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LM3500 Datasheet(PDF) 16 Page - Texas Instruments

Part No. LM3500
Description  Synchronous Step-up DC/DC Converter for White LED Applications
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM3500 Datasheet(HTML) 16 Page - Texas Instruments

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ICL
KD'
IOUT
VIND
2LFSW
-
|
IOUT
KD'
IL(AVE)
|
LM3500
SNVS231F – MAY 2004 – REVISED APRIL 2007
www.ti.com
(6)
The maximum output current capability of the LM3500 can be estimated with the following equation:
(7)
where ICL is the current limit. Some recommended inductors include but are not limited to:
Coilcraft DT1608C series
Coilcraft DO1608C series
TDK VLP4612 series
TDK VLP5610 series
TDK VLF4012A series
CAPACITOR SELECTION
Choose low ESR ceramic capacitors for the output to minimize output voltage ripple. Multilayer X7R or X5R type
ceramic capacitors are the best choice. For most applications, a 1µF ceramic output capacitor is sufficient.
Local bypassing for the input is needed on the LM3500. Multilayer X7R or X5R ceramic capacitors with low ESR
are a good choice for this as well. A 1µF ceramic capacitor is sufficient for most applications. However, for some
applications at least a 4.7µF ceramic capacitor may be required for proper startup of the LM3500. Using
capacitors with low ESR decreases input voltage ripple. For additional bypassing, a 100nF ceramic capacitor can
be used to shunt high frequency ripple on the input. Some recommended capacitors include but are not limited
to:
TDK C2012X7R1C105K
Taiyo-Yuden EMK212BJ105 G
LAYOUT CONSIDERATIONS
The input bypass capacitor CIN, as shown in Figure 2, must be placed close to the device and connect between
the VIN and GND pins. This will reduce copper trace resistance which effects the input voltage ripple of the IC.
For additional input voltage filtering, a 100nF bypass capacitor can be placed in parallel with CIN to shunt any
high frequency noise to ground. The output capacitor, COUT, should also be placed close to the LM3500 and
connected directly between the VOUT and GND pins. Any copper trace connections for the COUT capacitor can
increase the series resistance, which directly effects output voltage ripple and efficiency. The current setting
resistor, RLED, should be kept close to the FB pin to minimize copper trace connections that can inject noise into
the system. The ground connection for the current setting resistor should connect directly to the GND pin. The
AGND pin should connect directly to the GND pin. Not connecting the AGND pin directly, as close to the chip as
possible, may affect the performance of the LM3500 and limit its current driving capability. Trace connections
made to the inductor should be minimized to reduce power dissipation, EMI radiation and increase overall
efficiency. It is good practice to keep the VSW routing away from sensitive pins such as the FB pin. Failure to do
so may inject noise into the FB pin and affect the regulation of the device. See Figure 3 and Figure 4 for an
example of a good layout as used for the LM3500 evaluation board.
16
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Copyright © 2004–2007, Texas Instruments Incorporated
Product Folder Links: LM3500


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