lt3571 Linear Technology Corporation, lt3571 Datasheet - Page 12

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

TYPICAL APPLICATIONS
LT3571
APPLICATIONS INFORMATION
Setting APD Current Limit
The LT3571 has a unique current loop to limit the APD
current. Choose the sense resistor R
and MONIN pins to set the APD current limit by using the
following formula:
where I
Layout Hints
The high speed operation of the LT3571 demands careful
attention to board layout. Advertised performance will not
be achieved with a careless layout. To prevent radiation
and high frequency resonance problems, proper layout of
the high frequency switching path is essential. Keep the
output switch (SW pin), diode and output capacitor as
12
V
5V
IN
R
SENSE
L: TDK VLF3010AT – 100MR49
C1: TDK X7R C1608X7R1C105KT
C2, C4: MURATA X7R GRM188R72A104KA35
C3: AVX X7R 06031C103K
C5: MURATA X7R GRM155R71H103K
APD
OFF ON
C1
1μF
R
12.1k
1MHz
=
is the APD current limit.
T
1.2 × I
5V to 45V APD Bias Power Supply
SHDN
CTRL
R
SYNC
GND
V
APD
REF
T
200mV
V
MON
IN
(mA) + 0.3mA
LT3571
C5
10nF
10μH
L1
R3
10k
SW
MONIN
APD
V
OUT
R4
49.9Ω
FB
SENSE
50V
45V
across V
C3
10nF
C4
0.1μF
R1
1M
R
20Ω
R2
20.5k
SENSE
OUT
3571 TA03
C2
0.1μF
close together as possible. Minimize the length and area
of all traces connected to the switch pin, and always use
a ground plane under the switching regulator to minimize
interplane coupling. The high speed switching current path
is shown in Figure 6. The signal path, including the switch,
output diode and output capacitor contains nanosecond rise
and fall times and should be kept as short as possible.
V
IN
500
450
400
350
300
250
200
150
100
50
0
L1
Figure 6. High Frequency Path
0
Input Power vs APD Current
SWITCH
NODE
0.5
APD CURRENT (mA)
1
CIRCULATING
FREQUENCY
HIGH
PATH
1.5
2
2.5
3571 TA04
3
LOAD
3571 F06
V
3571f
OUT