DN231 - Tiny Regulators Drive White LED Backlights

Tiny Regulators Drive White LED Backlights – Design Note 231
Dave Kim
Introduction
The emergence of color LCD displays in handheld
information appliances has created the need for a
small, bright white backlight. Fortunately, the recent
commercialization of high intensity white LEDs provides
the perfect solution. These tiny LEDs are capable of
delivering ample white light without the fragility problems and costs associated with fluorescent backlights
commonly used in notebook computers. However, they
do pose a problem—the forward voltage of white LEDs
can be as high as 4V, precluding powering them directly
from a single lithium-ion cell.
This Design Note describes several different circuits that
may be used to boost and regulate the Li-Ion battery
voltage to power white LEDs. These circuits provide
sufficient power to drive multiple white LEDs and are
small enough to easily fit within cellular telephones and
handheld computers.
VIN
3V TO
4.2V
LTC1754-5
C3
10μF
C1
1μF
5
VIN SHDN
3
4
C–
GND
2
C+
VOUT
6
OFF ON
C2
10μF
1
R1
110Ω
D1
R2
110Ω
D2
R3
110Ω
D3
R4
110Ω
D4
C1: TAIYO YUDEN TMK316BJ105ML
C2, C3: TAIYO YUDEN EMK316BJ475ML
D1 TO D4: NICHIA NSPW500BS
DN231 F01
Figure 1. LTC1754 White LED Driver
VIN
3V TO
4.2V
Circuit Descriptions
Figure 1 depicts a very small charge pump DC/DC
converter that is capable of powering four white LEDs
at 15mA each (the typical forward current used for
backlighting). The LTC®1754-5 used in this application is
a tiny SOT-23 device that delivers a regulated 5V output
without any inductors. The entire circuit occupies less
than 0.1 in2 of board space (excluding the LEDs) and
may be powered directly from a lithium-ion cell.
A constant-current backlight supply may be constructed
using the LTC1682. This architecture has several
advantages: first, it directly controls the LED current
regardless of the forward voltage drop across the LED.
Additionally, the LTC1682’s output is regulated by an
on-chip linear regulator to deliver a very low noise
output to the LEDs. Output ripple is less than 4mV and
eliminates the risk of RF interference in sensitive cellular
phone applications. The LTC1682’s high current output
is capable of powering five parallel LEDs at 15mA each
from a single lithium-ion cell.
Applications requiring more white LEDs or higher efficiency can use an LT®1615 boost converter to drive
a series connected string of LEDs. The high efficiency
circuit shown in Figure 3 can provide a constant-current
drive for up to eight LEDs. Driving eight white LEDs in
series requires approximately 29V at the output and
is possible due to the internal 36V, 350mA switch in
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
LTC1682
C3
0.47μF
C4
0.47μF
6
VIN SHDN
2
7
C–
VOUT
1
C+
FB
5
8
C1
1μF
CPO
GND
3
4
OFF ON
D1
D2
D3
D4
R1
82.5Ω
R2
82.5Ω
R3
82.5Ω
R4
82.5Ω
DN231 F02
C1, C2: TAIYO YUDEN TMK316BJ105ML
C3, C4: TAIYO YUDEN TMK316BJ474ML
D1 TO D5: NICHIA NSPW500BS
Figure 2. LTC1682 Low Noise White LED Driver
05/00/231_conv
D5
C2
1μF
R5
82.5Ω
D9
MBR0530
L1
4.7μH
VIN
3V TO
4.2V
C2
4.7μF
LT1615
5
VIN
SW
GND
OFF ON
4
SHDN
FB
C1
1μF
35V
3
2
D1
D3
D5
D7
D2
D4
D6
D8
1
C3
1000pF
R1
80.6Ω
DN231 F03
C1: TAIYO YUDEN GMK316BJ105KL
C2: TAIYO YUDEN LMK316BJ475ML
D1 TO D8: NICHIA NSPW500BS
Figure 3. LT1615 White LED Driver
the LT1615. The constant-current design of the circuit
guarantees the same LED current through all series
LEDs, regardless of the forward voltage differences
between the LEDs. Although this circuit is designed to
operate from a single Li-Ion battery (2.5V to 4.2V), the
LT1615 is also capable of operating from inputs as low
as 1V with commensurate output power reductions.
VOUT
1
8)*5&-&%
LTC1682
FB
DAC
OUTPUT
R1
3
R2
BRIGHTNESS
CONTROL
ttt
RPROGRAM
%/'
Brightness Control
The brightness of the LED can be controlled by applying
a PWM signal to the SHDN pin on any of the backlight
circuits shown as long as a couple of precautions are
taken. Because of the “soft-start” circuitry incorporated
in these DC/DC converters, the output voltage will not
immediately rise to full output after the SHDN pin is
taken high. Consequently, a PWM signal in the range of
200Hz is recommended—much faster and brightness
control will be nonlinear; much slower and flicker may
be observed. It may also be desirable to place a resistor
between the DC/DC converter output and ground (in
parallel with the LED load) to discharge the output during
shutdown. Select a resistor that will draw approximately
1mA when the DC/DC converter is operational. (A parallel resistor is not required with the LTC1682 because it
contains internal discharge circuitry.)
As an alternative to using PWM control, a DAC output
can also be used to control the brightness of the LEDs
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Figure 4. Brightness Control Using DAC Output
in Figures 2 and 3. As depicted in Figure 4, the DAC
output controls the brightness of the LED by varying
the voltage across RPROGRAM. Since the regulator holds
the feedback voltage constant, varying the DAC voltage
will affect the current flowing through the LEDs. A lower
DAC voltage will result in higher brightness, while a high
DAC voltage will result in lower brightness.
Summary
The circuits shown are several examples of very tiny
step-up regulators that are suitable for driving white
LEDs. These circuits contain such desirable features as
constant-current LED drive, low noise (important for
cellular telephone applications), brightness control and
low voltage input (LT1615). Consult Linear Technology
for additional applications assistance with white LED
circuits.
For applications help,
call (408) 432-1900
dn231f_conv LT/TP 0500 370K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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© LINEAR TECHNOLOGY CORPORATION 2000
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