LT3482 LINER [Linear Technology], LT3482 Datasheet - Page 7

• Manufacturer Part Number: LT3482
• Description: 90V Boost DC/DC Converter with APD Current Monitor
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Switching Frequency
The LT3482 can operate at either 650kHz nominal or
1.1MHz nominal; the voltage at the f
frequency is used. At 1.1MHz, a physically smaller induc-
tor and capacitor can be used in a given application, but
higher frequencies will slightly decrease effi ciency and
maximum duty cycle. Generally if effi ciency and maximum
duty cycle are crucial, the lower switching frequency should
be selected by connecting f
and cost are more important, connect f
the higher switching frequency.
Inrush Current
The LT3482 has built-in Schottky diodes for the boost
and charge pump. When supply voltage is applied to the
V
generates inrush current fl owing from input through the
inductor and the Schottky diode (D1 in the Functional
Diagram) to charge the output capacitor. The selection of
inductor and capacitor value should ensure the peak of
the inrush current to be below 1A. In addition, the LT3482
turn-on should be delayed until the inrush current is less
than the maximum current limit. The peak inrush current
can be estimated as follows:
where L is the inductance and C is the output capacitance.
Table 1 gives inrush peak currents for some component
selections.
IN
I
P
pin, the voltage difference between V
=
V
IN
L
C
– .
–
0 6
1
• exp –
U
⎛
⎜
⎜
⎜
⎜
⎝
2
U
SET
π
L
C
to GND. If application size
–
1
⎞
⎟
⎟
⎟
⎟
⎠
W
SET
Figure 1. Output Voltage Feedback Connection
SET
pin selects which
to V
IN
U
IN
and V
to select
13
CTRL
OUT1
LT3482
V
OUT2
FB
Table 1. Inrush Peak Current
Setting Output Voltage
The LT3482 is equipped with both an internal 1.235V refer-
ence and an auxiliary reference input (the CTRL pin). This
allows users 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 chip is operating
to alter the output voltage of LT3482 for purposes such as
APD’s bias voltage adjustment. To use the internal 1.235V
reference, the CTRL pin should be held higher than 1.5V,
which can be done by tying it to V
pin is between 0V and 1.2V, the LT3482 will regulate the
output such that the FB pin voltage is equal to the CTRL
pin voltage.
To set the output voltage, select the values of R1 and R2
(see Figure 1) according to the following equation:
where V
V
selected to load the output to maintain a constant switching
frequency when the APD load is very low. Preventing entry
into pulse skipping mode is an important consideration
for post fi ltering the regulator output.
3482 F01
REF
4
14
R
V
1
= CTRL if CTRL is between 0V and 1.2V. R2 can be
IN
=
R1
R2
5
5
(V)
R
REF
2
⎛
⎝ ⎜
= 1.235V if the internal reference is used or
V
V
OUT
REF
2
L (µH)
–
10
22
1
⎞
⎠ ⎟
C (µF)
1
1
IN
. When the CTRL
LT3482
I
P
0.87
0.68
(A)
3482fa
7