SP7682EB Exar Corporation, SP7682EB Datasheet - Page 16

• Manufacturer Part Number: SP7682EB
• Description: Display Modules & Development Tools Backlight & Flash Driver in QFN 3x3 mm
• Manufacturer: Exar Corporation

Specifications of SP7682EB

• Product: Display Modules

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 (V
applications, it may be necessary to drive high-
V
illustrate low V
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.
shown in competitors’ datasheets and is shown
Sept 27-07 rev G
F
type LEDs. The typical characteristic curves
The first case is what is generally
F
Flash LEDs using the Lumi-
F
) levels. In flash
SP7682: Backlight and Flash Driver
Page 16
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 V
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 V
is less than the V
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.
V
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 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
milliohms for ceramic capacitors and can range
to more than several Ohms for tantalum or
aluminum electrolytic capacitors.
OUT
efficiency = V
IN
(V
is higher than the V
OUT
ESR
-V
APPLICATIONS INFORMATION
LED
F
OUT
is
) •I
(and less than the voltage
•I
OU
typically
OUT
t/(V
© 2007 Exar Corporation
/(V
IN
F
IN
•I
of the LEDs (and
IN
•I
measured
IN
)•100%
)•100%
IN
in