Jul31-07 RevT SP7611A/7612A/7614A Low Dropout Linear LED driver 2007 Sipex Corporation
Page 10
applications. Any of the SP761X series of LED
drivers can be used in the applications presented
in this document, due to their similar operation.
Example 1:
Drive low V
F
white or blue LEDs directly from
single cell Li-Ion
When using white or blue low V
F
LEDs, and
utilizing the driver's low voltage drop, only 3.4V V
IN
is needed for the full 20mA LED current. At 3.1V,
there is still 5mA of typical current available for the
LEDs.
The single cell Li-Ion battery is utilized in many
applications like cell phones or digital still
cameras. In most cases, the Li-Ion battery voltage
level only goes down to 3.0V, and not down to the
full discharge level (2.7V) before requesting the
charger.
V
DROP
< 0.3V. This is the dropout voltage for the
SP7611A.
- V
F
= 3.1V. Low V
F
white LED forward voltage
drop
- V
IN
(at 20mA) needs to be greater than
(V
DROP
+V
F
)
3.1V+.3V= 3.4V
Key Advantages
1) No boost circuit needed for the LCD or
keyboard backlight.
2) Drivers directly connected to a Li-Ion battery.
3) No EMI, no switching noise, no boost converter
efficiency loss, 1 capacitor, and no inductor.
Example 2:
Drive high V
F
white or blue LEDs from existing
bus ~ 4.0V to 5.5V
High V
F
LEDs have a forward voltage drop in the
range of 3.2V to 4.0V. In order to drive these LEDs
with the maximum current of 20mA, enabling
maximum brightness usually requires a boost
circuit for a single cell Li-Ion power supply. The
SP761X series is capable of driving high V
F
white
or blue LEDs with its ultra-low dropout feature.
The V
IN
needs to be only 300mV higher than the
highest V
F
in the circuit.
- V
DROP
< 0.3V. Dropout voltage of the SP7611A
- V
F
(at 20mA) < 3.3V to 4.0V (High VF)
- V
IN
(at 20mA) = V
DROP
+ V
F
= 3.6V to 4.3V
- V
IN
(at 5mA typical) ~ 3.3V
Where V
IN
= Existing bus = 3.3V to 4.3V
Key Advantages
1) No boost circuit needed for the LCD or
keyboard backlight.
2) Drivers utilize existing bus.
3) Ultra-low voltage drop provides the full 20mA
LED current at the lowest possible voltage level.
Example 3:
Drive white, blue, red, amber LED string
In a boost circuit, or existing voltage bus, the
SP761X series of LED drivers can be used to
drive a whole string of LEDs and achieve flexible
brightness control - whether using analog or PWM.
V
DROP
= 0.3V. Dropout voltage of the SP7611A
- V
IN_MIN
= N x V
F
+ V
DROP
- V
IN_MIN
= N x VF + 0.3V
Where V
IN_MIN
= Existing bus, boost Voltage
‘N’ number of LEDs Per Branch
VF = Forward voltage of the LED
Key Advantages
1) No need for current matching resistors
and discrete transistors for brightness
control.
APPLICATION INFORMATION
(10 paginas)
(362 paginas)
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