SP7614EB Exar Corporation, SP7614EB Datasheet - Page 10

• Manufacturer Part Number: SP7614EB
• Description: Display Modules & Development Tools LDO LED Drvr for any Color LED w/A Shtdwn
• Manufacturer: Exar Corporation

Specifications of SP7614EB

• Product: Display Modules

Setting the LED Current
The LED current is controlled by I
R
I
Gain depends on mode of operation
V
V
The voltage V
I
performance
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
Look up the V
graph “Ctrl Current vs. Control Voltage”.
Once V
find R
vs. Cathode voltage graph needs to be used to
determine gain.
SP7614 and ~435 for SP7612 when the part is
operating in the linear region.
R
For an SP7614 with V
current of 20mA.
I
operation) = 22uA.
Using the graph on page 5 in the typical
performance section, the V
control pin will be about 1.1V, then R
determined by:
PWM FUNCTION
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
Jan 09 -08 RevT
LED
SET
SET
CONTROL
CTRL
SET
SET
SET
= Gain X (V
vs Control voltage graph located in the typical
according to the following formula:
Example
= 20mA/870 (870 is the typical gain in linear
SET
is the voltage at the pin of the device
CTRL
R
R
The
= I
R
. The gain is not linear, the LED current
SET
SET
SET
is the voltage going into R
LED
is known, use the following formula to
= (3V - 1.1V) / 22uA
= 86KΩ
= (V
LED's
CTRL
CTRL
characteristics
/ Gain.
CONTROL
Typically, the gain is ~870 for
CONTROL -
can be determined using the
pin voltage using I
brightness
CONTROL
- V
CTRL
V
CTRL
CTRL
SP7612/7614 Low Dropout Linear LED driver
section.
) / R
of 3V and LED
) / I
voltage on the
can
CONTROL
SET
SET
SET
SET
SET
also
As
will be
on the
and
an
be
Page 10
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:
where T
T
Ө
I is the LED current and
VF is the LED forward voltage.
When the temperature rises and the cathode
voltage increases, SP7612/4 reduces the current
through LEDs.
Cathode
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
The lower the V
efficiency. Efficiency can be further improved
using a higher V
example 3.
architecture
efficiency compared to using simple ballast
resistors.
Application Information
The ultra-low voltage drop across the SP7612/4
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
applications. Any of the SP7612/4 series of LED
drivers can be used in the applications presented
in this document, due to their similar operation.
Example 1:
Drive low V
single cell Li-Ion
A
JA
is the ambient temperature,
is the junction to ambient thermal resistance,
T
J
J
is the LED junction temperature,
Voltage"
= T
SP7612 SP7614
APPLICATION INFORMATION
F
The SP7612/4 driver's low dropout
A
IN
white or blue LEDs directly from
can
+ Ө
CATHODE
- V
Refer to "LED Current vs. LED
IN
© 2008 EXAR Corporation
CATHODE
JA
with more LEDs as shown in
significantly
graph under
x I x V
, the higher the system
) / V
F
IN
improve
the Typical
the