DESIGN IDEAS Multi-Output Supply Drives White LEDs, Provides LCD or OLED Bias in a 3mm × 3mm DFN Package by Gurjit Thandi Introduction Many of today’s cell phones, PDAs and digital still cameras contain a high-resolution TFT-LCD display and sometimes an additional secondary OLED (Organic Light-Emitting Diode) display. OLED displays are fast becoming the secondary display of choice because they are brighter, thinner and more responsive than equivalent LCDs. The LT3466-1 is a dual switching regulator designed to meet the power supply requirements of small displays, including LCD-bias, white LED backlight and OLED displays. The LT3466-1 integrates a full featured white LED driver and a boost converter in a low profile 3mm × 3mm DFN package. It provides space and component savings with integrated 44V power switches and Schottky diodes. The LED driver can be configured to drive up to 10 white LEDs in series from a single Li-Ion battery. The white LED driver features a low 200mV reference for programming the LED current, thereby minimizing the power loss in the current setting resistor for better efficiency. The boost converter can be used for generating the main LCD bias voltages, or for providing the OLED bias supply. The boost converter achieves ±1.5% out- 3V TO 5V L1 33µH 6 LEDs COUT1 1µF EFFICIENCY (%) VIN SW2 VOUT1 LT3466-1 VOUT2 FB2 SHUTDOWN AND DIMMING CONTROL 1 RT GND 63.4k CIN 1µF COUT1 1µF VIN 5 10 15 20 25 30 OUTPUT CURRENT (mA) Figure 2. Efficiency versus load current for the circuit in Figure 1 Linear Technology Magazine • December 2005 –15V 10mA COUT3 1µF 15V 10mA VOUT2 LT3466-1 OFF ON D1 SW2 VOUT1 CTRL1 RT RFB1 13.3Ω C1 0.1µF L2 33µH L1 33µH 70 0 a 200kHz to 2MHz range. Additional features include output overvoltage protection, internal compensation and internal soft-start. The LT3466-1 operates from a wide input voltage range of 2.7V to 24V, making it suitable for a variety of applications. FB1 VIN = 3.6V 6 LEDs VOUT2 = 16V R2 24.9k 3V TO 5V BOOST CONVERTER 55 R1 475k CTRL2 SHUTDOWN AND DIMMING CONTROL 2 put voltage accuracy by the use of an internal precision 0.8V reference. The LT3466-1 also provides independent dimming and shutdown control of the two converters. The operating frequency of LT3466-1 can be set with an external resistor over LED DRIVER 60 16V 30mA Figure 1. The LT3466-1 powers a main LCD backlight and a secondary OLED display. It provides a 20mA drive for the six-white-LED LCD backlight and a 16V output for the OLED display. SW1 65 COUT2 1µF CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK316BJ105 L1, L2: 33µH TOKO D52LC 75 50 SW1 CTRL1 RFB1 10Ω 8 LEDs 80 1µF FB1 90 85 L2 33µH 475k COUT2 1µF FB2 CTRL2 63.4k 26.7k OFF ON CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2, COUT3: TAIYO YUDEN GMK316BJ105 C1: TAIYO YUDEN UMK212BJ104 L1, L2: 33µH TOKO D52LC D1: PHILIPS BAT54S Figure 3. High efficiency, Li-Ion powered complete TFT-LCD supply (bias and backlighting). The LT3466-1 drives eight white LEDs at 15mA to provide the backlight and generates dual, ±15V, outputs for the LCD-bias. 39 DESIGN IDEAS Dual Display Power Supply for Cell Phones A typical application for the LT3466-1 is as a driver for dual displays in cell phones. Present day, clam-shell cell phones typically use a color TFT-LCD main display and a secondary OLED display. Figure 1 shows the LT34661 powering the main LCD backlight and the secondary OLED display. The 86 VIN = 3.6V 8 LEDs +15V/10mA –15V/10mA EFFICIENCY (%) 84 Low Cost, Complete LCD Bias and White LED Backlighting Solution for Small TFT Displays 82 80 Small, active-matrix, TFT-LCD displays, used in cell phones, PDAs and other handheld devices generally require four to ten white LEDs for providing the backlight and fixed +15V and –15V supply voltages to bias the LCD. Figure 3 shows LT3466-1 powered complete TFT-LCD supply with minimal external components 78 76 74 0 2.5 LT3466-1 drives 6 white LEDs at 20mA for backlighting the main LCD panel and generates 16V output for powering the OLED. The LT3466-1 allows for independent dimming control of the main and secondary displays via the respective CTRL1 and CTRL2 pins. Figure 2 shows the efficiency versus output current for both the LED driver and the boost converter. The typical efficiency at 3.6V input supply is 84% with the white LEDs and the OLED driven at 20mA. 5 7.5 10 LED CURRENT (mA) 12.5 15 Figure 4. Efficiency versus LED current for the circuit in Figure 3. The circuit achieves greater than 83% efficiency driving eight LEDs at 15mA from 3.6V input. and high efficiency. The LT3466-1 drives eight white LEDs at 15mA and generates 15V boost output powered from a single Li-Ion supply. A discrete charge pump produces the secondary output of –15V. As seen in Figure 4, the circuit achieves greater than 83% efficiency driving eight LEDs at 15mA from 3.6V input. Conclusion The LT3466-1 integrates a full featured white LED driver and a boost converter in a space saving 3mm × 3mm DFN package. Integrated power switches and Schottky diodes reduce the overall system cost and size making it an excellent fit for handheld applications. Features like internal compensation, soft-start, Open LED protection enables LT3466-1 to provide complete TFT-LCD supply (bias and white LED backlight) for handheld devices with minimal external components and high efficiency. LT3477, continued from page 27 C1 3.3µF ISP1 ISN1 SW VIN IADJ1 IADJ2 SHDN SHDN D1 R2 402k LT3477 ISP2 75 RLOAD R4 0.2Ω ISN2 VREF FBP –5V FBN VC 4.75k L2 10µH EF(FICIENCY (%) R1 0.05Ω VIN 5V 85 C2 0.47µF L1 10µH C5 3.3µF 55 45 RT GND 65 SS 0 100 200 300 400 LOAD CURRENT (mA) 500 Figure 11. Efficiency of the Cuk converter. C3 22nF R3 100k 100pF C4 33nF R5 18.2k R6 10k C1, C5: TAIYO YUDEN LMK316BJ335ML D1: DIODES INC. DFL5120L L1, L2: TOKO A915AY-100M (D53LC SERIES) Figure 10. Negative output voltage Cuk converter. using a Cuk topology for 5V to –5V conversion. The first current sense amplifier is used for input current limit, and the second current sense amplifier is used for ground rail current sense to accurately limit the load current at 500mA. Even though the two current sense amplifiers are used, 40 efficiency up to 81% at 500mA output load can still be achieved. Figure 11 shows the efficiency. Conclusion The rail-to-rail constant-current/constant-voltage operation of the LT3477 makes the device an ideal choice for a variety of constant-current designs, including negative outputs. The dual current-sense amplifiers allow flexible configuration for input current limit, constant output current and fail-safe protection, along with excellent output voltage regulation. A wide input voltage range and the ability to produce outputs up to 42V make the LT3477 extremely versatile. Authors can be contacted at (408) 432-1900 Linear Technology Magazine • December 2005