L DESIGN FEATURES Boost Converters for Keep-Alive Circuits Draw Only 8.5μA of by Xiaohua Su Quiescent Current Introduction Industrial remote monitoring systems and keep-alive circuits spend most of their time idle. Many of these systems use batteries, so to maximize run time power losses,even during low power idle modes, must be minimized. Even at no load, power supplies draw some current to produce a regulated voltage for keep-alive circuits. The LT8410/-1 DC/DC boost converter features ultralow quiescent current and integrated high value feedback resistors to minimize the draw on the battery when electronics are idle. An entire boost converter takes very little space, as shown in Figure 1. Ultralow Quiescent Current Low Noise Boost Converter with Output Disconnect When a micropower boost converter is in regulation with no load, the input current depends mainly on two things—the quiescent current (required to keep regulation) and the output feedback resistor value. When the output voltage is high, the output feedback resistor can easily dissipate more power than the quiescent current of the IC. The quiescent current of the LT8410/-1 is a low 8.5µA, while the integrated output feedback resistors have very high values (12.4M/0.4M). This enables the LT8410/-1 to dissipate very little power in regulation at no load. In fact, the LT8410/-1 can regulate a 16V output at no load from 3.6V input with about 30µA of average input current. Figures 2, 3 and 4 show the typical quiescent and input current in regulation with no load. The LT8410/-1 controls power delivery by varying both the peak inductor current and switch off time. This control scheme results in low output voltage ripple as well as high efficiency over a wide load range. As shown in Figure 5, even with a small 0.1µF output capacitor, the output ripple is typically less than 10mV. The part also features output disconnect, which disconnects the output voltage from the input during shutdown. This output disconnect circuit also sets a maximum output current limit, allowing the chip survive output shorts. An Excellent Choice for High Impedance Batteries A power source with high internal impedance, such as a coin cell battery, may show normal output voltage on a voltmeter, but its voltage can collapse under heavy current demands. This makes it incompatible with high switch-current DC/DC converters. The LT8410/-1 has an integrated power switch and Schottky diode, and the switch current limits are very low (25mA for the LT8410 and 8mA for the LT8410-1). This low switch current limit enables the LT8410/-1 to operate very efficiently from high impedance sources, such as coin cell batteries, without causing inrush current problems. Figure 6 shows the LT8410-1 charging an electrolytic capacitor. Without any additional external circuitry, the input current for 12 10 1000 8 6 4 2 AVERAGE INPUT CURRENT (µA) 10 QUIESCENT CURRENT (µA) QUIESCENT CURRENT (µA) Figure 1. The LT8410/-1 is designed to facilitate compact board layout. 8 6 4 2 VCC = 3.6V 100 VCC = 3.6V 0 –40 0 40 80 TEMPERATURE (°C) Figure 2. Quiescent current vs temperature—not switching 22 120 0 0 4 8 12 VCC VOLTAGE (V) Figure 3. Quiescent current vs VCC voltage—not switching 16 10 0 10 20 30 OUTPUT VOLTAGE (V) 40 Figure 4. Average input current in regulation with no load Linear Technology Magazine • March 2009 DESIGN FEATURES L 100µH VIN 2.5V to 16V 2.2µF SW CAP VCC VOUT VOUT = 16V 0.1µF* LT8410 VREF SHDN CHIP ENABLE 604K GND 0.1µF FBP 412K *HIGHER VALUE CAPACITOR IS REQUIRED WHEN THE VIN IS HIGHER THAN 5V 100 10 VIN = 3.6V 6 4 2 R1 1.30 • 1 + R2 VIN = 12V 90 8 EFFICIENCY (%) VOUT PEAK-TO-PEAK RIPPLE (mV) the SHDN pin below 0.3V shuts down the part and reduces input current to less than 1µA. When the part is on, and the SHDN pin voltage is close to 1.3V, 0.1µA current flows out of the SHDN pin. A programmable enable voltage can be set up by connecting external resistors as shown in Figure 7. The turn-on voltage for the configuration is: 0.1µF VIN = 5V 80 and the turn-off voltage is: VIN = 3.6V 70 R1 (1.24 − R3 • 10 −7 ) • 1 + − (R1• 10 −7 ) R2 60 50 0 0.01 0.1 1 LOAD CURRENT (mA) 40 0.01 10 0.1 1 10 LOAD CURRENT (mA) 100 Figure 5. General purpose bias with wide input voltage and low output voltage ripple the entire charging cycle is less than 8mA. Tiny Footprint with Small Ceramic Capacitors Available in a tiny 8-pin 2mm × 2mm DFN package, the LT8410/-1 is internally compensated and stable for a wide range of output capacitors. For most applications, using 0.1µF output capacitor and 1µF input capacitor is sufficient. An optional 0.1µF capacitor at the VREF pin implements a soft-start feature. The combination of small package size and the ability to use small ceramic capacitors enable the VIN 2.5V to 16V LT8410/-1 to fit almost anywhere. Figure 1 shows the size of a circuit similar to that shown in Figure 4, illustrating how little board space is required to build a full featured LT8410/-1 application. where R1, R2 and R3 are resistance in Ω. Programming the turn-on/turnoff voltage is particularly useful for applications where high source impedance power sources are used, such as energy harvesting applications. By connecting an external capacitor (typically 47nF to 220nF) to the VREF pin, a soft-start feature can be implemented. When the part is brought continued on page 29 ENABLE VOLTAGE SHDN Pin Comparator and Soft-Start Reset Feature R1 An internal comparator compares the SHDN pin voltage to an internal voltage reference of 1.3V, giving the part a precise turn-on voltage level. The SHDN pin has built-in programmable hysteresis to reject noise and tolerate slowly varying input voltages. Driving R3 CONNECT TO SHDN PIN R2 Figure 7. Programming the enable voltage by using external resistors L1 220µH C1 2.2µF TURN ON/OFF SW CAP VCC VOUT LT8410-1 VREF SHDN GND FBP C2 1.0µF C3 10000µF R1 604k R2 412k C1: 2.2μF, 16V, X5R, 0603 C2: 1.0μF, 25V, X5R, 0603* C3: 10000μF, Electrolytic Capacitor C4: 0.1μF, 16V, X7R, 0402 L1: COILCRAFT LPS3008-224ML * HIGHER CAPACITANCE VALUE IS REQUIRED FOR C2 WHEN THE VIN IS HIGHER THAN 12V SHDN VOLTAGE 2V/DIV C4 0.1µF VOUT = 16V VOUT VOLTAGE 10V/DIV INPUT CURRENT 5mA/DIV INDUCTOR CURRENT 10mA/DIV VIN = 3.6V 20s/DIV Figure 6. Capacitor charger with the LT8410-1 and charging waveforms Linear Technology Magazine • March 2009 23 DESIGN FEATURES L and expensive solution than typical microprocessor-controlled methods. The simplest scheme uses a resistor divider from the VREF pin to the CTRL pin, where the top resistor in the divider is an NTC (negative temperature coefficient) resistor. While simple, this method suffers from nonlinear temperature coefficient of the NTC resistor. A more precise method uses a transistor network as shown in Figure 7. The PTC (Positive Temperature Coefficient) of the CTRL pin voltage is realized by an emitter follower of Q1 and a VBE multiplier of Q2. Assuming: VBE(Q1) = VBE(Q2) = VBE dT dT VCTRL = VREF − = 2mV °C R8 V R7 BE with PTC = 56 dVCTRL R8 2mV = • dT R7 °C 52 50 48 R8 = R7 46 44 42 40 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 Figure 8. Temperature response of the circuit shown in Figure 7 LT8410, continued from page 23 out of shutdown, the VREF pin is first discharged for 70µs with a strong pull down current, and then charged with 10µA to 1.235V. This achieves soft start since the output is proportional to VREF. Full soft-start cycles occur even with short SHDN low pulses since VREF is discharged when the part is enabled. In addition, the LT8410/-1 features a 2.5V to 16V input voltage range, up R1 = R2 dVOUT 2mV + • VOUT dT °C −1 2mV • VREF °C to 40V output voltage and overvoltage protection for CAP and VOUT. Conclusion The LT8410/-1 is a smart choice for applications which require low quiescent current and low input current. The ultralow quiescent current, combined with high value integrated feedback resistors, keeps the average input current very low, significantly EFFICIENCY (%) 90 = 2mV °C The LT3571 is a highly integrated, compact solution to APD bias supply design. It provides a useful feature set and the flexibility to meet a variety of challenging requirements, such as low noise, fast transient response speed, and temperature compensation. With a high level of integration and superior performance, the LT3571 is the natural choice for APD bias supply design. L extending battery operating time. Low current limit internal switches (8mA for the LT8410-1, 25mA for the LT8410) make the part ideal for high impedance sources such as coin cell batteries. The LT8410/-1 is packed with features without compromising performance or ease of use and is available in a tiny 8-pin 2mm × 2mm package. L The accurate programmable output current limit of the LT3653 and LT3663 eliminates localized heating from an output overload, reduces the maximum current requirements on the power components, and makes for a robust power supply solutions. L VIN = 8V VIN = 15V 80 VIN = 30V 70 60 50 40 0.1 dT Conclusion 100 LT3653/63, continued from page 21 of handling 60V transients. Figure 4 shows the circuit efficiency at multiple input voltages. The current limit of the application is set to 1.2A, therefore, the power path components are sized to handle 1.2A maximum. To reduce the application footprint, the LT3663 includes internal compensation and a boost diode. The RUN pin, when low, puts the LT3663 into a low current shutdown mode. VREF VOUT 2mV VBE + • °C dVOUT dT VBE • dVBE(Q2) Conclusion Given VOUT at room and dVOUT/DT, the R1/R2 and R8/R7 can be calculated as follows 54 VAPD (V) dVBE(Q2) = Simulation using LTspice always gives a good starting point. The circuit shown in Figure 7 is designed to have VAPD = 50V (VOUT = 55V) at room and dVAPD/dT = 100mV/°C (dVOUT/dT = 100mV/°C). The measured temperature response is shown in Figure 8, which is very close to the design target. then the CTRL pin voltage is 60 58 = dVBE(Q1) dT and dVBE(Q1) Resistors R5–R9 are selected to make I(Q1) = I(Q2) ≈ 10µA, and 0.3 0.5 0.7 0.9 OUTPUT CURRENT (A) 1.1 1.3 Authors can be contacted at (408) 432-1900 Figure 4. Efficiency of the circuit in Figure 3 Linear Technology Magazine • March 2009 29