9
LT3473/LT3473A
3473f
Inrush Current
The LT3473 has an integrated Schottky power diode.
When supply voltage is abruptly applied to the V
IN
pin
while the output capacitor is discharged, the voltage
difference between V
IN
and CAP generates inrush current
flowing from the input through the inductor and the
internal Schottky diode to charge the output capacitor at
the CAP pin. The maximum current the LT3473’s Schottky
can sustain is 2A. The selection of inductor and capacitor
values should ensure that the peak inrush current is less
than 2A. Peak inrush current can be calculated as follows:
I
V
L
r
L
LC
r
L
P
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
=
+
=
–.
•
• exp – • arctan •sin arctan
.
•
•
–
•
06
15
2
1
4
2
ω
α
ω
ω
α
ω
α
α
ω
where L is the inductance, r is the resistance of the
inductor and C is the output capacitance. For a low DCR
inductor, which is usually the case for this application, the
peak inrush current can be simplified as follows:
I
V
L
P
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
–.
•
• exp – •
06
2ω
α
ω
π
A large abrupt voltage step at V
IN
and/or a large capacitor
at the CAP pin generate larger inrush current. Table 3 gives
inrush peak currents for some component selections. An
inductor with low saturation current could generate very
large inrush current. For this case, inrush current should
be measured to ensure safe operation. Note that inrush
current is not a concern if the input voltage rises slowly.
Table 3. Inrush Peak Current
V
IN
(V) R (Ω)L (µH) C (µF) I
P
(A)
5 0.05 6.8 0.47 0.86
10 0.05 6.8 0.47 1.83
3.6 0.05 6.8 0.47 0.58
3.6 0.05 4.7 0.47 0.67
Setting the Output Voltages
The LT3473 has both an internal 1.25V reference and an
external reference input. This allows the user to select
between using the built-in reference and supplying an
external reference voltage. The voltage at the CTRL pin can
be adjusted while the device is operating to alter the output
voltage for purposes such as display dimming or contrast
adjustment. To use the internal 1.25V reference, the CTRL
pin must be held higher than 1.5V. When the CTRL pin is
held between 0V and 1.2V, the LT3473 will regulate the
output such that the FB pin voltage is equal to the CTRL pin
voltage.
The CAP pin should be used as the feedback node. To set
the output voltage, select the values of R1 and R2 accord-
ing to the following equation.
VV
R
R
INT REF
=+
⎛
⎝
⎜
⎞
⎠
⎟
•1
2
1
where V
REF
= 1.25V if the internal reference is used, or
V
REF
= V
CTRL
if V
CTRL
is between 0V and 1.2V.
To maintain output voltage accuracy, 1% resistors are
recommended.
Soft-Start
The SHDN pin also functions as soft-start. Use an RC filter
at the SHDN pin to limit the start-up current. The small bias
current of the SHDN pin allows using a small capacitor for
a large RC time constant.
APPLICATIO S I FOR ATIO
WUUU
Figure 4. Soft-Start Circuitry
SHDN
100nF
ON/OFF
3473 F04
20k
LT3473
Output Disconnect Considerations
The LT3473 has an output disconnect PNP that isolates
the load from the input during shutdown. The drive circuit
maintains the PNP at the edge of saturation, adaptively
according to the load, thus yielding the best compromise
between V
CESAT
and quiescent current to minimize power
loss. To remain stable, it requires a bypass capacitor
connected between the OUT pin and the CAP pin or
