Feb 2001 Charge Pump Powers White LEDs

DESIGN IDEAS
Charge Pump Powers White LEDs
by Steven Martin
Introduction
Most of today’s portable equipment
uses a liquid crystal display to convey
information to its user. Until recently,
those displays have been monochrome
and have used a low voltage light
emitting diode (LED) such as red or
green for backlighting. With the advent
of color liquid crystal displays, a white
backlight source is needed. Recently,
a revolution in LED technology has
lead to the long-coveted white LED.
However, white LEDs, which are
actually blue LEDs with a special
phosphor in the lens, operate at a
considerably higher voltage than red
or green LEDs. White LEDs can
require anywhere from 3.5V to 3.9V
to operate at 15mA. In battery powered systems, it’s impractical to drive
the LEDs directly from the battery
and still control the LED current. To
properly control the LED current a
somewhat larger voltage is needed,
where the excess is used for control.
DESIGN IDEAS
Charge Pump Powers White LEDs
................................................... 29
Steven Martin
48V Hot Swap Circuit Blocks Reverse
Battery Voltage ........................... 30
Mitchell Lee
Using the LTC1662 3µ A DAC to Cure
RF Implementation Ills ................ 31
Derek Redmayne
LTC1628-SYNC Minimizes Input
Capacitors in a Multioutput, High
Current Power Supply ................. 34
Figure 1. LTC3200 evaluation circuit
Figure 1 shows the LTC3200-5
constant frequency voltage doubler
used to drive five white LEDs. Figure
2 is the schematic diagram of this
circuit. The LTC3200-5 produces a
regulated 5V output from an input as
low as 3V. Switching at 2MHz, the
constant frequency operation of the
charge pump is ideal for low noise
environments such as cellular telephones or internet communication
devices.
Since it produces a regulated 5V
output, the additional voltage dropped
across the resistors controls the LED
current. The resistors also provide
ballasting to ensure that the LEDs
Wei Chen
continued on page 37
1µF
Reduce EMI with Ultralow Noise 48V
to 5V, 10W DC/DC Converter ........ 36
4
Rick Brewster
5
3V TO 4.4V
Li-Ion
BATTERY
1µF
C–
VIN
6
C+ 1
VOUT
UP TO 5 LEDS
1µF
LTC3200-5
3
ON OFF
Authors can be contacted
at (408) 432-1900
run at similar currents despite moderate differences in forward voltage.
Figure 3 shows how the adjustable
LTC3200 can be used to control the
LED current directly by controlling
the voltage on the ballast resistors.
The LTC3200 regulates the anodes of
the LEDs until the FB pin comes to
balance at 1.268V. The feedback LED’s
current is precisely controlled and
the remaining LEDs are moderately
well controlled by virtue of their similarity and the 1.268V ballast voltage.
Since the current is more precisely
controlled, up to six LEDs can be
powered by the adjustable LTC3200.
SHDN
(APPLY PWM WAVEFORM FOR
ADJUSTABLE BRIGHTNESS CONTROL)
GND
VSHDN
100Ω
100Ω
100Ω
100Ω
100Ω
2
t
Figure 2. Li-Ion battery-powered 5V white or blue LED driver
Linear Technology Magazine • February 2001
29
DESIGN IDEAS
A
200µV/DIV
voltage. The CS pin provides the feedback for pulse-by-pulse current
control and slew control. A large signal on CS, indicative of a fault, also
shuts the MOSFETs off.
Converter efficiency is improved
by use of a bootstrap winding that
powers the part when the converter is
up and running. Efficiency at the low
noise setting is approximately 77%.
200µVP-P
B
20mV/DIV
Conclusion
The LT1683 provides a unique way to
produce an efficient, ultralow noise
supply. Novel control circuitry quiets
the switcher, allowing a new supply
solution for sensitive electronic systems. The use of external MOSFET
switches allows the voltage and current ratings of the supply to be tailored
to the application.
5µs/DIV
Figure 3. 5V output noise (bandwidth = 100MHz)
with undervoltage lockout, ensuring
that the input is up and running
before the converter is allowed to start.
In addition, the GCL pin prevents
excessive gate voltage on the MOSFET
and protects against the MOSFETs
turning on without sufficient gate
Because of the voltage slew control,
clamps or snubbers on the MOSFET
drains are not required and switch
ringing is greatly reduced. Figure 3
shows the noise at the outputs. The
output noise is a very low 200µVP-P.
The SHDN pin provides the supply
LTC3200, continued from page 29
1µF
1
2
3V TO 4.4V
Li-Ion
BATTERY
1µF
C+
3
C–
VOUT
VIN
1µF
LTC3200
FB
ON OFF
6
(APPLY PWM WAVEFORM
FOR ADJUSTABLE
BRIGHTNESS CONTROL)
SGND
SHDN
VSHDN
UP TO 6 LEDS
8
PGND
7
5
82Ω
82Ω
82Ω
82Ω
82Ω
82Ω
4
t
Figure 3. White or blue LED driver with LED current control
Conclusion
In either constant voltage or current controlled applications of the
LTC3200, the LED brightness can be
controlled by applying a PWM signal
(approximately 100Hz) to the SHDN
pin. Varying the pulse width from 4%
to 100% gives the LEDs a linear
appearance of brightness control from
full-on to full-off.
Linear Technology Magazine • February 2001
In the tiny 6-pin SOT or 8-pin
MSOP packages, the LTC3200 family
of charge pumps provides a simple
solution for powering white LEDs. Its
small size, low external parts count
and low noise, constant frequency
operation is ideally suited for both
communications and other portable
products.
http://www.linear-tech.com/ezone/zone.html
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