T6311A Taiwan Memory Technology, T6311A Datasheet - Page 6

• Manufacturer Part Number: T6311A
• Description: Constant-Current White LED Bias Supply
• Manufacturer: Taiwan Memory Technology

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Functional Description
supply for white LED designs. The T6311A
uses a single resistor to set the bias current for
up to three LEDs. LED bias currents are
matched to 0.3% by the T6311A current
matching architecture (Fig. 1). Supply current
(I
0.05uA when disabled. The T6311A offers
several advantages over using ballast resistors,
such as improved LED to LED brightness
matching, lower bias variation with supply
voltage changes, significantly lower dropout
voltage,
significantly
T6311A achieves a 200mV dropout with a
9mA load on each output. For circuits
requiring only one or two LEDs, leave unused
LED outputs unconnected.
Enable Input
EN high (> 2.5V) to enable the device; drive
EN low (< 2.2V) to disable the device. When
driven high, EN draws 40µA to power the IC.
Driving EN low forces LED1, LED2, LED3,
LED4 and SET into a high-impedance state.
Setting the Output Current
flowing into LED1, LED2, LED3 and LED4
is 240 times greater than the current flowing
into SET. Set the output current as follows:
where V
voltage between 1.8V and 5.5V, and R
the resistor connected between V
SET (Fig. 1).
TM Technology Inc. reserves the right
to change products or specifications without notice.
EN
EN powers the input of the T6311A. Drive
SET controls the LED bias current. Current
I
The T6311A provides constant current bias
LED
) is a low 40uA in normal operation and
= 240 ( (V
SET
and
CH
TE
= 1.2V, V
improved
CTRL
in
- V
SET
some
CTRL
) / R
efficiency.
is an external
SET
applications,
)
CTRL
SET
is
The
and
Preliminary Product spec. T6311A
P.6
Applications Information
(Fig. 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 T6311A
current regulating architecture and low
dropout greatly minimize this effect compared
to using simple ballast resistors. The enable
function of the T6311A turns on and off the
LEDs. An existing low-dropout regulator is
used as V
(Fig. 3). A DAC is used as V
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.
Very Low-Cost, High-Efficiency Solution
Brightness Adjustment Using a DAC
CTRL
.
Publication Date: JUN. 2005
CTRL
Revision: C
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