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.
User's Guide
SNVA086A–May 2004–Revised April 2013
AN-1321 LM2796 Evaluation Board
1.2 R
: Setting LED Currents
SET
The resistance of the R
I
= 100 X (1.25V / Rset) (1)
Dxx
The default R
on the evaluation board is 8.3kΩ and gives a DC output current of 15mA (typ.).
SET
resistor sets the DC output currents of the LM2796 according to equation:
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 pulsewidth 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 AN-1321 LM2796 Evaluation Board
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C
IN
1 PF
3/2× Charge Pump
C1
1 PF
C2
1 PF
C
POUT
1 PF
EN ENA ENBC1+ C1- C2+ C2-
P
OUT
D1A D2A D3A D4A D1B D2B D3B
I
SET
V
IN
V
IN
2.7V to 5.6V
R
SET
GND
LM2796
A
I = 6 I
Dxx
LEDS_ON+
LEDS_ON-
Board Operation
It is recommended that ENA and ENB pins be used for PWM brightness adjustment (dimming). Toggling
the voltage on these pins turns the internal LM2796 current sources on and off, and the charge pump
stays ON continuously. Placing a PWM signal on the EN pin repeatedly turns the internal charge pump
ON and OFF. Each time the charge pump is activated, significant inrush current can be expected as the
large external capacitors are quickly recharged. This could inject noise on the input line.
1.4 Using the LEDS on Headers to Measure Output Currents or to Drive Different LEDS
By removing the LEDS_ON jumper, LM2796 output currents can easily be measured. Removing the
jumper disconnects the cathodes of all LEDs from GND, breaking the LED current paths. By placing a
current meter between the two header pins, as shown on the following page in Figure 1, the sum total of
all LED currents can be measured.
With the LEDS_ON jumper removed, the current of an individual output can be measured by placing a
current meter between a Dxx header and GND, as shown in Figure 2.
With such a connection, the voltage on pin Dxx will be almost 0V because the series resistance of the
current meter is likely to be quite small. Since the regulated output currents of the LM2796 are almost
completely independent of Dxx pin voltage (provided V
measurement will still be quite accurate.
With the LEDS_ON jumper removed, the LM2796 can drive external LEDs simply by connecting each
LED between a Dxx output and GND. The LEDs on the evaluation board need not be removed for this
type of test/evaluation.
is not too high to inhibit regulation), this
Dxx
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2
Figure 1. Measuring Current of all LEDs by Removing LEDs_ON Jumper and Placing a Current Meter
AN-1321 LM2796 Evaluation Board SNVA086A–May 2004–Revised April 2013
Between the Two LEDS_ON Header Posts
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C
IN
1 PF
3/2× Charge Pump
C1
1 PF
C2
1 PF
C
POUT
1 PF
EN ENA ENBC1+ C1- C2+ C2-
P
OUT
D1A D2A D3A D4A D1B D2B D3B
I
SET
V
IN
V
IN
2.7V to 5.6V
R
SET
GND
LM2796
A
V
D1A
| 0V
I = I
D1A
LEDS_ON+
I = 0
ALL LEDs OFF
A
Series LED or Resistor
Optional to Set V
Dxx
| 3.6V
V
D3B
| V
LEDm
I = I
D3B
LED
M
www.ti.com
Board Operation
Figure 2. Measuring Current of All Individual DxxOutputs by Removing the LEDS_ON Jumper and Placing
Current Meters Between the DxxPins and GND
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Copyright © 2004–2013, Texas Instruments Incorporated
3
C
IN
1 PF
3/2× Charge Pump
C1
1 PF
C2
1 PF
C
POUT
1 PF
EN ENA ENBC1+ C1- C2+ C2-
P
OUT
D1A D2A D3A D4A D1B D2B D3B
I
SET
V
IN
V
IN
2.7V to 5.5V
R
SET
or R
SET
¶
GND
LM2796
LEDS_ON
EN ENA ENB
D1A-D4A
D1B-D3B
+
-
+
-
+
-
Schematic
2 Schematic
www.ti.com
3 Bill of Materials
Component Value Package Dimensions Temperature Manufacturer Part #
Symbol [U.S. (Metric)] (mm) Characteristic
LM2796 -- YZR0018 2.1 x 2.4 x 0.6 -- Texas LM2796
Cin, Cout 2.2µF, 6.3V 0603 (1608) 1.6 x 0.8 x 0.8 X5R TDK C1608X5R0J225K
C1, C2 1µF, 10V 0603 (1608) 1.6 x 0.8 x 0.8 X5R TDK C1608X5R1A105K
Dxx White LEDs -- 1.5 x 2.3 x 1.4 -- OSRAM LWM67C-T1U1-3C5D
Rset 8.3kΩ 0603 (1608) 1.6 x 0.8 x 0.8 -- Vishay-Dale CRCW06048251F
Rset' (optional) -- -- -- -- --
DSBGA Instruments
4
AN-1321 LM2796 Evaluation Board SNVA086A–May 2004–Revised April 2013
Copyright © 2004–2013, Texas Instruments Incorporated
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www.ti.com
4 LM2796 Evaluation Board Layout
LM2796 Evaluation Board Layout
Figure 3. Top Layer
Figure 4. Bottom Layer (top view, unmirrored)
SNVA086A–May 2004–Revised April 2013 AN-1321 LM2796 Evaluation Board
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