DN267 - High Efficiency White LED Driver Guarantees Matching LED Brightness

High Efficiency White LED Driver Guarantees Matching LED
Brightness – Design Note 267
Dave Kim
Introduction
White LEDs are widely used in small LCD backlight
applications for simplicity, high reliability and low cost.
However, due to variations in the forward voltage drop of
white LEDs, matching the brightness becomes a major
design consideration for multiple LED applications.
Since LED brightness is proportional to current rather
than voltage, a constant-current source driving LEDs in
series should be used to ensure the same illumination
of each LED. The LT®1932 uses a constant-current stepup architecture that directly regulates the LED current
and guarantees a constant light intensity in each LED,
regardless of differences in their forward voltage drops.
Its unique internal current source accurately regulates
LED current even when the input voltage is higher than
the LED voltage, greatly simplifying the battery/adapter
power designs. The internal 36V switch is capable of
driving up to eight LEDs in series with 20mA of LED
current.
Li-Ion LED Driver for Four White LEDs
The white LED driver shown in Figure 1 is an ideal solution for Li-Ion powered, 4-LED backlight applications.
The LT1932 and five external components require less
than 0.65cm2 of printed circuit board space.
The constant-frequency step-up topology provides low
noise, high efficiency and matched LED brightness along
with adjustable current control. The LT1932 permits no
LED current during shutdown, resulting in less than
1μA of battery current in the standby mode. The LED
current is easily programmed over a range of 5mA to
40mA by selecting the value of RSET as shown in Table
1. By applying an additional DC voltage or a pulse-width
modulated (PWM) signal to the RSET pin, the LED current can be adjusted for dimming or brightness control.
Table 1. RSET Resistor Values
ILED (mA)
RSET VALUE (Ω)
40
562
30
750
20
1.13k
15
1.50k
10
2.26k
5
4.53k
The LT1932 comes in the tiny 1mm, ThinSOT™ package.
It operates at a constant 1.2MHz switching frequency,
permitting the use of tiny, low profile chip inductors
and capacitors to minimize circuit size and cost in
space-conscious portable applications such as cellular
telephones and handheld computers.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
L1
6.8μH
VIN
2.7V TO 4.2V
Efficiency
D1
C1
4.7μF
6
1
VIN
SW
LT1932
PWM
DIMMING
CONTROL
5
SHDN
RSET
4
RSET
1.50k
LED
GND
3
C2
1μF
15mA
EFFICIENCY (%)
85
80
VIN = 4.2V
75
VIN = 2.7V
70
65
2
60
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN EMK212BJ105
D1:ZETEX ZHCS400
L1: SUMIDA CLQ4D106R8 OR PANASONIC ELJEA6R8
DN267 F01a
55
0
5
10
15
LED CURRENT (mA)
Figure 1. Li-Ion (or 5V) LED Driver for Four White LEDs
09/01/267_conv
20
DN267 F01b
Dimming Control
The brightness of the LED can be easily adjusted using
a PWM signal, a filtered PWM signal, a logic signal or
a DC voltage. Five LED dimming schemes are shown
in Figure 2. The LT1932 using PWM brightness control
provides the widest dimming range and the purest white
LED color over the entire dimming range. This results
in better than a 20:1 dimming ratio without the undesirable blue tint common to white LED backlights. PWM
controlled LED current is shown in Figure 3. Average LED
current changes with duty cycle by switching between
full current and zero current. This ensures that when
the LEDs are on, they can be driven at the appropriate
current to give the purest white light (at 15mA or 20mA)
while the light intensity changes with the PWM duty
cycle. See the LT1932 data sheet for more information
about the dimming control methods shown in Figure 2.
the light intensity matching in each LED. The constant
current topology and 36V internal switch make the
LT1932 an ideal solution for multiple white LED driver
applications. The LT1932 features the purest white LED
color dimming control, less than 1μA standby mode
quiescent current, selectable current level, guaranteed
LED brightness matching and extremely small circuit
size making it well suited for portable cellular phone
and handheld computer applications.
Conclusion
For multiple white LED backlight applications, a constant current, series LED driver is required to ensure
50μs/DIV
LT1932
LT1932
RSET
SHDN
5
4
VPWM
2V/DIV
ILED
10mA/DIV
Figure 3. PWM Dimming Using the SHDN Pin
LT1932
RSET
4
RPWM
PWM
LT1932
RSET
10k
4
RPWM
PWM
LT1932
RSET
RSET
0.1μF
4
RADJ
PWM
RSET
DN267 F03
RINCR
VDC
RSET
RMIN
LOGIC
SIGNAL
DN267 F02
PWM
PWM
FILTERED PWM
DC VOLTAGE
LOGIC
Figure 2. Five Methods of LED Dimming
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn267f_conv LT/TP 0901 341.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2001
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