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Low-Dropout, Constant-Current
Triple White LED Bias Supply
V+
TO OTHER
Setting the Output Current
SET controls the LED bias current. Current flowing into
LED1, LED2, and LED3 is 230 times greater than the
EXISTING V CTRL
LDO
CIRCUITS
current flowing into SET. Set the output current as fol-
lows:
ON
SET
LED1 LED2 LED3
I LED _ = 230
(V CTRL ? V SET )
R SET
OFF
EN
MAX1916
GND
where V SET = 1.215V, V CTRL is an external voltage
between 1.8V and 5.5V, and R SET is the resistor con-
nected between V CTRL and SET ( Figure 1).
Applications Information
Figure 2. Very Low-Cost, High-Efficiency Solution
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
DAC
V CTRL
V+
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
SET
LED1 LED2
LED3
of the MAX1916 turns on and off the LEDs. An exist-
ON
OFF
EN
MAX1916
ing low-dropout regulator is used as V CTRL .
2) Brightness Adjustment Using a DAC (Figure 3). A
GND
Figure 3. Brightness Adjust Using DAC
V BATT
DAC is used as V CTRL such that the LED brightness
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
EXISTING
REGULATOR
5.0V
V+
TO OTHER
CIRCUITS
provide enough voltage headroom such that the
LEDs will maintain constant brightness for any bat-
tery voltage. Use the existing regulated supply as
V CTRL .
ON
OFF
SET
EN
LED1 LED2
MAX1916
LED3
TRANSISTOR COUNT: 220
Chip Information
Figure 4. Existing 5V Supply Circuit
GND
PROCESS: BiCMOS
6
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