DESIGN FEATURES
`Constant-Current DC/DC Converter
`Drives White LEDs with 80% Efficiency
`
`Introduction
`The new LT1932 DC/DC converter is
`specifically designed to provide high
`efficiency, constant-current drive for
`white LED backlights. To achieve the
`best possible brightness matching,
`the LT1932 drives the LEDs in a
`series string, which inherently guar-
`antees current matching and therefore
`equal brightness among all LEDs in
`the string. Competing charge-pump
`designs generate a fixed output volt-
`age and require ballast resistors to
`set the LED current. Not only is this
`inefficient, as power is wasted in the
`resistors, but variations in LED
`forward voltage cause unequal current
`
`VIN
`2.7V TO 5V
`
`flow in the LEDs, leading to brightness
`mismatch. Typical efficiencies of 75%
`to 80% are obtained using the LT1932,
`significantly higher than the 40% to
`50% efficiencies obtained in parallel-
`driven charge-pump solutions.
`A single external resistor sets the
`LED current between 5mA and 40mA,
`which can then be easily adjusted to
`provide dimming using either a DC
`voltage or a pulse-width-modulated
`(PWM) signal. With an input voltage
`range of 1V to 10V, the LT1932 works
`from a variety of input sources; it can
`even regulate the LED current when
`the input voltage is higher than the
`L1
`10µH
`
`D1
`
`6
`VIN
`
`1
`SW
`
`LT1932
`
`SHDN
`RSET
`4
`
`3
`
`LED
`GND
`2
`
`15mA
`
`C2
`1µF
`
`C1
`4.7µF
`
`5
`
`80.6k
`
`3.3V DC
`DIMMING
`CONTROL
`
`RSET
`1.50k
`
`C1: TAIYO YUDEN JMK212BJ475
`C2: TAIYO YUDEN EMK212BJ105
`D1:ZETEX ZHCS400
`L1: SUMIDA CLQ4D10100
`
`OR MURATA LQH3C100M24
`
`(408) 573-4150
`
`(516) 543-7100
`(847) 956-0667
`(814) 237-1431
`
`Figure 1. Li-Ion driver for four white LEDs
`
`by Bryan Legates
`
`output voltage. The 36V switch rating
`allows output voltages as high as 35V
`to be generated, easily driving up to
`eight white LEDs in series. When the
`LT1932 is turned off, the LEDs are
`disconnected from the output, ensur-
`ing a true low current shutdown with
`no excess battery leakage or light
`output. The 1.2MHz switching fre-
`quency allows the use of inexpensive,
`low profile inductors and capacitors,
`which, along with the LT1932’s 1mm-
`tall TSOT-23 package, provide a tiny,
`low cost LED driver solution. The
`LT1932’s small size and high effi-
`ciency, and its ability to provide
`uniform brightness, make it an excel-
`lent choice for applications using white
`LEDs to backlight or frontlight color
`liquid crystal displays (LCDs).
`
`VIN = 4.2V
`
`VIN = 2.7V
`
`0
`
`5
`
`10
`LED CURRENT (mA)
`
`15
`
`20
`
`85
`
`80
`
`75
`
`70
`
`65
`
`60
`
`55
`
`EFFICIENCY (%)
`
`Figure 2. Efficiency of Figure 1’s circuit
`
`VIN = 4.2V
`
`VIN = 2.7V
`
`0
`
`5
`
`10
`LED CURRENT (mA)
`
`15
`
`20
`
`85
`
`80
`
`75
`
`70
`
`65
`
`60
`
`55
`
`EFFICIENCY (%)
`
`VIN
`2.7V TO 5V
`
`C1
`4.7µF
`
`5
`
`L1
`10µH
`
`D1
`
`6
`VIN
`
`1
`SW
`
`LT1932
`
`SHDN
`RSET
`4
`
`3
`
`LED
`GND
`2
`
`C2
`1µF
`
`80.6k
`
`3.3V DC
`DIMMING
`CONTROL
`
`RSET
`1.50k
`
`15mA
`
`C1: TAIYO YUDEN JMK212BJ475
`C2: TAIYO YUDEN TMK316BJ105
`D1: ZETEX ZHCS400
`L1: MURATA LQH3C100M24
`
`(408) 573-4150
`(408) 573-4150
`(631) 543-7100
`(814) 237-1431
`
`Figure 3. Li-Ion driver for six white LEDs
`
`Figure 4. Efficiency of Figure 3’s circuit
`
`Linear Technology Magazine • May 2001
`
`21
`
`Kinetic Technologies, Inc.
`Ex. 1010 Page 1
`
`

`
`DESIGN FEATURES
`
`L1
`4.7µH
`
`D1
`
`6
`VIN
`
`1
`SW
`
`LT1932
`
`VIN
`1.8V TO 3V
`
`C1
`4.7µF
`
`5
`
`SHDN
`RSET
`4
`
`3
`
`LED
`GND
`2
`
`15mA
`
`C2
`1µF
`
`60.4k
`
`2.5V DC
`DIMMING
`CONTROL
`
`RSET
`1.50k
`
`C1: TAIYO YUDEN JMK212BJ475
`C2: TAIYO YUDEN EMK212BJ105
`D1: ZETEX ZHCS400
`L1: MURATA LQH3C4R7M24
`
`(408) 573-4150
`(408) 573-4150
`(631) 543-7100
`(814) 237-1431
`
`VIN
`1V TO 1.5V
`
`C1
`4.7µF
`
`5
`
`L1
`4.7µH
`
`D1
`
`6
`VIN
`
`1
`SW
`
`LT1932
`
`SHDN
`RSET
`4
`
`3
`
`LED
`GND
`2
`
`15mA
`
`C2
`2.2µF
`
`24.9k
`
`2.5V PWM
`DIMMING
`CONTROL
`
`RSET
`1.50k
`
`C1: TAIYO YUDEN JMK212BJ475
`C2: TAIYO YUDEN LMK212BJ225
`D1: ZETEX ZHCS400
`L1: MURATA LQH3C4R7M24
`
`(408) 573-4150
`(408) 573-4150
`(631) 543-7100
`(814) 237-1431
`
`Figure 5. 2-cell driver for four white LEDs
`
`Figure 7. Single-cell driver for two white LEDs
`
`seen in Figure 8. For very small LCDs,
`the LT1932 can drive a single LED
`from a single-cell alkaline battery with
`an efficiency of 72%.
`Conclusion
`The constant-current operation of the
`LT1932 makes it ideal for driving
`LEDs. The device provides uniform
`LED brightness at efficiencies up to
`80%, making it a significant improve-
`ment over charge-pump solutions.
`With a wide input voltage range and
`the ability to drive up to eight white
`LEDs in series, the LT1932 works
`well in a variety of applications.
`
`References
`1. Kim, Dave. “Tiny Regulators Drive White LED
`Backlights.” Linear Technology Corporation Pub-
`lication: Design Note 231 (May 2000).
`2. LT1932 Data Sheet. Linear Technology Corpora-
`tion: 2001.
`
`VIN = 1.5V
`
`VIN = 1.1V
`
`0
`
`2.5
`
`5
`7.5
`10
`LED CURRENT (mA)
`
`12.5
`
`15
`
`80
`
`75
`
`70
`
`65
`
`60
`
`55
`
`50
`
`EFFICIENCY (%)
`
`Figure 8. Efficiency of Figure 7’s circuit
`
`Linear Technology Magazine • May 2001
`
`Li-Ion Driver for
`Six White LEDs
`For bigger displays, more light may
`be needed to provide adequate back-
`lighting. Capable of driving six white
`LEDs from a single Li-Ion cell, the
`circuit in Figure 3 is ideal for LCDs
`needing a higher level of light output.
`Efficiency for this circuit, shown in
`Figure 4, exceeds 77%. Li-Ion sys-
`tems needing between two and eight
`LEDs can use similar circuits with
`efficiencies of 75% to 80%. Consult
`the LT1932 data sheet for several
`other LED driver circuits that operate
`from a single Li-Ion cell.
`2-Cell Driver for
`Four White LEDs
`The circuit in Figure 5 drives four
`white LEDs from two alkaline cells,
`making it a good choice for personal
`digital assistants (PDAs) and many
`other handheld instruments with
`small LCDs. Figure 6 shows the effi-
`ciency, which reaches 76%. 2-cell
`systems that need to drive two, three
`or five LEDs can use similar circuits
`(see LT1932 data sheet) with efficien-
`cies reaching 75%.
`Single-Cell Driver
`for Two White LEDs
`For applications that must operate
`from a single-cell alkaline battery,
`the circuit in Figure 7 is capable of
`lighting two white LEDs. Ideal for
`pagers and many MP3 players, this
`solution achieves 74% efficiency, as
`
`Li-Ion Driver
`for Four White LEDs
`The circuit in Figure 1 is a common
`choice for many portable devices, driv-
`ing four white LEDs from a single
`Li-Ion cell with efficiencies reaching
`80%. Figure 2 shows that the effi-
`ciency remains high over the full
`voltage range of the battery and over
`the entire range of LED current. As
`shown, dimming is accomplished
`using a DC voltage, but logic-con-
`trolled or PWM-controlled dimming
`can also be used. Consult the LT1932
`data sheet for detailed information on
`LED dimming. The overall simplicity
`of the LT1932 solution can easily be
`seen in Figure 1, as only a few exter-
`nal components are needed to provide
`a complete LED driver.
`
`VIN = 3V
`
`VIN = 1.8V
`
`0
`
`5
`
`10
`LED CURRENT (mA)
`
`15
`
`20
`
`85
`
`80
`
`75
`
`70
`
`65
`
`60
`
`55
`
`EFFICIENCY (%)
`
`Figure 6. Efficiency of Figure 5’s circuit
`
`22
`
`Kinetic Technologies, Inc.
`Ex. 1010 Page 2