Jul31-07 RevT SP7611A/7612A/7614A Low Dropout Linear LED driver 2007 Sipex Corporation
Page 9
APPLICATION INFORMATION
Setting the LED Current
The LED current is controlled by I
CONTROL
and
R
SET
according to the following formula:
I
LED
= Gain X (V
CONTROL
- V
CTRL
) / R
SET
Gain depends on mode of operation
V
CONTROL
is the voltage going into R
SET
V
CTRL
is the voltage at the pin of the device
The voltage V
CTRL
can be determined using the
I
SET
vs Control voltage graph located in the typical
performance characteristics section. As an
example, to set the LED current for an SP761x the
procedure is:
Take the LED current and divide it by the Gain to
get I
SET
= I
LED
/ Gain.
Look up the V
CTRL
pin voltage using I
SET
on the
graph “Ctrl Current vs. Control Voltage”.
Once V
CTRL
is known, use the following formula to
find R
SET
. The gain is not linear, the LED current
vs. Cathode voltage graph needs to be used to
determine gain. Typically, the gain is ~870 for
SP7614A and ~435 for SP7611A/12A when the
part is operating in the linear region.
R
SET
= (V
CONTROL -
V
CTRL
) / I
SET
R
SET
Example
For an SP7614A with V
CONTROL
of 3V and LED
current of 20mA.
I
SET
= 20mA/870 (870 is the typical gain in linear
operation) = 22uA.
Using the graph on page 5 in the typical
performance section, the V
CTRL
voltage on the
control pin will be about 1.1V, then R
SET
will be
determined by:
R
SET
= (3V - 1.1V) / 22uA
R
SET
= 86KΩ
PWM FUNCTION
The LED's brightness can also be
adjusted by driving either the ENABLE or the
CTRL pin with a PWM signal. The driving signal
frequency should be greater than 100Hz to avoid
flickering, increasing to more than 1MHz, if
necessary.
Temperature Considerations
LEDs are very sensitive to temperature. In most
cases the maximum allowed junction temperature
is 100ºC. The case of overtemperature due to
power dissipation is de-scribed by the following:
T
J
= T
A
+ Ө
JA
x I x V
F
where T
J
is the LED junction temperature,
T
A
is the ambient temperature,
Ө
JA
is the junction to ambient thermal resistance,
I is the LED current and
V
F
is the LED forward voltage.
When the temperature rises and the cathode
voltage increases, SP761X reduces the current
through LEDs. Refer to "LED Current vs. LED
Cathode Voltage" graph under the Typical
Performance Characteristics section.
Efficiency
The system efficiency, defined as the ratio
between the LED's power and the input supplied
power can be calculated as follows:
Efficiency = (V
IN
- V
CATHODE
) / V
IN
The lower the V
CATHODE
, the higher the system
efficiency. Efficiency can be further improved
using a higher V
IN
with more LEDs as shown in
example 3. The SP761X driver's low dropout
architecture can significantly improve the
efficiency compared to using simple ballast
resistors.
Application Information
The ultra-low voltage drop across the SP761X
series of LED drivers allows the devices to drive
white, blue, and other color LEDs in a wide range
of input voltages. The driver can be used in many
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