DESIGN IDEAS LT1610 Micropower Step-Up DC/DC Converter Runs at 1.7MHz by Steve Pietkiewicz Single-Cell to 3V The LT1610’s input voltage ranges DC/DC Converter Introduction When designing portable electronics, be it a pager, handheld computer or cell phone, “footprint” is one of the most important specifications of any component. Most such products use at least one DC/DC converter to generate regulated voltages from a battery. The LT1610, a micropower DC/DC converter IC, addresses the issue of footprint in several ways. First, the switching frequency is 1.7MHz, allowing the use of small, inexpensive, minimal-height inductors and capacitors. Second, the frequencycompensation components have been integrated, eliminating the requirement for an external RC network in most applications. Finally, the device comes in LTC’s 8-lead MSOP package, one-half the size of the 8-lead SO package. L1 4.7µH 3 + 1 CELL C1 22µF 5 SW FB SHDN COMP GND VC VOUT 3V 30mA R1 1M 2 LT1610 8 continued on page 35 D1 6 VIN 7 R2 681k + C2 22µF VOUT 50mV/DIV DC OFFSET I L1 100mA/DIV 31mA 1mA ILOAD PGND 4 1 A 1V to 3V boost converter is shown in Figure 1. The specified components take up very little board space. The 4.7µ H Murata inductor specified measures 2.5mm by 3.2mm and is only 2mm high. The 22µF AVX “A” case tantalum capacitors measure 1.6mm by 3.2mm and are 1.6mm tall. Circuit efficiency, which reaches 77%, is detailed in Figure 2. Transient response to a 1mA to 31mA load step is pictured in Figure 3. The device features Burst Mode operation at light loads. This can be seen at a load of 1mA. When the load is increased to 31mA, the device shifts to constantfrequency switching and peak switch current is controlled to achieve output regulation. from 1V to 8V, and the 30V, 300mA switch allows several different configurations, such as boost, SEPIC and flyback, to be successfully implemented. Output voltage can be up to 28V in boost mode. Operating quiescent current is 50µ A unloaded; grounding the shutdown pin reduces the current to 0.5µA. The device can generate 3V at 30mA from a single (1V) cell, or 5V at 100mA from two cells (2V). Configured as a Li-Ion cell to 3.3V SEPIC converter, the LT1610 can deliver 100mA. In boost mode, efficiency ranges from 60% at a 100µA load to 83% at full load. 500µs/DIV VIN = 1.25V VOUT = 3V C1, C2: AVX TAJA226M010R D1: MOTOROLA MBR0520 L1: MURATA LQH3C4R7M24 1610 TA01 Figure 3. Transient load response of single-cell converter, load stepped from 1mA to 31mA Figure 1. Single cell to 3V converter delivers 30mA. L1 4.7µH D1 VOUT 5V/100mA 85 VOUT = 3V 6 80 VIN = 1.25V VIN VIN = 1.5V 3 EFFICIENCY (%) 75 + 70 5 SW VIN = 1V 2 CELLS 65 C1 15µF FB SHDN 1M 2 332k LT1610 8 COMP VC 60 1 GND 7 + C2 15µF PGND 4 55 50 0.1 1 10 LOAD CURRENT (mA) 100 1610 TA02 Figure 2. Single-cell converter efficiency reaches 77%. 32 C1, C2: AVX TAJA156M010R D1: MOTOROLA MBR0520 L1: SUMIDA CD43-4R7 MURATA LQH3C4R7M24 1610 TA04 Figure 4. 2 cell to 5V converter delivers 100mA at 2V input. Linear Technology Magazine • May 1998 DESIGN IDEAS Although single-tone distortion measurements are a good indicator of circuit performance in single-carrier applications, they do not provide any insight into amplifier linearity when processing more that one tone at a time. An effective tool in gauging dynamic performance in these applications is 2-tone intermodulation. Figure 4 illustrates the performance of Figure 1’s circuit with two sine waves at 600kHz and 700kHz. The frequency spectrum displayed is representative of both DMT and CAP downstream operation, and the two tones were chosen to show both 2nd and 3rd order IMD products (2IMD and 3IMD) that fall in-band. With a 1:1 turns-ratio transformer, the output level of the circuit was adjusted to produce an 18.9VP-P envelope across the 100Ω load. This output voltage level implies a peak differential voltage across the line driver outputs of an amplifier’s dynamic power dissipation. The supply voltage should not be reduced below a level that causes the amplifier output stage to clip the peak transmitted signal, however. The best method for gauging dynamic performance is to monitor the biterror-rate (BER) performance of the modem. Under normal DMT or CAP operation (downstream or upstream), the supply voltage and quiescent currents of the line-driver amplifiers can be reduced until the system BER degrades beyond an acceptable minimum. For additional information on a complete line of driver solutions, featuring the LT1210 (1.1A), the LT1206 (250mA) and the LT1497 (125mA), please consult the LTC factory. approximately 38V P-P. With each amplifier operating at a supply current of 13mA, the circuit achieves a spur-free dynamic performance of 63dBc, sufficient for peak power operation in CAP-based systems. Improved performance at lower supply currents can be achieved with a transformer turns ratio greater than 1:1, whereby amplifier output current drive is substituted for amplifier output voltage drive. Conclusion Under DC voltage or digital control, the quiescent supply current of the line-driver CFAs can be adjusted (statically or dynamically) to reduce their static power dissipation without sacrificing either downstream or upstream dynamic performance. In addition, this supply-current control can be coupled with a reduction of the line-driver supply voltage to reduce Note: 1 Hoskins, Kevin. “The LT1207: An Elegant Dual 60MHz, 250mA Current Feedback Amplifier.” Linear Technology VI:2 (May 1996), pp. 9–13. LT1610, continued from page 32 2-Cell to 5V DC/DC Converter Li-Ion to 3.3V SEPIC Converter By simply changing the feedback resistor values, the LT1610 can generate 5V. Figure 4’s circuit generates 5V at a load of up to 100mA from a 2-cell input. Figure 5’s graph shows efficiency the of the circuit, which reaches 83%. This circuit is also suitable for 3.3V to 5V conversion, supplying over 200mA. Figure 6 employs the SEPIC (single ended primary inductance converter) topology to provide a regulated 3.3V output from an input that can range above or below the output voltage. Although the circuit requires two inductors and a ceramic coupling capacitor, the total footprint of this solution is still attractive compared with alternative methods of generating 3.3V, such as a boost converter followed by a linear regulator. The circuit can supply up to 100mA. Efficiency, while lower than that of a standard boost converter, reaches approximately 73%. Unlike a boost converter, this topology provides input-to-output isolation. The output is completely disconnected from the battery in shutdown mode, preventing inadvertent battery discharge through the load. The LT1610’s subµA shutdown current reduces standby losses, increasing battery life. 90 VIN = 3V EFFICIENCY (%) 80 VIN = 2V INPUT Li-ION 3V to 4.2V 6 VIN VIN = 1.5V 70 1 + 60 50 C1 22µF 6.3V 5 SW 1 10 100 LOAD CURRENT (mA) 1000 1610 TA05 Figure 5. 2-cell converter efficiency reaches 83%. 2 COMP 7 C1, C2: AVX TAJA226M010R C3: AVX 1206YG475 D1: MOTOROLA MBR0520 L1, L2: MURATA LQH3C4R7M24 VOUT 3.3V 100mA L2 4.7µH 604k LT1610 8 D1 1M FB VC GND 0.1 C3 4.7µF CERAMIC L1 4.7µH SHDN 3 + C2 22µF 6.3V PGND 4 1610 TA06 SHUTDOWN Figure 6. Li-Ion to 3.3V SEPIC converter delivers 100mA. Linear Technology Magazine • May 1998 35