DESIGN FEATURES Current-Limited DC/DC Converter Simplifies USB Power Supplies by Bryan Legates Introduction currents when the device is plugged into the USB port; when first plugged in, the device must draw less than 100mA from the port and, for high power devices, the current drawn from the port can increase to 500mA only after it is given permission to do so by the USB controller. These requirements can be easily met using the Many portable Universal Serial Bus (USB) devices power themselves from the USB host or hub power supply when plugged into the USB port. Several requirements must be met to ensure the integrity of the bus: the USB specification dictates that the input capacitance of a device must be less than 10µF to minimize inrush L1 10µH C1 4.7µF 3 VOUT 12V 2 7 ISN SW R1 909k ISP LT1618 8 3.3V OFF ON 0V 90 D1 9 1 FB VIN C2 4.7µF R2 107k SHDN IADJ GND 20k 4 VC 5 13k 0 20 40 60 80 100 120 140 160 LOAD CURRENT (mA) 10nF Figure 2. USB to 12V boost efficiency (408) 573-4150 (602) 244-6600 (847) 956-0667 3 C3 0.47µF L1 10µH 0.1Ω C1 4.7µF VOUT 2V/DIV 8 9 20k D1 VOUT 5V 2 7 ISN SW L2 10µH ISP LT1618 5 VC C2 10µF Figure 4. USB to 5V SEPIC during start-up 10 2k 13k 1ms/DIV R2 107k SHDN IADJ GND IIN 50mA/DIV R1 316k 1 FB VIN 4 3.3V 100mA 500mA 0V 70 65 Figure 1. USB to 12V boost converter with selectable 100mA/500mA current limit 3.3V OFF ON 0V 75 10 2k C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN EMK316BJ475 D1: ON SEMICONDUCTOR MBR0520 L1: SUMIDA CR43-100 VIN 5V 80 60 3.3V 100mA 500mA 0V IIN 85 EFFICIENCY (%) 0.1Ω VIN 5V LT1618 DC/DC converter, which provides an accurate input current control ideal for USB applications. The LT1618 combines a traditional voltage feedback loop with a unique current feedback loop to operate as a constant-current, constant-voltage source. VOUT 2V/DIV 10nF 50mA/DIV C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN JMK316BJ106 C3: TAIYO YUDEN EMK212BJ474 D1: ON SEMICONDUCTOR MBR0520 L1: SUMIDA CR43-100 (408) 573-4150 (408) 573-4150 (408) 573-4150 (800) 282-9855 (847) 956-0666 Figure 3. USB to 5V SEPIC converter 6 1ms/DIV Figure 5. USB to 5V SEPIC start-up with shorted output Linear Technology Magazine • February 2001 DESIGN FEATURES 80 L1 10µH VIN 2.7V TO 5V D1 2.49Ω 20mA EFFICIENCY (%) 75 C1 4.7µF 9 70 10kHz TO 50kHz PWM BRIGHTNESS ADJUST 65 60 8 7 VIN SW SHDN ISP 50 100 150 200 250 LOAD CURRENT (mA) 300 350 R3 5.1k LT1618 4 IADJ GND USB to 12V Boost Converter Figure 1 shows a 5V to 12V boost converter ideal for USB applications. The converter has a selectable 100mA/500mA input current limit, allowing the device to be easily switched between the USB low and high power modes. Efficiency, shown 80 75 VIN = 4.2V EFFICIENCY (%) 70 VIN = 3.3V 65 VIN = 2.7V 60 55 50 45 40 0 5 10 15 20 LED CURRENT (mA) Figure 8. Li-Ion white LED driver efficiency Linear Technology Magazine • February 2001 R1 2M 1 FB VC 10 C2 1µF C3 0.1µF R2 100k CC 0.1µF Figure 6. USB to 5V SEPIC efficiency In addition to providing an accurate input current limit, the LT1618 can also be used to provide an accurately regulated output current for current-source applications. Driving white LEDs is one application for which the device is ideally suited. With an input voltage range of 1.6V to 18V, the LT1618 works from a variety of input sources. The 36V switch rating allows output voltages of up to 35V to be generated, easily driving up to eight white LEDs in series. The 1.4MHz switching frequency allows the use of low profile inductors and capacitors, which, along with the LT1618’s MSOP-10 package, helps to minimize board area. 2 ISN 5 0 3 C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN TMK316BJ105 D1: ON SEMICONDUCTOR MBR0530 L1: SUMIDA CLQ4D10-100 (408) 573-4150 (408) 573-4150 (800) 282-9855 (847) 956-0666 Figure 7. Li-Ion white LED driver in Figure 2, exceeds 85%. If the load demands more current than the converter can provide with the input current limited to 100mA (or 500mA), the output voltage will simply decrease and the LT1618 will operate in constant-current mode. For example, with an input current limit of 100mA, about 35mA can be provided to the 12V output. If the load increases to 50mA, the output voltage will reduce to approximately 8V to maintain a constant 100mA input current. USB to 5V SEPIC Converter with Short-Circuit Protection Unlike boost converters, SEPICs (single-ended primary inductance) converters) have an output that is DC-isolated from the input, so an input current limit not only helps soft start the output, but also provides excellent short-circuit protection. The 5V SEPIC converter shown in Figure 3 is ideal for applications that need the output voltage to go to zero during shutdown. The accurate input current limit ensures USB device compliance even under output fault conditions. Figure 4 shows the startup characteristic of the SEPIC converter with a 50mA load. By limiting the input current to 100mA, the output is effectively soft started, smoothly increasing and not overshooting its final 5V value. Figure 5 shows that the input current does not exceed 100mA even with the out- put shorted to ground (thus the flat output voltage waveform in the oscilloscope photo). Efficiency is shown in Figure 6. This converter also has a selectable input current limit of either 100mA or 500mA, making it ideal for high power USB applications. Li-Ion White LED Driver The circuit in Figure 7 is capable of driving six white LEDs from a single Li-Ion cell. LED brightness can be easily adjusted using a pulse width modulated (PWM) signal, as shown, or using a DC voltage to drive the IADJ pin directly, without the R3, C3 lowpass filter. If brightness control is not needed, simply connect the IADJ pin to ground. The typical output voltage is about 22V and the R1, R2 output divider sets the maximum output voltage to around 26V to protect the LT1618 if the LEDs are disconnected. The LT1618’s constant current loop regulates 50mV across the 2.49Ω sense resistor, setting the LED current to 20mA. Efficiency for this circuit, shown in Figure 8, exceeds 70%, which is significantly higher than the 30% to 50% efficiencies obtained when using a charge pump for LED drive. No current flows in the LEDs when the LT1618 is turned off. Their high forward voltages prevent them from turning on, ensuring a true low current shutdown with no excess battery leakage or light output. continued on page 23 7 DESIGN FEATURES 1MHz Series-Resonant Crystal Oscillator with Square and Sinusoid Outputs Figure 5 shows a classic 1MHz seriesresonant crystal oscillator. At series resonance, the crystal is a low impedance and the positive feedback connection brings about oscillation at the series resonance frequency. The RC feedback to the – input ensures that the circuit does not find a stable DC operating point and refuse to oscillate. The comparator output is a 1MHz square wave (top trace of Figure 6), with jitter measured at 28psRMS on a 5V supply and 40 psRMS on a 3V supply. At pin 2 of the comparator, on the other side of the crystal, is a clean sine wave except for the presence of the small high frequency glitch (middle trace of Figure 6). This glitch is caused by the fast edge of the comparator output feeding back through crystal capacitance. Amplitude stability of the sine wave is maintained by the is the bottom trace of Figure 6. Distortion was measured at –70dBc and –55dBc on the second and third harmonics, respectively. A 3V/DIV B 1V/DIV Conclusion C 1V/DIV 200ns/DIV Figure 6. Oscillator waveforms with VS = 3V: Trace A = comparator output; Trace B = crystal feedback to pin 2 of the LT1713; Trace C = buffered, inverted and bandpass filtered output of LT1806 fact that the sine wave is basically a filtered version of the square wave. Hence, the usual amplitude-control loops associated with sinusoidal oscillators are not necessary.2 The sine wave is filtered and buffered by the fast, low noise LT1806 op amp. To remove the glitch, the LT1806 is configured as a bandpass filter with a Q of 5 and unity gain center frequency of 1MHz. The final sinusoidal output The fully differential rail-to-rail inputs of the new LT1711 family of fast comparators make them useful across a wide variety of applications. The high speed, low jitter performance of this family, coupled with their small package sizes and 2.4V operation, makes them attractive where PCB real estate is at a premium and bandwidth-topower ratios must be optimized. 1 Using the design value of R2 + R3 = 2.653k rather than the implementation value of 2.55k + 124Ω = 2.674k. 2 Amplitude will be a linear function of comparator output swing, which is supply dependent and therefore adjustable. The important difference here is that any added amplitude stabilization or control loop will not be faced with the classical task of avoiding regions of nonoscillation vs clipping. LT1618, continued from page 7 L1 10µH VIN 2.7V TO 5V D1 0.619Ω 80mA 90 85 9 10kHz TO 50kHz PWM BRIGHTNESS ADJUST 8 7 VIN SW SHDN ISP ISN R3 5.1k 4 VIN = 5V 80 3 2 R1 2M LT1618 IADJ FB VC GND 5 1 C2 1µF CC 0.1µF Linear Technology Magazine • February 2001 VIN = 2.8V 65 50 51Ω 51Ω 51Ω 51Ω 10 20 30 40 50 60 70 80 LED CURRENT (mA) Figure 10. High power white LED driver efficiency Figure 9. High power white LED driver For larger LCD displays where a greater amount of light output is needed, multiple strings of LEDs can be driven in parallel. When driving parallel strings, ballast resistors should be added to compensate for LED forward voltage variations. The amount of ballasting needed depends on the LEDs used and how well they 70 55 R2 121k (408) 573-4150 (408) 573-4150 (800) 282-9855 (847) 956-0666 High Power White LED Driver VIN = 3.3V 75 60 10 C3 0.1µF C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN TMK316BJ105 D1: ON SEMICONDUCTOR MBR0530 L1: SUMIDA CR43-100 EFFICIENCY (%) C1 4.7µF are matched. The circuit in Figure 9 is ideal for larger displays, providing constant current drive for twenty white LEDs from a single Li-Ion cell. Efficiency reaches a respectable 82%, as seen in Figure 10. Conclusion The constant-current/constant-voltage operation of the LT1618 makes the device an ideal choice for a variety of constant-current designs. The device provides accurate output current regulation or input current limiting, along with excellent output voltage regulation. With a wide input voltage range and the ability to produce outputs up to 35V, the LT1618 works well in many different applications. References 1. Kim, Dave. “Tiny Regulators Drive White LED Backlights.” Linear Technology Design Note 231 (May 2000). 23