EUP2794
I
LED
Current Selection Procedures
The following procedures illustrate how to set and
adjust output current levels. For constant brightness
or analog brightness control, go to “Brightness
control using BRGT”. Otherwise refer to “Brightness
control using PWM”.
Brightness Control Using PWM
1. Set the BRGT pin to 0V
2. Determine the maximum desired I
LED
current.
Use the I
LED
equation to calculate R
SET
by setting
BRGT to 0V or use Table 1 to select a value for
R
SET
when BRGT equals 0V.
3. Brightness control can be implemented by
pulsing a signal at the SD pin. LED brightness is
proportional to the duty cycle (D) of the PWM
signal. For linear brightness control over the full
duty cycle adjustment range, the PWM frequency
(f) should be limited to accommodate the turn-on
time (T
ON
= 100µs) of the device.
D *
(
1/f
)
>
TON
f MAX
=
D MIN
÷
TON
TABLE 1. R
SET
Values
LED Current
10mA
15mA
185.2
122.8
376.0
250.6
567.0
375.0
752.0
503.0
939.0
625.0
1.129K
756.0
1.319K
881.0
BRGT
0.0V
0.5V
1.0V
1.5V
2.0V
2.5V
3.0V
5mA
375.0
760.0
1.139K
1.526K
1.9K
2.32K
2.663K
20mA
91.9
187.8
282.2
375.0
467.0
565.0
660.0
TABLE 2. LED Current
R
SET
Values
1.32K
881
1.52
2.20
2.92
4.31
4.36
6.47
5.73
8.57
7.20
10.72
8.64
12.81
10.02
15.00
If the PWM frequency is much less than 100Hz,
flicker may be seen in the LEDs. For the EUP2794,
zero duty cycle will turn off the LEDs and a 50%
duty cycle will result in an average I
LED
being half of
the programmed LED current. For example, if R
SET
is
set to program 15mA, a 50% duty cycle will result in
an average I
LED
of 7.5mA.
Brightness Control Using BRGT
1. Choose the maximum I
LED
desired and determine
the max voltage to be applied to the BRGT pin.
For constant brightness, set BRGT to a fixed
voltage between 0V to 3V.
2. Use Table 1 to determine the value of R
SET
required or use the I
LED
equation above to
calculate R
SET
.
3. Use Table 2 as a reference for the dimming
profile of the LEDs, when BRGT ranges from 0V
to 3V
BRGT
0.0V
0.5V
1.0V
1.5V
2.0V
2.5V
3.0V
2.66K
0.82
1.51
2.21
2.91
3.60
4.29
5.01
660
2.88
5.71
8.59
11.39
14.22
17.07
20.00
Charge Pump Output (P
OUT
)
The EUP2794 charge pump is an unregulated
switched capacitor converter with a gain of 1.5. The
voltage at the output of the pump (the P
OUT
pin) is
nominally 1.5
×
V
IN
. This rail can be used to deliver
additional current to other circuitry. A ballast resistor
sets the current through each LED, and LED current
matching is dependent on the LED forward voltage
matching. Because of this, LEDs driven by P
OUT
are
recommended for functions where brightness
matching is not critical, such as keypad backlighting.
DS2794 Ver0.5 Mar. 2006
9
EUTECH [ EUTECH MICROELECTRONICS INC ]
