DESIGN FEATURES Tiny Device Drives 20 White LEDs from by Gurjit Thandi a Single Cell Li-Ion Battery Introduction White LEDs are gaining popularity as the backlighting source for the LCD displays used in handheld devices, mainly due to their improved efficiency and shrinking costs. White LEDs are also making inroads into the larger LCD displays used in automotive instrument panels and car radios. The LT3466 simplifies the task of fitting the LED driver circuitry into the latest devices by providing a dual high efficiency, constant current white LED driver in a space-saving 3mm × 3mm DFN package. The LT3466 is designed to drive up to 20 white LEDs from a single cell Li-Ion battery input with greater than 80% efficiency. It also provides space- and component-sav- ings with integrated Schottky diodes and internal compensation. 200mV, high accuracy (±4%) reference voltage is provided to program the LED current. The step-up converters use a current mode topology to provide excellent line and load transient response. Internal feedback loop compensation of LT3466 allows the use of small ceramic capacitors at the output. The built-in over-voltage protection circuit clamps the output of either converter to 42V if the LED string connected to that output fails open-circuited. Internal soft-start is provided for each stepup converter, thus minimizing inrush current during start-up. The switching frequency of LT3466 can be programmed over a 200kHz About the LT3466 Figure 1 shows a block diagram of the LT3466 with its two independent, but identical, step-up converters capable of driving asymmetric LED strings. The step-up converters are designed to drive the series connected LEDs with a constant current, thus ensuring uniform brightness and eliminating the need for ballast resistors. LT3466 incorporates internal 44V power switches and Schottky diodes. Switch current limit is guaranteed to be greater than 320mA over the full operating temperature range. A low, VIN C1 RT L1 1 C2 3 8 2 4 VIN RT SW1 VOUT1 L2 SW2 VOUT2 5 C3 OVERVOLT DETECTION OVERVOLT DETECTION OSC DRIVER DRIVER OSC PWM LOGIC Q1 + OSC + A3 A3 RSNS1 EA – A1 CONVERTER 1 10 + + – 0.2V 0.2V REF 1.25V SHDN + + – – EA A2 A1 CONVERTER 2 FB2 FB1 RFB1 PWM COMP + Σ Σ + A2 OSC RSNS2 – – PWM COMP PWM LOGIC Q2 RAMP GEN 20k 80k 80k START-UP CONTROL 9 CTRL1 7 CTRL2 20k 6 RFB2 EXPOSED PAD 11 3466 F02 Figure 1. LT3466 block diagram Linear Technology Magazine • May 2004 13 DESIGN FEATURES inputs and a single inverting input. An internal 200mV (±4%) reference voltage is connected to one of its noninverting inputs. An input voltage equal to 0.2 • VCTRL is connected to the second noninverting input of A1. The inverting input of A1 is connected to the cathode of the lowest LED in the string and the feedback resistor. The LED current in each string is given by: VIN = 3V TA = 25°C 200 150 100 50 0 1 0.5 1.5 CONTROL VOLTAGE (V) 0 2 Figure 2. Correlation of feedback voltage (VFB) to control voltage. The current (dimming) in the LED string is given by ILED = VFB/RFB. to 2MHz range by means of a single resistor from the R T pin to ground. The LT3466 operates from a wide 2.7V to 24V input voltage range, making it suitable for a wide range of applications. The device features independent shutdown and dimming control of the two LED strings. The current in each LED string can be shut off by pulling the respective control (CTRL1 or CTRL2) pin voltage below 50mV. Dimming for each LED string is achieved by applying a DC voltage to its respective control pin. When both CTRL1 and CTRL2 pin voltages are pulled below 50mV, the device enters total shutdown.The dimming feature for the LT3466 can be best understood by referring to the block diagram in Figure 1. The amplifier A1 (present in both converters) has two noninverting 3V TO 5V CIN 1µF L1 15µH COUT1 0.47µF SW1 L2 15µH VIN SW2 VOUT1 VOUT2 COUT2 1µF LT3466 FB1 RFB1 10Ω CTRL1 FB2 RT CTRL2 RFB2 10Ω 38.3k 1% CIN: TAIYO YUDEN JMK107BJ105 COUT1: TAIYO YUDEN EMK212BJ474 COUT2: TAIYO YUDEN LMK212BJ105 L1, L2: MURATA LQH32CN150 Figure 3. Low profile (max height < 1.7mm), single cell Li-Ion powered, six (4/2) white LED driver circuit 14 ILED = VFB R FB Thus, a linear change in the feedback voltage results in a linear change in the LED current. The amplifier A1 regulates the feedback pin voltage as a function of the control voltage as given by: VFB = 0.2 • VCTRL , When 0.2V < VCTRL < 1V VFB = 0.2V , When VCTRL > 1.6V As the voltage at the control pin is ramped from 0.2V to 1.6V, the respective feedback pin voltage changes from 40mV to 200mV. When the control voltage is taken above 1.6V, it does not affect the feedback pin voltage. Figure 2 shows the correlation between the feedback voltage and the control pin voltage. Main and Sub-Display Backlighting for Cell Phones A typical application of the LT3466 is as a driver for dual backlights in a cell phone. Present day, flip style cell phones typically use four white LEDs (with the phone open) for backlighting the main display and two white LEDs (with the phone closed) for a sub-display. Each of the backlights requires independent dimming and shutdown control. Figure 3 shows a Li-Ion battery powered 6-LED (4-LED main and 2-LED sub) backlight system. LT3466 allows for independent dimming control of the main and sub display via the CTRL1 and CTRL2 pins. Board real estate is at a premium in cell phones and the circuit shown in Figure 3 minimizes the number of external components and provides a complete system solution with maximum component height under 1.7mm. The LT3466 is designed to run at a 85 VIN = 3.6V 4/2 LEDs 80 75 EFFICIENCY (%) FEEDBACK VOLTAGE (mV) 250 70 65 60 55 50 0 5 10 15 20 LED CURRENT (mA) Figure 4. Efficiency for Figure 3’s circuit 1.25MHz switching frequency via the selection of the R T resistor. The choice of high 1.25MHz switching frequency allows the use of space saving lowprofile inductors and tiny 0805 size ceramic capacitors, while maintaining high system efficiency. Figure 4 shows the efficiency of the circuit. The typical efficiency at 3.6V input supply is 81% with both the LED strings being run at 20mA. Figure 5 shows the transient response of the circuit to a step in the current of the 4-LED string from 10mA to 20mA. The inductor current transition is smooth and has a well-defined steady state ripple, which results in a lower output voltage ripple. This reduces the size and cost of the output filter capacitor and allows the use of a small 0.47µF (16V, X7R dielectric) 0805 case size ceramic output capacitor. Single Cell Li-Ion-Powered, 20-White-LED Driver Circuit Using all Ceramic Capacitors Large color LCD displays used in present day GPS systems and other handheld devices may require up to 20 white LEDs for backlighting while VOUT1 (ACCOUPLED) 1V/DIV ILI 200mA/ DIV CTRL1 2V/DIV 50µs/DIV Figure 5. Transient response for Figure 3’s circuit. Current in the 4-LED string is stepped from 10mA to 20mA Linear Technology Magazine • May 2004 DESIGN FEATURES 90 3V TO 5V CIN 1µF COUT1 1µF SW1 85 EFFICIENCY (%) L1 68µH L2 68µH VIN SW2 VOUT1 VOUT2 COUT2 1µF LT3466 80 75 70 65 FB1 CTRL1 Lighting up Automotive Instrument Panels: A 50-White-LED Driver Operates from a 12V Supply VIN = 3.6V 10/10 LEDs FB2 RT 60 CTRL2 4 0 8 12 LED CURRENT (mA) 147k 1% Figure 7. Efficiency for Figure 6’s circuit RFB1 RFB2 16.5Ω 16.5Ω CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN UMK325BJ105 L1, L2: TOKO A920CY-680M Figure 6. High efficiency, single cell Li-Ion powered twenty (10/10) white LED circuit uses all ceramic capacitors running off a single Li-Ion cell. The LT3466, with its internal 44V power switches and Schottky diodes, is well suited to drive up to ten white LEDs in series at each output. In order to drive ten white LEDs in series, the converter needs to generate up to a 40V output voltage (the forward voltage drop of a white LED being 3V to 4V). Figure 6 shows 20 white LEDs powered by single cell Li-Ion battery. To drive ten LEDs per output from a single Li-Ion cell, the converter must run at a high duty cycle of 94% (typical). The unique architecture of LT3466 allows it to achieve high duty cycles by switching at a lower frequency. In the circuit shown in Figure 6, the LT3466 is designed to run at a switching frequency of 350kHz. The circuit of Figure 6 uses low profile inductors and all ceramic capacitors. Figure 7 shows the efficiency vs LED current for the circuit. The typical efficiency at 3.6V input supply is 83% with both the LED strings being run at 12mA. If either of the 10-LED strings must be run at greater than 12mA, then it is necessary to power the LT3466 with a higher input supply voltage. The LT3466 is capable of driving 20 white LEDs at 20mA when powered from two Li-Ion cells connected in series. Consult the LT3466 data sheet for more details on the application circuit. The LT3466’s wide input voltage range makes it ideal for automotive applications. White LEDs are commonly used for providing the backlight for automotive instrument panels and car radio displays. In these applications, the white LEDs must be powered by a constant current to guarantee consistent light intensity and uniform brightness. Figure 8 shows the LT3466 powering 50 (two banks of 25) white LEDs from a 12V input supply. The circuit is configured as a voltage tripler to produce output voltages in excess of 90V. This allows a string of 25 LEDs to be connected at each output, resulting in constant current and uniform brightness. In Figure 8, the LT3466 is configured to operate at a 2MHz switching frequency by the choice of the 20.5kΩ R T resistor. This ensures that the radiated switching noise falls outside the AM radio band. High switching frequency also allows the use of lowprofile inductors and surface mount ceramic capacitors. Figure 9 shows the efficiency for the circuit. In this application, LT3466 delivers 2.4W output power with 83% efficiency. The thermally enhanced 3mm × 3mm DFN packaging (with exposed pad) of the continued on page 18 VIN 12V D5 VLED1 C4 0.1µF C5 0.1µF L1 33µH D6 D7 25 LEDs CIN 1µF C2 0.1µF L2 33µH D1 D2 C8 0.1µF C3 0.1µF SW1 VIN SW2 VOUT1 VOUT2 LT3466 FB1 CTRL1 CIN: TAIYO YUDEN EMK316BJ105 C3-C5, C8-C10: TAIYO YUDEN UMK212BJ104 C2, C7: TAIYO YUDEN HMK316BJ104 C6, C11: TAIYO YUDEN UMK316BJ224 D1-D8: PHILIPS BAV99 L1, L2: MURATA LQH32CN330 FB2 RT CTRL2 20.5k 1% VLED2 C9 0.1µF D3 D4 D8 C6 0.22µF RFB1 13.3Ω C7 0.1µF C10 0.1µF 25 LEDs C11 0.22µF RFB2 13.3Ω 3466 TA10a Figure 8. 50 white LEDs powered by a 12V input using low profile surface-mount components Linear Technology Magazine • May 2004 15 DESIGN FEATURES VINA VINB VOCM VINC + – LTC1992-2 – + RG 3V TO 10V RF V2 CF V1 RS 10M VOCM 0.1µF VIN 3 IPD V1 = VINA + VINB – VINC V2 = –VINA + VINB + VINC Figure 4. Easy arithmetic processing of single-ended signals 8 2 IR PHOTODIODE (SFH213FA) namic and has extremes that are 50% of the input swing (VINCM is ±2.5V in the Figure 3 example). The VICM equation above is used with both the upper and lower dynamic VINCM values to verify single-ended operability. 7 1 + – VMID V 6 RG VOUT LTC1992 –OCM+ 0.01µF RS – 5 + 4 The configurable-gain LTC1992 makes it possible to extend input commonmode capability to well outside the supply range by selecting gain below unity and/or introducing commonmode shunt-resistors (see RS in Figure 6). The drawback to the shunt-resistor method is that component tolerances of RG and RS become magnified by approximately the gain of the circuit, leading to reduced CMRR performance for a given resistor tolerance. For low-gain operation, common-mode extension to beyond 35V is realizable with the use of high-accuracy resistor networks. Conclusion The LTC1992 family of differential amplifiers offers easy-to-use building blocks that provide simple, minimum component-count solutions for a wide range of applications, including convenient methods of transforming signals Conclusion The LT3466 is a dual white LED driver designed to drive up to 20 white LEDs from a single Li-Ion input. Integrated power switches, Schottky diodes, and availability in a space-saving (3mm × 3mm) DFN package make LT3466 an excellent fit for handheld applications. The wide operating voltage range and high frequency capability of the LT3466 enables it to meet the backlighting needs for automotive to/from differential form, providing component-free gain, or generating DC level-shifting functions. The versions that include on-chip precision resistors save space and reduce costs by eliminating expensive precision resistor networks. The configurablegain LTC1992 saves cost by allowing single-supply applications to support input signal swings that exceed the supply-voltage window without additional design complexity. VLEDI (AC COUPLED) 1V/DIV VIN = 12V 25/25 LEDs 80 SW1 20V/DIV 75 70 ILI 100mV/ DIV 65 60 0.2µs/DIV 55 50 A • m • VINCM m • VOCM + A+m A+m Figure 6. Extending input common-mode range TRANSIMPEDANCE: VOUT/IPD = 20MΩ COMPENSATION: CF < 1pF 85 RS RG + RS EXAMPLE: ASSUME A = 1, m = 0.1, 5V SUPPLY, VOCM = 2.5V THUS: RF = RG = 30.1k, RS = 3.32k, –3.6V < VINCM < 38V FOR 0.1% RESISTORS, CMRR ≥ 48dB CF EFFICIENCY (%) LT3466 enables it to drive as many as 50 white LEDs from a 12V input supply. Figure 10 shows the switching waveforms for the circuit. VOUT RF R GAIN: A = F RG VICM = 10M LT3466, continued from page 15 LTC1992 – + COMMON MODE SCALING: m = Figure 5. Fully differential transimpedance amplifier topology Common-Mode Input Range Extension VMID + – 0 10 5 LED CURRENT (mA) Figure 10. Switching waveforms for Figure 8’s Circuit. Each set of 25 white LEDs driven at 15mA 15 Figure 9. Efficiency for Figure 8’s circuit instrument panels and car radio displays as well. Features like internal soft-start, open LED protection and internal loop compensation reduce the number of external components, thus reducing the overall cost and size of the white LED driver circuit. To view this and past issues of LT Magazine online, see http://www.linear.com/go/ltmag 18 Linear Technology Magazine • May 2004