SP7614 datasheet(10/14 Pages) EXAR

10 / 14 page

Jan 09 -08 RevT

SP7612/7614 Low Dropout Linear LED driver

Page 10

SP7612 SP7614

APPLICATION INFORMATION

Setting the LED Current

The LED current is controlled by

ICONTROL and

RSET according to the following formula:

ILED = Gain X (VCONTROL - VCTRL) / RSET

Gain depends on mode of operation

VCONTROL is the voltage going into RSET

VCTRL is the voltage at the pin of the device

The voltage VCTRL can be determined using the

ISET vs Control voltage graph located in the typical

performance

characteristics

section.

example, to set the LED current for an SP761x the

procedure is:

Take the LED current and divide it by the Gain to

get

ISET = ILED / Gain.

Look up the VCTRL pin voltage using

ISET on the

graph “Ctrl Current vs. Control Voltage”.

Once VCTRL is known, use the following formula to

find RSET. 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

SP7614 and ~435 for SP7612 when the part is

operating in the linear region.

RSET = (VCONTROL - VCTRL) /

ISET

RSET Example

For an SP7614 with VCONTROL of 3V and LED

current of 20mA.

ISET = 20mA/870 (870 is the typical gain in linear

operation) = 22uA.

Using the graph on page 5 in the typical

performance section, the VCTRL voltage on the

control pin will be about 1.1V, then RSET will be

determined by:

RSET = (3V - 1.1V) / 22uA

RSET = 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:

TJ = TA + ӨJA x I x VF

where TJ is the LED junction temperature,

TA is the ambient temperature,

ӨJA is the junction to ambient thermal resistance,

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. 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 = (VIN - VCATHODE) / VIN

The lower the VCATHODE , the higher the system

efficiency. Efficiency can be further improved

using a higher VIN with more LEDs as shown in

example 3. The SP7612/4 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 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 VF white or blue LEDs directly from

single cell Li-

Ion

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