Texas Instruments LM2796 User Manual

1 Board Operation
1.1 Basic Connections
To operate the LM2796 evaluation board, connect a supply voltage (2.7V-5.5V) between board connectors
VIN and GND.
Default Jumper Connections:
• EN: Connects the "+" post to the middle post of the EN header strip. This connects VIN to the EN pin
of the LM2796, enabling the part.
• ENA: Connects the "+" post to the middle post of the ENA header strip. This connects VIN to the ENA
pin of the LM2796, enabling D1A-D4A outputs
• ENB: Connects the "+" post to the middle post of the ENB header strip. This connects VIN to the ENB
pin of the LM2796, enabling D1B-D3B outputs.
• LEDS_ON: Jumper connects the two posts of the LEDS_ON header strip. This connects the cathodes
of all 7 LEDs to GND, establishing the LED current path.
When these connections are all made correctly, all LEDs will be ON.
1.2 R : Setting LED Currents
SET
The resistance of the R
I = 100 X (1.25V / Rset)
Dxx
The default R on the evaluation board is 8.3kΩ and gives a DC output current of 15mA (typ.).
SET
Component Rset' is an optional leaded resistor replacement for the surface mount Rset, provided for ease
of use.
1.3 EN, ENA, and ENB Headers: LED Activation and PWM Brightness Control
The header strips EN, ENA, and ENB can be used to enable/disable the LM2796 and/or the LED (output)
currents. The connections to the ENx pins provided by these posts can also be used to connect pulse-
width modulated (PWM) signals to the LM2796 in order to adjust the average brightness of the LEDs.
On each of these header strips, the post labeled "+" is connected to VIN. The post labeled "-" is connected
to GND. The middle post connects to EN, ENA, and ENB, respectively.
Jumpers can be used to connect each ENx pin to either VIN or GND. Connecting EN to VIN enables the
charge pump and other internal circuitry of the LM2796. Connecting EN to GND places the part in
Shutdown mode.
When the part in enabled (EN = VIN), connecting ENA to VIN enables the D1A-D4A LEDs. Connecting
ENA to GND disables these LEDs. Similarly, connecting ENB to VIN enables the D1B-D3B LEDs, and
connecting ENB to GND disables them.
A pulse signal (PWM) can be connected to the ENA and/or ENB pins to adjust the brightness of the
respective LED banks. The duty cycle of the pulse signal determines the net brightness, as perceived by
the human eye. For example, with a duty cycle of 50%, the LEDs will only be ON for 50% of the time, and
the perceived brightness will be approximately half of what the brightness is when the output current flows
continuously through the LEDs. Recommended frequency range for PWM signals: 100Hz to 1kHz.
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SNVA086A – May 2004 – Revised April 2013
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AN-1321 LM2796 Evaluation Board
resistor sets the DC output currents of the LM2796 according to equation:
SET
Copyright © 2004–2013, Texas Instruments Incorporated
User's Guide
SNVA086A – May 2004 – Revised April 2013
AN-1321 LM2796 Evaluation Board
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