LT3482 LINER [Linear Technology], LT3482 Datasheet - Page 8

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

LT3482
APPLICATIO S I FOR ATIO
Inductor Selection
The inductors used with the LT3482 should have a satura-
tion current rating of 0.3A 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
60mA or more. For a given V
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 in-
ductor should be selected to force the LT3482 work in
discontinuous conduction mode (DCM). The inequality
is true when the LT3482 is operating in discontinuous
condition 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. For applications with out voltage less than
45V, intermediate output pin V
the output pin. Typically use a 2µF capacitor for output
voltage less than 25V and 1µF capacitor for output voltage
between 25V and 45V. When output voltage goes beyond
45V, a charge pump must be formed with cascaded 0.47µF
8
L
L
D
=
<
=
• ƒ 60
ƒ
LIMIT
V
D V
D V
OUT
•
•
•
I
V
LIMIT
OUT
mA
1
is the switch current limit. Operating in DCM
IN
IN
+
1
1
+
–
1
V
U
IN
U
IN
OUT1
and V
W
can directly serve as
OUT1
, the inductor
U
capacitors C1 and C2 at the output nodes. A typical 0.1µF
capacitor is used as the fl ying capacitor C
charge pump. Always use a capacitor with suffi cient volt-
age rating.
Either ceramic or solid tantalum capacitors may be used
for the input decoupling capacitor, which should be placed
as close as possible to the LT3482. 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
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 switch-
ing power supply can interfere with the photodiode DC
measurement. To suppress this noise, a 0.1µF capacitor
is recommended at APD pin. 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 level signals. For applications requiring
fast current monitor response time, a RC lowpass fi lter at
MONIN pin is used to replace the 0.1µF capacitor at APD
pin, as illustrated in Figure 2.
LT3482
MONIN
V
V
OUT2
OUT1
APD
Figure 2
3482 F02
R
C
MON
MON
C2
C1
FLY
PL
to form the
.
3482fa