June 21-07 revH

SP7680: Complete Backlight Solution



2007 Sipex Corporation

Page 13

LED Selection

The SP7680 is designed as a driver for backlight

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

main and sub LCD display backlighting, camera

photo-flash applications, color (RGB) LEDs,

infrared (IR) diodes for remotes, and other loads

benefiting from a controlled output current

generated from a varying input voltage. Since

the D1 to D6 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 low dropout current-sinks in

the SP7680 make it capable of driving main

LEDs with forward voltages as high as 4.0V at

full current from an input supply as low as 3.2V.

LED current-sink inputs can be paralleled to

drive high-current LEDs without complication.

Device Switching Noise Performance

The SP7680 operates at a fixed frequency of

approximately 2MHz

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 is

20mV peak-to-peak, typically ten times less than

inductor-based DC/DC boost converter white

LED backlight solutions. The SP7680 soft-start

feature

prevents

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.

The 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 you can

see in the curves, the LED efficiency is greatest

higher than the voltage required on the constant

current-sink outputs of the LEDs) and that is

required on the constant current-sink outputs of

the LEDs) the SP7680 is in the 1.5x mode and

in this mode the input current is 1.5 times the

output current and therefore the efficiency will be

reduced.

LED efficiency =

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 SP7680. 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

is

typically

measured

in

milliohms for ceramic capacitors and can range

to more than several Ohms for tantalum or

aluminum electrolytic capacitors.

APPLICATIONS INFORMATION