lt3571 Linear Technology Corporation, lt3571 Datasheet - Page 10

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

APPLICATIONS INFORMATION
LT3571
Inductor Selection
The inductors used with the LT3571 should have a
saturation current rating of 0.4A, or greater. If the device
is used in an application where the input supply will be
hot-plugged, the saturation current rating should be equal
to, or greater than, the peak inrush current. For best loop
stability, the inductor value selected should provide a
ripple current of 80mA or more. For a given V
the inductor value to use in continuous conduction mode
(CCM) is estimated by the formula:
Where:
and f is the switching frequency.
To achieve low output voltage ripple, a small value inductor
should be selected to force the LT3571 to operate in
discontinuous conduction mode (DCM). The inequality
is true when the LT3571 is operating in discontinuous
conduction mode.
where I
reduces the maximum load current and the conversion
effi ciency.
Capacitor Selection
Low ESR capacitors should be used at the output to
minimize the output voltage ripple. Use only X5R and X7R
types, because they retain their capacitance over wider
voltage and temperature ranges than other types. High
output voltages typically require less capacitance for loop
stability. Typically, use a 1μF capacitor for output voltage
less than 25V, and a 0.22μF capacitor for output voltage
beyond 25V. Place the output capacitor as close as possible
to the V
10
L
D
L
=
=
<
• ƒ 80
V
ƒ
LIMIT
OUT
D V
D V
OUT
•
V
•
•
I
OUT
LIMIT
lead and to the GND of the IC.
+
is the switch current limit. Operating in DCM
mA
IN
IN
1
+
–
1
V
IN
IN
and V
OUT
,
Either ceramic or solid tantalum capacitors may be used
for the input decoupling capacitor, which should be placed
as close as possible to the LT3571. A 1μF capacitor is
suffi cient for most applications.
Phase Lead Capacitor
A small value capacitor (i.e., 10pF to 22pF) can be added
in parallel with the resistor between the output and the FB
pin to reduce output perturbation due to a load step and
to improve transient response. This phase lead capacitor
introduces a pole-zero pair to the feedback that boosts the
phase margin near the crossover frequency. The APD is
very sensitive to a noisy bias supply. To lowpass fi lter noise
from the internal reference and error amplifi er, a 0.1μF
phase lead capacitor can be used. The corner frequency
of the noise fi lter is R1 • C
APD Current Monitor
The power supply switching noise associated with a
switching power supply can interfere with the photodiode
DC measurement. To suppress this noise, a 0.1μF capacitor
is recommended at the APD pin. An additional series
resistor is necessary to ensure enough high frequency
compensation at the APD pin over the full operating range
of the LT3571, as shown in Figure 1. An additional output
lowpass fi lter, a 10k resistor and a 10nF capacitor in
parallel at MON pin can further reduce the power supply
noise, and other wide band noise, which might limit the
measurement accuracy of low current levels.
For applications requiring fast current monitor response
time, an RC lowpass fi lter at the MONIN pin is used to
replace the 0.1μF capacitor at the APD pin, as illustrated
in Figure 3.
Figure 3. RC Filter at MONIN Pin
LT3571
MONIN
V
APD
OUT
PL
.
3571 F03
R
SENSE
C
FILT
C1
3571f