
operation.
If any additional load is added, ensure that the regulator is
capable of supplying that load. As the load is increased,
the boost regulator will switch into continuous mode op-
eration. 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
IN
.
APPLICATIONS INFORMATION
V
DD
Capacitor Selection
Low ESR (Equivalent Series Resistance) capacitors should
be used on V
DD
to minimize the output ripple voltage.
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
IN
.
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
B
and its ESR create an LCR network. The input LCR can
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
B
should be:
C
B
≥
4 • L
IN
R
ESR
2
where C
B
is the capacitor value, R
ESR
is the capacitor’s
ESR and L
IN
is the inductance of the input lines.
While damped ringing is not necessarily bad, it may pro-
duce unexpected results as the LT4351 ideal diode reacts
to the varying V
IN
to OUT voltage. Typically an electrolytic
or tantalum low ESR capacitor would be used. Figure 7a
illustrates V
IN
for a low value of C
B
and Figure 7b shows
it with a correctly sized value.
Figure 7a. Example of Input Voltage Ringing
with Low C
IN
Capacitor at MOSFET Turn Off
Figure 7b. Example of Input Voltage with
Sufficient C
IN
Capacitor at MOSFET Turn Off
10µs/DIV
V
IN
200mV
4351 F07a
10µs/DIV
V
IN
200mV
4351 F07b
