LTC3873_15 Datasheet, PDF (12/16 Pages) Linear Technology – Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller

LTC3873
APPLICATIONS INFORMATION
In a flyback converter, the input flows in pulses placing
severe demands on the input capacitors. Select an input
capacitor with a ripple current rating greater than:
I_RMS
=
P_IN
V_IN(MIN)
1– D_MAX
D_MAX
Duty Cycle Considerations
The LTC3873 imposes a maximum duty cycle limit of
80% typical. For a flyback 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_OUT(MAX)
=
V_IN(MIN)
1– 0.8
–
V_D
Current and voltage stress on the power switch and
synchronous rectifiers, 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_IDEAL
=
V_OUT
V_IN
•
1– D
D
=
V_OUT
V_IN
Output Diode Selection
To maximize efficiency, 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.
V_IN
36V TO 72V
T1
4.7μF
•
100V
D2
221k
UPS840
•
2.2nF
15k
I_TH
NGATE
LTC3873
GND
V_CC
0.1μF
RUN/SS IPRG
12.06k
V_FB = 1.2V SW
R_FB*
21.5k
V_OUT
Q1
FAN2512
51Ω
4.7μF
10V
*FOR 5V OUTPUT CHANGE R_FB TO 42.2k
• _D1
BAS516
68mΩ
100μF
6.3V
s3
V_OUT*
3.3V
3A
0.1μF
3873 F07
Figure 7. 3.3V Output Nonisolated Telecom DC/DC Converter
3873fa
12