DESIGN FEATURES Micropower 600kHz Fixed-Frequency DC/DC Converters Step Up from a 1-Cell or 2-Cell Battery by Steve Pietkiewicz Linear Technology introduces two new micropower DC/DC converters designed to provide power from a single-cell or higher input voltage. The LT1308 features an onboard switch capable of handling 2A with a voltage drop of 300mV and operates from an input voltage as low as 1V. The LT1317, intended for lower power requirements, operates from an input voltage as low as 1.5V. Its internal switch handles 600mA with a drop of 360mV. Both devices feature Burst Mode operation at light load; efficiencies are above 70% for load currents of 1mA. Both devices switch at 600kHz; this high frequency keeps associated power components small and flat; additionally, troublesome interference problems in the sensitive 455kHz IF band are avoided. The LT1308 is intended for generating power on the order of 2W–5W. This is sufficient for RF power amplifiers in GSM or DECT terminals or for digitalcamera power supplies. The LT1317, with its smaller switch, can generate 100mW to 2W of power. The LT1317 is available in LTC’s smallest 8-lead package, the MSOP. This package is approximately one-half the size of a standard 8-lead SO package. The LT1308 is available in the 8-lead SO package. Single Li-Ion Cell to 5V/1A DC/DC Converter for GSM GSM terminals have emerged as a worldwide standard. A common requirement for these products is an efficient, compact, step-up converter to develop 5V from a single Li-Ion cell to power the RF amplifier. The LT1308 performs this function with a minimum of external components. The circuit is detailed in Figure 1. Many designs use a large aluminum electrolytic capacitor (1000µF to 3300µF) at the DC/DC converter output to hold up the output voltage during the transmit time slice, since the amplifier can require more than 1A. The 3V TO 4.2V SHDN VIN LBI SW C1 100µF LBO NiCD CELL 5V 1A FB VC C1 10µF LBO R2 100k + + C2 100µF 3.3V 400mA R2 100k + C1: CERAMIC C2: AVX TPS SERIES D1: IR 10BQ015 L1: COILTRONICS CTX5-1 COILCRAFT DO3316-472 1308_01,eps Figure 1. Single Li-Ion cell to 5V/1A DC/DC converter C2 100µF 1308_04.eps Figure 4. Single NiCd cell to 3.3V/400mA DC/DC converter 90 95 V IN = 3.6V VIN = 1.2V VOUT = 3.3V R1 = 169k 85 V IN = 4.2V 80 VOUT 200mV/DIV AC COUPLED 85 EFFICIENCY (%) 90 EFFICIENCY (%) D1 GND RC 47k CC 22nF 2200µF C1,C2: AVX TPS SERIES D1: INTERNATIONAL RECTIFIER 10BQ015 L1: COILTRONICS CTX5-1 COILCRAFT DO3316-472 80 V IN = 3V 75 INDUCTOR CURRENT 1A/DIV 70 75 70 65 60 1ms/DIV 65 R1 169k FB VC GND RC 47k CC 22nF SW LT1308 D1 L1 4.7µH VIN SHDN LBI R1 301k LT1308 Li-Ion CELL L1 4.7µH 55 50 1 10 100 LOAD CURRENT (mA) 1 1000 10 100 LOAD CURRENT (mA) 1000 1308 G01 1308 F01a Figure 2. Efficiency of Figure 1’s circuit reaches 90% 8 Figure 3. Transient response of DC/DC converter: VIN = 3V, 0A–1A load step Figure 5. Efficiency of Figure 4’s circuit reaches 81% Linear Technology Magazine • February 1998 DESIGN FEATURES VOUT 200mV/DIV AC COUPLED VOUT 200mV/DIV AC COUPLED IL1 1A/DIV ILOAD 400mA 50mA ILOAD 400mA 50mA 100µs/DIV 20ms/DIV Figure 6. DECT load transient response: with a single NiCd cell, the LT1308 provides 3.3V with a 400mA pulsed load. The pulse width = 416µs. output capacitor, along with the LT1308 compensation network, serves to smooth out the input current demanded from the Li-Ion cell. Efficiency, which reaches 90%, is shown in Figure 2. Transient response of a 0A to 1A load step with typical GSM profiling (1:8 duty cycle, 577µs pulse duration) is depicted in Figure 3. Voltage droop (top trace) is 200mV. Inductor current (bottom trace) increases to 1.7A peak; the input capacitor supplies some of this current, with the remainder drawn from the Li-Ion cell. Efficiency, reaching 81% from a 1.2V input, is pictured in Figure 5. Transient response of a typical DECT load of 50mA to 400mA is detailed in Figure 6. Output voltage droop (top trace) is under 200mV. Figure 7 zooms in on a single pulse to show the output voltage and inductor current responses more clearly. 2-Cell Digital Camera Supply Produces 3.3V, 5V, 18V and –10V Power supplies for digital cameras must be small and efficient while generating several voltages. The DSP and logic need 3.3V, the ADC and LCD display need 5V and biasing for the CCD element requires 18V and –10V. The power supplies must also be free of low frequency noise, so that postfiltering can be done easily. The obvious approach, to use a separate DC/DC converter IC for each output voltage, is not cost-effective. A single Single NiCd Cell to 3.3V/ 400mA Supply for DECT Only minor changes are required in Figure 1’s circuit to construct a singlecell NiCd to 3.3V converter. The large output capacitor is no longer required as the output current can be handled directly by the LT1308. Figure 4 shows the DECT DC/DC converter circuit. 8 VIN C1 + 100µF C6 10µF 2 SW VC C8 1nF R4 47k C7 22nF 90 3 L1C 3 N = 0.3 R3 340k SHDN LT1308 85 L1B N = 0.7 D1 D2 4 FB GND R1 100k R2 2.01M 80 5V 200mA + C2 100µF + 3.3V 200mA C3 100µF D3 CCD BIAS 18V 10mA 7 L1D N = 3.5 + 6 D1, D2 = IR 10BQ015 D3, D4 = BAT-85 L1 = COILTRONICS CTX02-13973 + L1E N=2 5 1308_08.eps D4 Figure 8. This digital camera power supply delivers 5V/200mA, 3.3V/200mA, 18V/ 10mA and –10V/10mA from two AA cells. Linear Technology Magazine • February 1998 C4 10µF 100mA LOADS 75 70 65 150mA LOADS 60 6 C1, C2, C3 = AVX TPS C4, C5 = AVX TAJ C6 = CERAMIC LT1308, along with an inexpensive transformer, generates 3.3V/200mA, 5V/200mA, 18V/10mA and –10V/ 10mA from a pair of AA or AAA cells. Figure 8 shows the circuit. A coupledflyback scheme is used, actually an extension of the SEPIC (single ended primary inductance converter) topology. The addition of capacitor C6 clamps the SW pin, eliminating a snubber network. Both the 3.3V and 5V outputs are fed back to the LT1308 FB pin, a technique known as split feedback. This compromise results in better overall line and load regulation. The 5V output has more influence than the 3.3V output, as can be seen from the relative values of R2 and R3. Transformer T1 is available from Coiltronics, Inc. (561-241-7876). Efficiency vs input voltage for several load currents on both 3.3V and 5V outputs is pictured in Figure 9. The CCD bias voltages are loaded with 10mA in all cases. EFFICIENCY (%) VIN 1.6V TO 6V L1A N=1 10µH 1 Figure 7. DECT load transient response: faster sweep speed (100µs/DIV) details VOUT and inductor current of a single DECT transmit pulse. C5 10µF CCD BIAS –10V 10mA 200mA LOADS 55 50 1 1.5 2 2.5 3 3.5 4 INPUT VOLTAGE (V) 4.5 5 1308_09.EPS Figure 9. Camera power supply efficiency reaches 78%. 9 DESIGN FEATURES LT1317 2-Cell to 5V DC/DC Converter Figure 10 shows a simple 2-cell to 5V DC/DC converter using the LT1317. This device generates a clean, low ripple output from an input voltage as low as 1.5V. Designed for 2-cell applications, it offers better performance than its 1-cell predecessor, the LT1307. More gain in the error amplifier results in lower Burst Mode ripple, and an internal preregulator eliminates oscillator variation with input voltage. For comparison, Figure 11 details transient responses of both the LT1307 and the LT1317 generating 5V from a 3V input. The load step is 5mA to 200mA. Output capacitance in both cases is 33µF. The LT1307 has low frequency ripple of 100mV, whereas the LT1317 Burst Mode ripple of 20mV is the same as the 600kHz ripple resulting from the output capacitor’s ESR with a 200mA load. pass through C1. Since C1 is ceramic, its ESR is low and there is no appreciable efficiency loss. C5 is charged to –VOUT when the switch is off, then its bottom plate is grounded when the switch turns on. The negative output is fairly well regulated, since the diode drops tend to cancel. The circuit is switching continuously at rated load, where efficiency is 75%. Output ripple is under 40mV and can be reduced further with conventional postfiltering techniques. Single Li-Ion Cell to ±4V DC/DC Converter By again employing the SEPIC topology, a ±4V supply can be designed with one IC. Figure 12’s circuit generates 4V at 70mA and –4V at 10mA from an input voltage ranging from 2.5V to over 5V. Maximum component height is 2mm. This converter uses two separate inductors (L1 and L2), so it is an uncoupled SEPIC converter. This reduces the overall cost, but requires that all output current Conclusion The LT1308 and LT1317 provide low noise compact solutions for contemporary portable-product power supplies. SHUTDOWN SW LBI 2 CELLS C1 10µF 10V VOUT LT1307 100mV/DIV 5V OFFSET L1 22µH VIN SHDN R1 1M LT1317 LBO D1 VC VOUT LT1317 100mV/DIV 5V OFFSET 5V 200mA FB GND R2 324k 1% RC 100k CC 680pF + C2 33µF ILOAD 200mA 5mA 500µs/DIV C1: CERAMIC D1: MOTOROLA MBRO520L L1: 22 µH SUMIDA CD43-220 1308_10.eps Figure 11. The LT1317 has reduced Burst Mode ripple compared to the LT1307. Figure 10. 2-cell to 5V boost converter using the LT1317 D2A D2B –VOUT –4V/10mA VIN 2.5V–5V C5 1µF SHDN SHUTDOWN C1 10µF SW VIN C3 15µF LB1 LT1317 D1 R1 1M +VOUT 4V/70mA FB LB0 VC C4 1µF + L1 22µH GND + R3 47k R2 442k C2 33µF L2 22µH C6 680pF L1, L2 =MURATA LQH3C220 C1 =MURATA GRM235Y5V106Z01 D1 =MBR0520 D2 =BAT54S (DUAL DIODE) C2 =AVX TAJB33M6010 C3 =AVX TAJA156MO1O C4, C5 =CERAMIC Figure 12. This single Li-Ion cell to ±4V DC/DC converter has a maximum height of 2mm. 10 Linear Technology Magazine • February 1998