DESIGN IDEAS Single-Inductor, Positive-Output Buck/Boost Converter Uses No RSENSE Controller by Christopher B. Umminger DESIGN IDEAS Single-Inductor, Positive-Output Buck/Boost Converter Uses No R SENSE Controller .....................28 Christopher B. Umminger Designing the L T1167 Instrumen tation Amplifier into a Single 5V Supply Application .... 29 Adolfo A. Garcia Low Voltage PowerPath Driver Switches from a 3.3V or 5V Supply to Battery Backup ........................31 Peter Guan and Tim Skovmand 20A Constant Current Sourc e/Battery Charger is 95% Efficient ..............32 Goran Perica LTC2400 Differential Bridge Digitizers ..........................34 Kevin R. Hoskins and Derek Redmayne 4.5 µ A Li-Ion Battery Protection Circu it ............36 Albert Lee provides a 12V output with inputs that can range from 18V down to 6V. All of the circuitry to the left of the inductor is identical to that of a typical buck converter implemented with the LTC1625. However, the output (right) side of the inductor is also switched, using an additional pair of MOSFETs (M3 and M4). During the first phase of each cycle, switches M1 and M3 are on while M2 and M4 are off. The input voltage is applied across the inductor and its current increases. In the second phase, M1 and M3 are turned off while M2 and M4 are turned on. Current is then delivered to the output with VOUT applied across the inductor. This type of converter has several significant differences compared to the buck topology that is usually used with the LTC1625. First, the duty cycle relationship is now equal to VOUT/(VIN + VOUT). When VIN is equal to VOUT, a fifty percent duty cycle is required to balance the volt-seconds across the inductor. Second, both the input and output capacitors must filter a square pulse current. This increases the required power handling capability of the output capacitors. Finally, the average value of the inductor current is equal to the sum of the input and output currents. Thus, the inductor is larger than that required by a pure buck or boost converter. This last point also has a bearing on the current-limit behavior. continued on page 30 100 VOUT = 12V 90 EFFICIENCY (%) The LTC1625 No RSENSE™ controller can be used in a power-converter topology that is capable of both up and down conversion and requires only a single inductor. An example of such a circuit, shown in Figure 1, 80 VIN = 6V 70 VIN = 12V 60 VIN = 18V 50 0.1 1 10 LOAD CURRENT (A) Figure 2. Efficiency vs load current for Figure 1’s circuit RF 1Ω 1 2 CSS 0.1µF 3 4 CC1 2.2nF RC 10k 5 CC2 220pF 6 7 R1 3.92k 8 LTC1625CS 16 VIN EXTVCC 15 TK SYNC 14 SW RUN/SS 13 TG FCB 12 BOOST ITH 11 INTVCC SGND 10 BG VOSENSE 9 PGND VPROG R2 35.7k L1: 7A, 18µH, Kool-Mµ 77120-A7 MAGNETICS, INC. (800) 245-3984 15 TURNS 17 GAUGE CIN: SANYO 20SA68M COUT: SANYO 16SA100M (619) 661-6835 + CF 0.1µF M1 Si4420 VIN CIN 6V–18V 68µF 20V ×2 *V IN 18 12 6 D2 MBRS340T3 VOUT 12V/* L1 18µH DB CMDSH-3 CB 0.33µF R1 100k + M4 Si4425 C1 510pF 1 CVCC 4.7µF M2 Si4420 IOUT 4.0 3.3 2.0 D1 MBRS340T3 Z1 MMBZ5240B 10V 3 1/2 LTC1693-2 8 D3 BAT85 7 1/2 2 LTC1693-2 4 5 M3 Si4420 + COUT 100µF 16V 0.03Ω ESR ×2 C2 0.1µF 6 D4 BAT85 D5 BAT85 Figure 1. Single-inductor, positive-output buck-boost converter 28 Linear T echnology Magazine • June 1999 DESIGN IDEAS This is given by: ( GAIN = 1 + 49.4kΩ RG Table 1. Performance summary of 5V single-supply instrumentation amplifier with rail-to-rail outputs ( × ( R3R2( A gain-of-100 composite configuration is realized with RG = 1.5k. Other gain settings can be realized with various values of RG, as illustrated in Table 1. Even though the inputs to the circuit are not required to operate at the positive rail or at ground, wide input common mode operation is always beneficial. In this configuration, the LT1167 input stage can accept signals up to 3.7V (common mode plus differential mode) with no loss of precision. In fact, at low circuit gains, the circuit’s common mode input voltage range spans 2.25V to 3.45V. This wide input common mode range allows room for the full-scale differential input voltage to drive the output ±2.5V about the reference point (VREF). Here’s another application hint regarding this circuit: though the LT1167’s input bias currents are less than 1nA, the differential input terminals still require a DC return path to ground. For more information regarding this topic, please consult the LT1167 data sheet. The static and dynamic performance of the composite instrumentation amplifier is summarized in Table 1. 0.1Hz to Bandwidth VOS, RTI* TCVOS, RTI* 10Hz Nonlinearity (kHz) Noise, RTI* (µV) (µV/˚C) w/o C1 (µVP-P) Circuit Gain RG (Ω) 10 20.5k 1300 6.5 < 0.006% 900 2 30 5.36k 450 2.3 < 0.006% 850 0.7 100 1.5k 160 0.8 < 0.006% 500 0.4 300 487 10 0 0.5 < 0.006% 160 0.3 1000 147 90 0.4 < 0.006% 40 0.3 *RTI is an acronym for error “ referred to input.” The transient response of the circuit as a function of gain and load is well behaved, and is attributable to the LT1498’s wideband rail-to-rail output stage . Its 10MHz gain-bandwidth product and 6V/µs slew rate ensure that the small-signal performance of the circuit is dominated by the L T1167. Capacitor C1 is recommended for low fr equency applications (signal bandwidths <20Hz) to eliminate or significantly reduce noise pickup. Noise can also be injected into the circuit via the input terminals of the LT1167, especially if the sensor is located remotely from the signal conditioning circuitry. This type of noise can cause a shift in the input offset voltage of the LT1167, thereby producing an error. This effect For more information on parts featured in this issue, see http://www.linear-tech.com/go/ltmag is commonly known as RFI rectification. A differential filter can be easily added to the LT1167’s input terminals to reduce the effects of RFI rectification. Please consult the LT1167 data sheet for additional information on this topic. Conclusion As this design idea illustrates, the precision DC performance of a dualsupply instrumentation amplifier can be successfully applied to single-supply, bridge-type sensor applications using a precision rail-to-rail dual operational amplifier. The combination of the LT1167, the LT1498 and the LT1634 yields a cost-effective solution for 14-bit signal conditioning applications. for the latest information on LTC products, visit www.linear-tech.com LTC1625, continued from page 28 The LTC1625 uses MOSFET VDS sensing to control the inductor current peaks. Thus, the controller limits the average value of the inductor current rather than the output current in this topology. Because the input current varies as VIN is changed, the limit on output current depends upon the input voltage. With VIN = 12V, the maximum output current is about 3.3A. Efficiency of the circuit is shown in Figure 2. 30 Nonoverlapping control signals for the switches M2 and M4 are generated from the LTC1625 and buffered by an LTC1693-2 dual MOSFET driver. Note that the control signal for the PFET M4 must be able to swing between ground and VOUT. Thus, the inverting half of the LTC1693-2 is powered from a diode-OR between INTVCC (for start-up) and VOUT. Several simplifications are possible for this circuit. The switch node can be connected directly to M3’s gate, provided that VIN remains below the maximum rated gate voltage. This eliminates R1, C1, Z1, D2 and the buffer portion of U2. The second stage could also be made nonsynchronous by replacing D2 with a larger diode, such as an MBRD835L, and eliminating M4, D4, D5, C2 and the inverting portion of U2. Nonsynchronous operation reduces the peak efficiency by two to three percent. Linear T echnology Magazine • June 1999