MAX1522/MAX1523/MAX1524
Simple SOT23 Boost Controllers
_______________________________________________________________________________________ 9
tance value two or three times larger than C
OUT(MIN)
is
typically needed. Output ripple due to ESR is:
at light and medium loads, and three times as great at
peak load.
Continue the CCM design procedure by going to the
Optional Feed-Forward Capacitor Selection section.
Design Procedure for DCM
On-Time Selection
The MAX1522/MAX1523/MAX1524 may operate in
DCM at any duty cycle as required by the application’s
input and output voltages. However, best performance
is achieved when the maximum duty cycle of the appli-
cation is similar to the MAX1522/MAX1523/MAX1524s’
maximum duty factor as set using the SET input.
Connect SET to GND for applications with maximum
duty cycles less than 67%. Connect SET to V
CC
for
applications with maximum duty cycles between 67%
and 99%.
Inductor Selection
For DCM, the ideal inductor value is given by:
If L
IDEAL
is not a standard value, choose the next lower
nominal value. The above formula already includes a
factor for ±30% inductor tolerance. Values higher than
ideal may not supply the maximum load when the input
voltage is low, while values much lower than ideal will
have poorer efficiency.
Calculate the Peak Inductor Current
For DCM, the peak inductor current is given by:
The saturation rating of the selected inductor should
meet or exceed the calculated value for I
PEAK
,
although most coil types can be operated up to 20%
over their saturation rating without difficulty. In addition
to the saturation criteria, the inductor should have as
low a series resistance as possible. The power loss in
the inductor resistance is approximately given by:
Due to the MAX1522/MAX1523/MAX1524s’ high switch-
ing frequencies, inductors with a ferrite core or equiva-
lent are recommended. Powdered iron cores are not
recommended due to their high losses at frequencies
over 50kHz.
Switching Frequency
In DCM, the switching frequency is proportional to the
load current and is approximately given by:
Note that f
SWITCHING
is a function of load and input
voltage.
Output Capacitor Selection
In DCM, the MAX1522/MAX1523/MAX1524 may use
either a ceramic output capacitor (with very low ESR) or
other capacitors, such as tantalum or organic, with
higher ESR. For less than 2% output ripple, the mini-
mum value for ceramic output capacitors should be
greater than:
To control inductor current during soft-start, the maxi-
mum value for any type of output capacitors should be
less than:
where t
SS
= 3.2ms.
The capacitor should be chosen to provide an output
ripple between 25mV minimum and 2% of V
OUT
maxi-
mum. The output ripple due to capacitance ripple and
ESR ripple can be approximated by:
For output ripple close to 2% of V
OUT
, the optional
feed-forward capacitor may not be required. For lower
output ripple, a feed-forward capacitor is necessary for
stability and to control inductor current during soft-start.
(15 Seiten)
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