SP7680EB Exar Corporation, SP7680EB Datasheet - Page 13

• Manufacturer Part Number: SP7680EB
• Description: Display Modules & Development Tools Complete Backlight Solution
• Manufacturer: Exar Corporation

Specifications of SP7680EB

• Product: Display Modules

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
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.
shown in competitors’ datasheets and is shown
Sept 13-07 revI
prevents
The first case is what is generally
noise
transient
SP7680: Complete Backlight Solution
effects
Page 13
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
when V
higher than the voltage required on the constant
current-sink outputs of the LEDs) and that is
when the SP7680 is in the 1x mode. When V
is less than the V
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.
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 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
milliohms for ceramic capacitors and can range
to more than several Ohms for tantalum or
aluminum electrolytic capacitors.
OUT
efficiency = V
IN
(V
APPLICATIONS INFORMATION
is higher than the V
OUT
ESR
-V
LED
F
OUT
is
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OU
typically
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OUT
t/(V
/(V
IN
F
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of the LEDs (and
IN
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measured
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IN
in