Texas Instruments LM3431 Evaluation Board LM3431EVAL/NOPB LM3431EVAL/NOPB Manuale Utente
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LM3431EVAL/NOPB
User's Guide
SNVA309A – January 2008 – Revised May 2013
AN-1764 LM3431 Evaluation Board
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Introduction
The LM3431 is a combination switching boost voltage regulator and multi-channel LED current controller.
The LM3431 Evaluation Board is designed to drive four parallel strings of eight series LEDs at 140mA per
string from an input voltage range of 8V to 18V. The boost switcher regulates LED cathode voltage for
optimum efficiency, while allowing the output voltage to vary according to the LED forward voltage. The
LM3431 incorporates complete fault protection features including LED short and open protection, over-
current protection, over-voltage and under-voltage protection, and thermal shutdown. The error detection
flag can be monitored at the DLY post, which has a programmable delay time.
The LM3431 Evaluation Board is designed to drive four parallel strings of eight series LEDs at 140mA per
string from an input voltage range of 8V to 18V. The boost switcher regulates LED cathode voltage for
optimum efficiency, while allowing the output voltage to vary according to the LED forward voltage. The
LM3431 incorporates complete fault protection features including LED short and open protection, over-
current protection, over-voltage and under-voltage protection, and thermal shutdown. The error detection
flag can be monitored at the DLY post, which has a programmable delay time.
The board includes a DIM input terminal for a selectable digital or analog dimming control signal. PWM
dimming frequencies up to 1kHz at 100:1 contrast ratio and contrast ratios up to 500:1 at 200Hz are
possible.
dimming frequencies up to 1kHz at 100:1 contrast ratio and contrast ratios up to 500:1 at 200Hz are
possible.
The evaluation board is designed to operate at ambient temperatures up to 70°C.
The complete evaluation board schematic is shown in
. Typical waveforms and performance
curves are shown in
through
.
Although a wide variety of LED combinations and currents can be driven with the LM3431, the schematic
must be modified to achieve optimum performance with any given array. However, for initial evaluation
and demonstration, some modifications from the default settings are possible. A few of the possible
variations are described below and listed in
must be modified to achieve optimum performance with any given array. However, for initial evaluation
and demonstration, some modifications from the default settings are possible. A few of the possible
variations are described below and listed in
and
.
LED current can be set as low as 100mA per string without significantly degrading performance; the
number of strings can be reduced from 4 to 3 or 2; and the number of series LEDs can be set between 6
and 9.
number of strings can be reduced from 4 to 3 or 2; and the number of series LEDs can be set between 6
and 9.
To design for LED arrays other than the default, and for complete device and design information, refer to
the LM3431/31A/31Q/31AQ 3-Channel Constant Current LED Driver With Integrated Boost Controller
(
the LM3431/31A/31Q/31AQ 3-Channel Constant Current LED Driver With Integrated Boost Controller
(
) data sheet.
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Before Powering Up
Before powering up the LM3431 evaluation board, a few options must be selected.
First, verify that the default LED current setting of 140mA is appropriate to drive the LEDs. To drive LEDs
at other currents, refer to
at other currents, refer to
. If the required LED current is not listed, refer to the LM3431 data sheet
(
), as more extensive modifications will be required.
By default, the LM3431 evaluation board has channels 1-4 enabled. To drive fewer channels, first remove
the sense resistor or NPN on the channel to be disabled (R10-R13 or Q2-Q5). Then refer to
the sense resistor or NPN on the channel to be disabled (R10-R13 or Q2-Q5). Then refer to
and
make any other required changes. This table also gives options for driving fewer LEDs per string.
Finally, select the type of dimming control signal to be used. By default, R-MODE is installed, which
enables digital dimming control mode. To use this mode, connect a digital PWM signal to the DIM post.
The dimming signal must have a minimum high logic level of 2.3V.
enables digital dimming control mode. To use this mode, connect a digital PWM signal to the DIM post.
The dimming signal must have a minimum high logic level of 2.3V.
To enable analog dimming, remove the MODE resistor and connect an analog voltage level to the DIM
post. In analog mode, the dimming duty cycle is varied by the voltage at the DIM post within a range of
0.37V to 2.5V. The default analog dimming frequency is 1kHz. To modify this frequency, change C5 as
described in the datasheet.
post. In analog mode, the dimming duty cycle is varied by the voltage at the DIM post within a range of
0.37V to 2.5V. The default analog dimming frequency is 1kHz. To modify this frequency, change C5 as
described in the datasheet.
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SNVA309A – January 2008 – Revised May 2013
AN-1764 LM3431 Evaluation Board
Copyright © 2008–2013, Texas Instruments Incorporated