ltc3873 Linear Technology Corporation, ltc3873 Datasheet - Page 11

• Manufacturer Part Number: ltc3873
• Description: No Rsense Constant Frequency Current Mode Boost/flyback/sepic Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
In a fl yback converter, the input current fl ows in pulses
placing severe demands on the input capacitors. Select an
input capacitor with a ripple current rating greater than:
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:
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
I
V
RMS
OUT MAX
(
=
V
IN MIN
)
P
(
IN
=
V
1 0 8
)
IN MIN
– .
(
– 1
D
)
D
MAX
–
MAX
V
D
2.2nF
12.06k
*FOR 5V OUTPUT CHANGE R
0.1µF
V
Figure 7. 3.3V Output Nonisolated Telecom DC/DC Converter
OUT
15k
R
21.5k
FB
*
V
GND
RUN/SS
I
TH
FB
= 1.2V
LTC3873
36V TO 72V
NGATE
IPRG
V
V
FB
SW
IN
CC
TO 42.2k
FAN2512
221k
4.7µF
100V
4.7µF
10V
51Ω
Q1
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:
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.
N
68mΩ
IDEAL
•
T1
•
•
=
UPS840
BAS516
D2
D1
V
V
OUT
IN
•
– 1
100µF
6.3V
×3
0.1µF
3783 F07
D
D
=
V
3.3V
3A
OUT
V
V
OUT
*
IN
LTC3873
11
3873f