Setting the Output Current

SET controls the LED bias current. Current flowing into

LED1, LED2, and LED3 is 230 times greater than the

current flowing into SET. Set the output current as fol-

lows:

Applications Information

1) Very Low-Cost, High-Efficiency Solution (Figure 2).

A battery (single Li+ or three NiMH cells) powers

the LEDs directly. This is the least expensive and

most efficient architecture. Due to the high forward

voltage of white LEDs (3.3V), the LED brightness

may dim slightly at the end of battery life. The

MAX1916’s current-regulating architecture and low

dropout greatly minimize this effect compared to

using simple ballast resistors. The enable function

of the MAX1916 turns on and off the LEDs. An exist-

2) Brightness Adjustment Using a DAC (Figure 3). A

may be dynamically adjusted to eliminate factory

calibration. A battery (single Li+ or three NiMH

cells) or a regulated power source drives the LEDs.

3) Existing 5V Supply (Figure 4). Use an existing sys-

tem regulator, such as the MAX684, to provide the

required LED voltage and provide power to other

circuits. Due to the high forward voltage of white

LEDs (3.3V), use a 3.6V to 5.5V regulated supply to

provide enough voltage headroom such that the

LEDs will maintain constant brightness for any bat-

tery voltage. Use the existing regulated supply as

Chip Information

TRANSISTOR COUNT: 220

PROCESS: BiCMOS

I

VV

R

LED

CTRL

SET

SET

_

()

=

−

230

Low-Dropout, Constant-Current

Triple White LED Bias Supply

6

_______________________________________________________________________________________

MAX1916

ON

EN

OFF

EXISTING

LDO

GND

SET

LED1 LED2 LED3

V+

TO OTHER

CIRCUITS

Figure 2. Very Low-Cost, High-Efficiency Solution

MAX1916

ON

EN

OFF

DAC

GND

SET

LED1 LED2

LED3

V+

Figure 3. Brightness Adjust Using DAC

MAX1916

ON

EN

OFF

EXISTING

REGULATOR

GND

SET

LED1 LED2

LED3

V+

5.0V

TO OTHER

CIRCUITS

Figure 4. Existing 5V Supply Circuit