C
IN
1
P
F
3/2× Charge Pump
C1
1
P
F
C2
1
P
F
C
POUT
1
P
F
EN
ENA
ENB
C1+
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-
Board Operation
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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
Dxx
is not too high to inhibit regulation), this
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.
Figure 1. Measuring Current of all LEDs by Removing LEDs_ON Jumper and Placing a Current Meter
Between the Two LEDS_ON Header Posts
2
AN-1321 LM2796 Evaluation Board
SNVA086A – May 2004 – Revised April 2013
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