LM3241
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SNOSB38B –JANUARY 2009–REVISED APRIL 2013
APPLICATION INFORMATION
Setting the Output Voltage
The LM3241 features a pin-controlled adjustable output voltage to eliminate the need for external feedback
resistors. It can be programmed for an output voltage from 0.6V to 3.4V by setting the voltage on the VCON pin,
as in the following formula:
VOUT = 2.5 x VCON
(1)
When VCON is between 0.24V and 1.36V, the output voltage will follow proportionally by 2.5 times of VCON.
If VCON is less than 0.24V (VOUT = 0.6V), the output voltage may be regulated. Refer to datasheet curve (Low
VCON Voltage vs. Output Voltage) for details. This curve exhibits the characteristics of a typical part, and the
performance cannot be guaranteed as there could be a part-to-part variation for output voltages less than 0.6V.
For VOUT lower than 0.6V, the converter might suffer from larger output ripple voltage and higher current limit
operation.
Inductor Selection
There are two main considerations when choosing an inductor: the inductor should not saturate, and the inductor
current ripple should be small enough to achieve the desired output voltage ripple. Different manufacturers follow
different saturation current rating specifications, so attention must be given to details. Saturation current ratings
are typically specified at 25°C so ratings over the ambient temperature of application should be requested from
manufacturer.
Minimum value of inductance to guarantee good performance is 0.3 µH at bias current (ILIM (typ.)) over the
ambient temperature range. Shielded inductors radiate less noise and should be preferred. There are two
methods to choose the inductor saturation current rating.
Method 1:
The saturation current should be greater than the sum of the maximum load current and the worst case average
to peak inductor current. This can be written as:
ISAT > IOUT_MAX + IRIPPLE
where
«
≈
«
≈
VIN - VOUT
VOUT
VIN
1
f
≈
≈
«
≈
«
IRIPPLE
=
x
x
2 x L
«
•
•
•
•
•
•
IRIPPLE: average-to-peak inductor current
IOUT_MAX: maximum load current (750 mA)
VIN: maximum input voltage in application
L: minimum inductor value including worst-case tolerances (30% drop can be considered for Method 1)
F: minimum switching frequency (5.7 MHz)
VOUT: output voltage
Method 2:
A more conservative and recommended approach is to choose an inductor that can handle the maximum current
limit of 1600 mA.
The inductor’s resistance should be less than around 0.1Ω for good efficiency. Table 1 lists suggested inductors
and suppliers.
Table 1. Suggested Inductors
Model
Size (W x L x H) (mm)
2.0 x 1.2 x 1.0
Vendor
FDK
MIPSZ2012D0R5
LQM21PNR54MG0
LQM2MPNR47NG0
2.0 x 1.25 x 0.9
2.0 x 1.6 x 0.9
Murata
Murata
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