Abstract: This reference design presents a circuit for addressing the power-supply and current-monitoring requirements of APD biasing applications. Based on the MAX15031 DC-DC converter, the application circuit provides a 70V, 4mA, DC-DC power converter with a 2.7V to 11V input supply-voltage range. Key specifications for this reference design are listed below, along with a detailed schematic (Figure 1) and bill of materials (Table 1) for the application. Design Specifications and Setup Wide, 2.7V to 11V input supply-voltage range 70V output voltage 4mA output current Fixed, 400kHz switching frequency -40°C to +125°C operating temperature range Tiny, 8mm x 12mm circuit footprint Schematic of the Reference Design Figure 1 presents the reference design schematic for a 2.7V to 5.5V input range. The same circuit can be used for inputs from 5.5V to 11V by connecting the CP pin to VIN and removing the charge-pump capacitor (C3). Figure 1. Schematic of the MAX15031 boost converter for FSW = 400kHz (fixed). Table 1. Bill of Materials (BOM) Designator Value Description Part Footprint Manufacturer Quantity C1 1µF, 10V Capacitor GRM155R61A105KE15 0402 Murata 1 C2, C4 0.1µF, 16V Capacitors GRM155R71C104KA88 0402 Murata 2 C3, C6 0.01µF, 25V Capacitors GRM155R71E103KA01D 0402 Murata 2 C5, C8 0.1µF, 100V Capacitors GCM21BR72A104KA37L 0805 Murata 2 C7 0 Capacitor Open 0402 Open 1 D1 100V, 150mA Schottky diode BAT46W-7-F SOD-123 Diodes Inc. 1 L1 4.7µH Inductor ME3220-472MLB 3mm x 3mm Coilcraft 1 R1 348kΩ Resistor SMD, 1%, 0.125W 0402 Vishay 1 R2 6.34kΩ Resistor SMD, 1%, 0.125W 0402 Vishay 1 R3, R6 10kΩ Resistors SMD, 1%, 0.125W 0402 Vishay 2 R4 100Ω Resistor SMD, 1%, 0.125W 0402 Vishay 1 R5 3.16kΩ Resistor SMD, 1%, 0.125W 0402 Vishay 1 U1 MAX15031 Boost converter MAX15031ATE+ 16-TQFN-EP Maxim 1 Waveforms Depicting the Performance of the Design Figure 2 and Figure 3 show the performance of the Figure 1 circuit. From the LX node voltage, it can be known that the converter is operating in a discontinuous mode of operation. The circuit is designed for a 70V output (VOUT) while the input (VIN) is kept at 3.3V. Figure 2. VIN (Channel 1), VOUT (Channel 2), and APD (Channel 3) at 3.3V input. Figure 3. LX node voltage (Channel 1), VOUT (Channel 2), and APD output (Channel 3) at VIN =3.3 V with an APD current of 4mA. Figure 4. Input ripple (Channel 1) and output ripple (Channel 2) at VIN = 3.3V with an APD current of 4mA. Figure 4 shows the input (VIN) and output (VOUT) ripple when input voltage is kept at 3.3V and the load current is 4mA. Figure 5 shows the voltage across RLIM when the current reaches the current limit. In this case, the current limit is set at 4mA. When the current limit is reached, the voltage across RLIM is equal to 1.245V. Figure 5. APD (Channel 2), VIN (Channel 3), and APD current-monitoring output across RLIM (Channel 4) at 4mA of APD current. Efficiency Measurements Efficiency of the total system is calculated from the test results with 3.3V and 5V inputs, and with the load current varied from 0 to 4mA. Figure 6 shows the efficiency curve for a 70V output. Figure 6. Efficiency versus load current with 3.3V and 5V inputs. Board Design Figure 7 shows the layout of this reference design, which fits on a tiny, 12mm x 8mm circuit board. Figure 8 and Figure 9 show the top-layer and bottom layer components, respectively. Figure 7. Board layout of the reference design, which measures 12mm x 8mm. Figure 8. Top-layer components. Figure 9. Bottom-layer components. Related Parts   APP 4374: Mar 31, 2009 MAX15031 80V, 300mW Boost Converter and Current Monitor for APD Bias Applications Full Data Sheet (PDF, 476kB) Free Samples Automatic Updates Would you like to be automatically notified when new application notes are published in your areas of interest? Sign up for EE-Mail™. We Want Your Feedback!   Download, PDF Format (148kB)  AN4374, AN 4374, APP4374, Appnote4374, Appnote 4374