EUP2794
ILED Current Selection Procedures
TABLE 1. RSET Values
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”.
LED Current
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
10mA
185.2
15mA
122.8
250.6
375.0
503.0
625.0
756.0
881.0
20mA
91.9
376.0
187.8
282.2
375.0
467.0
565.0
660.0
Brightness Control Using PWM
567.0
1. Set the BRGT pin to 0V
752.0
2. Determine the maximum desired ILED current.
Use the ILED equation to calculate RSET by setting
BRGT to 0V or use Table 1 to select a value for
RSET when BRGT equals 0V.
939.0
1.129K
1.319K
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 (TON = 100µs) of the device.
TABLE 2. LED Current
R
SET Values
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
1.32K
1.52
2.92
4.36
5.73
7.20
8.64
10.02
881
2.20
660
2.88
4.31
5.71
6.47
8.57
10.72
12.81
15.00
8.59
D*(1/f ) > T
ON
11.39
14.22
17.07
20.00
f
= D
÷ T
ON
MAX
MIN
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 ILED being half of
the programmed LED current. For example, if RSET is
set to program 15mA, a 50% duty cycle will result in
an average ILED of 7.5mA.
Charge Pump Output (POUT
)
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 POUT pin) is
nominally 1.5× VIN. 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 POUT are
recommended for functions where brightness
matching is not critical, such as keypad backlighting.
Brightness Control Using BRGT
1. Choose the maximum ILED 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 RSET
required or use the ILED equation above to
calculate RSET
.
3. Use Table 2 as a reference for the dimming
profile of the LEDs, when BRGT ranges from 0V
to 3V
DS2794 Ver0.5 Mar. 2006
9
EUTECH [ EUTECH MICROELECTRONICS INC ]
