Datasheet

LTC3873
12
3873fa
APPLICATIONS INFORMATION
In a fl yback converter, the input fl ows in pulses placing
severe demands on the input capacitors. Select an input
capacitor with a ripple current rating greater than:
I
P
V
D
D
RMS
IN
IN MIN
MAX
MAX
=
()
–1
Duty Cycle Considerations
The LTC3873 imposes a maximum duty cycle limit of
80% typical. For a fl yback converter, the maximum duty
cycle prevents the transformer core from saturation. In
a boost converter application, however, it sets a limit on
the maximum step-up ratio or maximum output voltage
with the given input voltage of:
V
V
V
OUT MAX
IN MIN
D()
()
–.
–=
108
Current and voltage stress on the power switch and
synchronous rectifi ers, input and output capacitor RMS
currents and transformer utilization (size vs power) are
impacted by duty factor. Unfortunately duty factor can-
not be adjusted to simultaneously optimize all of these
requirements. In general, avoid extreme duty factors since
this severely impacts the current stress on most of the
components. A reasonable target for duty factor is 50% at
nominal input voltage. Using this rule of thumb, the ideal
transformer turns ratio is:
N
V
V
D
D
V
V
IDEAL
OUT
IN
OUT
IN
==•
–1
Output Diode Selection
To maximize effi ciency, a fast switching diode with low
forward drop and low reverse leakage is desired. The output
diode in a boost converter conducts current during the
switch off-time. The peak reverse voltage that the diode
must withstand is equal to the regulator output voltage.
The average forward current in normal operation is equal
to the output current, and the peak current is equal to the
peak inductor current.
I
TH
NGATE
LTC3873
GND
RUN/SS
V
FB
= 1.2V
100µF
6.3V
s3
0.1µF
Q1
FAN2512
V
OUT
*
3.3V
3A
V
IN
36V TO 72V
4.7µF
100V
221k
4.7µF
10V
68m
*FOR 5V OUTPUT CHANGE R
FB
TO 42.2k
15k
2.2nF
0.1µF
T1
D2
UPS840
D1
BAS516
3873 F07
•
•
V
CC
SW
IPRG
12.06k
V
OUT
R
FB
*
21.5k
51
•
Figure 7. 3.3V Output Nonisolated Telecom DC/DC Converter