lt3571 Linear Technology Corporation, lt3571 Datasheet - Page 9

• Manufacturer Part Number: lt3571
• Description: 75v Dc/dc Converter For Apd Bias
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Switching Frequency
There are two methods to set the switching frequency of the
LT3571. Both methods require a resistor connected at the
R
pin with a capacitor. A resistor must always be connected
for proper operation. One way to set the frequency is simply
connecting an external resistor between the R
GND. See Table 1 or the Oscillator Frequency vs R
in the Typical Performance Characteristics for resistor
values and corresponding switching frequencies. The other
way is to make the LT3571 synchronize with an external
clock via the SYNC pin. For proper operation, a resistor
should be connected at the R
a switching frequency 20% lower than the external clock
when the external clock is absent.
Table 1. Switching Frequency vs R
Inrush Current
The LT3571 has a built-in Schottky diode for the boost
converter. When supply voltage is applied to the V
voltage difference between V
current fl owing from input through the inductor and the
Schottky diode (D1 in the Block Diagram), to charge the
output capacitor. The selection of inductor and capacitor
value should ensure the peak of the inrush current to below
1A. In addition, the LT3571 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.
T
pin. Do not leave the R
I
P
Switching Frequency (kHz)
=
V
IN
L
C
– .
1000
1500
2000
2500
–
250
500
0 9
1
• exp –
⎛
⎜
⎜
⎜
⎜
⎝
T
2
pin open. Also, do not load this
IN
π
L
C
T
T
and V
–
pin and able to generate
1
⎞
⎟
⎟
⎟
⎟
⎠
OUT
generates inrush
R
56.2
26.1
12.1
6.81
4.22
2.67
T
(k)
T
IN
pin and
T
pin, the
graph
Table 2 gives inrush peak currents for some component
selections.
Table 2. Inrush Peak Current
Setting Output Voltage
The LT3571 is equipped with both an internal 1V reference
and an auxiliary reference input (the CTRL pin). This
feature 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 LT3571
for purposes such as APD’s bias voltage adjustment. To
use the internal 1V reference, the CTRL pin should be held
higher than 1.2V, which can be done by tying it to V
When the CTRL pin is between 0V and 1V, the LT3571 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 2) according to the
following equation:
where V1 = 1V if the internal reference is used, or V1 =
CTRL if CTRL is between 0V and 1V. R2 can be 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.
R
V
1
IN
=
5
5
(V)
Figure 2. Output Voltage Feedback Connection
R
2
⎛
⎝ ⎜
V
MONIN
V
14
1
L (μH)
CTRL
10
22
–
LT3571
1
⎞
⎠ ⎟
MONIN
FB
3571 F02
15
3
C (μF)
R1
R2
1
1
LT3571
I
0.81
0.63
P
(A)
REF
9
3571f
.