LTC3873-5 Datasheet, PDF (11/16 Pages) Linear Technology – No RSENSETM Constant Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller

LTC3873-5
APPLICATIONS INFORMATION
Input Capacitors
The input capacitor of a boost converter is less critical due
to the fact that the input current waveform is triangular, and
does not contain large square wave currents as found in
the output capacitor. The input voltage source impedance
determines the size of the capacitor that is typically 10μF to
100μF. A low ESR is recommended although not as critical
as the output capacitor can be on the order of 0.3Ω.
The RMS input ripple current for a boost converter is:
I_RMS(CIN)
=
0.3
•
V_IN(MIN)
L•f
• D_MAX
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to the
input of the converter and solid tantalum capacitors can
fail catastrophically under these conditions.
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 LTC3873-5 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.
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