DESIGN FEATURES Constant Current from 3A DC/DC Converter with 2 Rail-to-Rail by Daniel Chen Current Sense Amplifiers Introduction Traditional DC/DC converters use voltage feedback for constant output voltage regulation. There are many applications, however, that need to regulate a constant output current. Driving LEDs in series is one such application. The LT3477 combines a traditional voltage feedback loop and two unique current feedback loops to operate as a constant-current, constant-voltage source. It is a current mode, 3A DC/DC converter with dual rail-to-rail 100mV current sense amplifiers that can be configured as a buck mode or buck-boost mode LED driver. It is versatile enough to also be configured as an input-output current limited boost, SEPIC or inverting converter. Both current sense voltages can be adjusted independently using the IADJ1 and IADJ2 pins. With two identical precision current sense amplifiers, the LT3477 can provide an accurate input current limit as well as an accurately regulated output current. With an input voltage range of 2.5V to 25V, the LT3477 works from a variety of input sources. The 42V switch rating allows an output voltage of up to 41V to be generated, easily The unique feature of the three-feedback-loop topology (two current and one voltage) is that it can support constant voltage and/or constant current applications. VADJ – + + A1 VADJ – + + A2 IA1 – + IA2 – IADJ2 FBP + FBN – VREF A3 – + A4 R S VA Σ SLOPE VREF 1.25V Q1 Q – ISN2 SW VC + IADJ1 ISP2 Figure 1 shows a block diagram of the LT3477. The voltage error amplifier has both FBP and FBN pins to allow a positive or negative output configuration. With the addition of two current feedback control loops, amplifier A3 becomes a summing point for three feedback loops. Depending on configuration, any of the loops can take over feedback control by sourcing or sinking current at the VC node. The unique feature of the three-feedbackloop topology (two current and one voltage) is that it can support constant voltage and/or constant current applications. + ISN1 How It Works driving up to ten white LEDs in series. The buck mode LED configuration is capable of driving multiple ten-LED strings in parallel if external current mirroring circuitry is added. The switching fequency is adjustable from 200kHz to 3.5Mhz, set by SS ISP1 a single resistor. The available high operating frequencies allow the use of low profile inductors and capacitors—important in applications where space is a premium. The wide available range makes it possible to optimize size and efficiency for your application. OSCILLATOR SHDN VIN RT Figure 1. LT3477 block diagram Linear Technology Magazine • December 2005 25 DESIGN FEATURES 120 PVIN 32V VCM = 10V C1 2.2µF R1 0.1Ω VOLTAGE SENSE (mV) 100 80 D2 1A 60 D5 • • • LED STRING C2 1µF 40 L1 20 0 0 100 200 300 400 500 600 700 800 IADJ VOLTAGE (mV) VIN 3.3V Figure 2. Current sense amplifier voltage sense level vs IADJ pin voltage R2 1k D2 ISN1 R1 0.1Ω D1 R5 200k SW1 ISN2 FBP RT GND SS C4 33nF R3 22k Schottky diode is connected between the SW and PVIN nodes. With high side current sense, the boost converter is effectively converted into a buck LED converter, which increases the part’s power handling capability. In addition, the VIN pin, which provides the chip operating current, can be tied to a lower voltage level such as 3.3V. As a result, the power consumption on the chip itself is also reduced, thus improving overall efficiency. Over 90% efficiency can be readily achieved with a wide range of inductor and frequency selections. FBN 90 85 LT3477 ISP2 ISN2 VREF RT GND C3 10nF SS C4 33nF VIN = 8V 80 EFFICIENCY (%) VC FBP ISP2 VREF LED drivers use a grounded current sense resistor to regulate current, but the LT3477 current sense amplifiers work in a high side sense scheme, so the sensed voltage for current feedback no longer needs to be ground referred. In buck mode configuration, the sense resistor is placed right at the input supply. The LEDs are placed between the sense resistor and the inductor and the VIN IADJ1 IADJ2 SHDN LT3477 Figure 3. Buck mode high current LED driver L1 4.7µH SHDN R6 10k C1: NIPPON UNITED CHEMICON NTS40X5R1H225M C2: TAIYO YUDEN GMK316BJ105ML C3: TAIYO YUDEN LMK316BJ475 L1: TOKO D1OFA814AY-330M D1: DIODES INC DFLS140 D3 LED BRIGHTNESS CONTROL 0mV TO 650mV FBN C5 4.7nF Buck Mode High Current LED Driver Figure 3 shows a typical application to drive high current LEDs. Traditionally, ISP1 VIN IADJ1 IADJ2 SHDN R5 309k SW ISN1 VC Applications C1 3.3µF C3 3.3µF SHDN Current sense levels are adjustable via sense resistors at the IADJ1 and IADJ2 pins. The default sense voltage is 100mV for each current sense amplifier if the IADJ1 and IADJ2 pins are tied to a potential higher than 650mV. If the potentials at the IADJ1 and IADJ2 pins are lower than 625mV, the LT3477 linearly adjusts the current sense level. Figure 2 shows the voltage sense level vs the IADJ pin voltage. For LED drivers, IADJ1 and IADJ2 pins can be used to adjust LED current levels. Rail-to-rail current sense amplifiers allow flexible current sense schemes. VIN 2.7V TO 16V ISP1 D1 R3 18k R6 10k C2 4.7µF 75 VIN = 4.2V 70 65 60 55 C1: TAIYO YUDEN LMK316BJ335ML C2: MURATA GRM31CR71E475KA88L D1: DIODES, INC. B320A L1: TOKO FDV0630-4R7M Figure 4. Buck-boost LED driver 26 50 0 0.2 0.4 0.6 IOUT (A) 0.8 1.0 Figure 5. Buck-boost LED driver efficiency Linear Technology Magazine • December 2005 DESIGN FEATURES L2 10µH C1 3.3µF D1 VIN IADJ1 IADJ2 R1 10k LT3477 ISP2 VC RT GND FBP SS C3 33nF C4 4.7nF 330mA R6 0.3Ω ISN2 VREF R4 1k 80 FBN SHDN SHDN 85 C2 3.3µF R2 200k SW ISN1 ISP1 90 EFFICIENCY (%) VIN 5V R3 22k LED1 55 LED2 50 LED3 LED4 VIN 3V TO 16V C1 3.3µF ISP1 ISN1 VIN IADJ1 IADJ2 SHDN SHDN 0.4 0.3 R4 0.15Ω 5.5V 670mA R5 34.8k L2 4.7µH 5.5V SEPIC Converter with Short-Circuit Protection Certain applications demand a converter output that is DC-isolated from the input. SEPICs (single-ended primary inductance converters) provide the solution. Figure 8 is an implementation which provides a 5.5V output with complete short-circuit protection. The current sense amplifier used for current sense not only provides excellent short-circuit protection, but also helps soft start the output. The accurate output current limit ensures the maximum current is set at 670mA. When the load demands more, the output voltage will droop while the 670mA output current is maintained. Efficiency is shown in Figure 9. Cuk Converter The LT3477 provides pins for both inputs to the voltage error amplifier, which enables negative output voltages. Figure 10 is an implementation continued on page 40 FBN LT3477 0.2 Voltage feedback is used for open LED protection. D1 SW 0.1 Figure 7. 4W LED driver efficiency 330mA LED Driver with Open LED Protection LT3477 can also be used for LED driver applications using a conventional boost topology with the current sense amplifier for current regulation. Figure 6 shows a typical application circuit, and Figure 7 shows the efficiency. Figure 6 uses a high side current sense configuration for feedback control. The current sense amplifier could also be used for a grounded current sense for this application, if desired, so the output can be tied to the LED string directly. ISP2 would be tied to the cathode side of the LEDs, and ISN2 is tied to ground. C2 10µF 0 IOUT (A) Figure 6. 4W LED driver L1 4.7µH 70 65 60 C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN TMK325BJ335MN D1: DIODES INC. DFLS120L L1: TOKO A915AY-100M Buck-Boost LED Driver In some applications, the input voltage might be comparable to the total LED voltage drop or the input voltage might fluctuate to higher or lower than the total LED voltage drop. A buckboost LED driver works well in this type of application. Figure 4 shows the LT3477 buck-boost LED driver. The cathode end of the LED string is tied back to the input voltage, which allows it to operate from a wide input voltage range. R5 and R6 in Figure 4 are used for open LED protection. Figure 5 is the efficiency measured for this circuit. 75 90 VIN = 3V 85 ISP2 ISN2 VREF R2 1k FBP RT GND SS C5 4.7nF C4 33nF R3 18.2k C3 10µF C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN LMK325BJ106MN C3: TAIYO YUDEN LMK316BJ106ZL D1: DIODES INC. DFL5120L L1, L2: TOKO FDV0630-4R7M Figure 8. 5.5V SEPIC converter with short-circuit protection Linear Technology Magazine • December 2005 R6 10k EFFICIENCY (%) 80 VC 75 70 65 60 55 50 0 0.1 0.2 0.3 0.4 IOUT (A) 0.5 0.6 0.7 Figure 9. 5.5V SEPIC converter with short-circuit protection efficiency 27 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