SP7682EB datasheet(16/19 Pages) EXAR

16 / 19 page

Sept 27-07 rev G

SP7682: Backlight and Flash Driver

Page 16

LED Selection

The SP7682 is designed as a driver for backlight

white LEDs and Flash white LEDs, but is

capable of driving other LED types with forward

voltage specifications ranging from 2.0V to 3.8V.

LED applications may include LCD display

backlighting, camera photo-flash applications,

infrared (IR) diodes for remotes, and other loads

benefiting from a controlled output current

generated from a varying input voltage. Since

the D1 to D4 output current-sinks are matched

with negligible voltage dependence, the LED

brightness will be matched regardless of the

specific LED forward voltage (VF) levels. In flash

applications, it may be necessary to drive high-

VF type LEDs. The typical characteristic curves

illustrate low VF Flash LEDs using the Lumi-

LEDs PWF4 LED, but the low dropout current-

sinks in the SP7682 make it capable of driving

LEDs with forward voltages as high as 4.0V from

an input supply as low as 3.2V but at a reduced

output current.

Device Switching Noise Performance

The SP7682 operates at a fixed frequency of

approximately 2.4MHz to control noise and

limit harmonics that can interfere with the RF

operation of cellular telephone handsets or other

communication devices. Back-injected noise

appearing on the input pin of the charge pump

can be much less than 100mV peak-to-peak,

typically less than inductor-based DC/DC boost

converter white LED backlight solutions. The

SP7682 soft-start feature helps prevent noise

transient effects associated with inrush currents

during startup of the charge pump circuit.

Power Efficiency

The charge pump efficiency shown in the typical

characteristic curves is shown for two cases.

The first case is called output efficiency which is

the power efficiency to the output as a ratio of

the output voltage power to the input voltage

power and expressed as a percentage. The

second case is called LED efficiency and is the

power efficiency to the LED outputs and is

expressed as a ratio of the power to the LEDs to

the input voltage power. These expressions are

shown at the end of this section in their

formulas. The first case is what is generally

shown in competitors’ datasheets and is shown

here for reference. The second case of LED

power efficiency is included to show the user the

true power delivered to the LEDs. As one can

see in the curves, the LED efficiency is greatest

when VIN is higher than the VF of the LEDs (and

higher than the voltage required on the constant

current-sink outputs of the LEDs) and that is

when the SP7682 is in the 1x mode. When VIN

is less than the VF (and less than the voltage

required on the constant current-sink outputs of

the LEDs) the SP7682 is in the 1.5x mode or 2x

mode and in these modes the input current is

1.5 times or 2 times the output current and

therefore the efficiency will be reduced.

VOUT efficiency = VOUT•

IOUT/(VIN•IIN)•100%

LED efficiency =

(VOUT -VLED) •

IOUt/(VIN•IIN)•100%

Refer to the Typical Characteristics section of

this document for measured plots of efficiency

versus input voltage and output load current

versus input voltage for given LED output

current options.

Capacitor Characteristics

Ceramic composition capacitors are highly

recommended over all other types of capacitors

for use with the SP7682. Ceramic capacitors

offer many advantages over their tantalum and

aluminum electrolytic counterparts. A ceramic

capacitor has very low ESR, is lower in cost, has

a smaller PCB footprint, and is non-polarized.

Low ESR ceramic capacitors help to maximize

charge pump transient response. Since ceramic

capacitors are non-polarized, they are not prone

to incorrect connection damage.

Equivalent Series Resistance (ESR)

ESR is an important characteristic to consider

when selecting a capacitor. ESR is a resistance

internal to a capacitor that is caused by the

leads, internal connections, size or area,

material composition, and ambient temperature.

Capacitor

ESR

typically

measured

milliohms for ceramic capacitors and can range

to more than several Ohms for tantalum or

aluminum electrolytic capacitors.

APPLICATIONS INFORMATION

SP7682EB Datasheet(PDF) 16 Page - Exar Corporation

SP7682EB Datasheet(HTML) 16 Page - Exar Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL