MAX1522/MAX1523/MAX1524
Simple SOT23 Boost Controllers
_______________________________________________________________________________________ 7
under light loads. The selection of 30% ripple current
causes this to happen at loads less than approximately
1/6th of maximum load.
There are two common reasons not to run in CCM:
1) High output voltage. In this case, the output-to-
input voltage ratio exceeds the level obtainable
by the MAX1522/MAX1523/MAX1524s’ maximum duty
factor. Calculate the application’s maximum duty cycle
using the equation in the Calculate the Maximum Duty
Cycle section. If this number exceeds 80%, you will
have to design for DCM.
2) Small output current. If the maximum output current
is very small, the inductor required for CCM may be
disproportionally large and expensive. Since I
2
R losses
are not a concern, it may make sense to use a smaller
inductor and run in DCM. This typically occurs when
the load current times the output-to-input voltage ratio
drops below a few hundred milliamps, although this
also depends on the external components.
Calculate the Maximum Duty Cycle
The maximum duty cycle of the application is given by:
where V
D
is the forward voltage drop of the Schottky
diode (about 0.5V).
Design Procedure for CCM
On-Time Selection
For CCM to occur, the MAX1522/MAX1523/MAX1524
must be able to exceed the application’s maximum
duty cycle. For applications up to 45% duty cycle, con-
nect SET to GND for 0.5µs on-time to get fast switching
and a smaller inductor. For applications up to 80% duty
cycle, it is necessary to connect SET to V
CC
for 3.0µs
on-time. For applications greater than 80% duty cycle,
CCM operation is not guaranteed; see the Design
Procedure for DCM section.
Switching Frequency
A benefit of CCM is that the switching frequency
remains high as the load is reduced, whereas in DCM
the switching frequency varies directly with load. This is
important in applications where switching noise needs
to stay above the audio band. The medium- and heavy-
load switching frequency in CCM circuits is given by:
Note that f
SWITCHING
is not a function of load and
varies primarily with input voltage. However, when the
load is reduced, a CCM circuit drops into DCM, and
the frequency becomes load dependent:
Calculate the Peak Inductor Current
For CCM, the peak inductor current is given by:
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