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LM3242 데이터시트(HTML) 15 Page - Texas Instruments

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부품명 LM3242
상세내용  6MHz, 750mA Miniature, Adjustable, Step-Down DC-DC Converter with Auto Bypass for RF Power Amplifiers
Download  28 Pages
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제조사  TI1 [Texas Instruments]
홈페이지  http://www.ti.com
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LM3242 데이터시트(HTML) 15 Page - Texas Instruments

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ISAT > IOUT_MAX + IRIPPLE
VOUT
VIN
©
§
VIN - VOUT
2 x L
©
§
x
IRIPPLE =
x
1
f
©
§
where
LM3242
www.ti.com
SNOSB48D – OCTOBER 2011 – REVISED MARCH 2013
APPLICATION INFORMATION
Setting the Output Voltage
The LM3242 features a pin-controlled adjustable output voltage to eliminate the need for external feedback
resistors. It can be programmed for an output voltage from 0.4V to 3.6V by setting the voltage on the VCON pin,
as in the following formula:
VOUT = 2.5 x VCON
(1)
When VCON is between 0.16V and 1.44V, the output voltage will follow proportionally by 2.5 times of VCON.
If VCON is less than 0.16V (VOUT = 0.4V), the output voltage may not be well 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 ensured as there could be a part-to-part variation for output voltages less than 0.4V.
For VOUT lower than 0.4V, the converter might suffer from larger output ripple voltage and higher current limit
operation.
FB
Typically the FB pin is connected to VOUT for regulating the output voltage maximum of 3.6V.
Inductor Selection
There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor
current ripple is 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:
where
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
(2)
Copyright © 2011–2013, Texas Instruments Incorporated
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