Does this sound familiar: you buy a small piece of equipment, such as a programming & debugging interface for a microcontrol-ler, and you have to use a dunky AC wall adapter to supply it with power? It’s even worse when you’re travelling and there’s no mains socket anywhere in sight. Of course, you can use the USB bus directly as a power source if the supply voltage is 5 V. If you need a higher voltage, you can use the USB converter described here. This small switch-mode step-up converter can gener-ate an output voltage of up to 15 V with a maximum output current of 150 mA. The 1M3578 is a general-purpose switch-mode voltage converter. Figure 1 shows its intemal block diagram. Here we use it as a step-up converter. The circuit diagram in Figure 2 shows the necessary compo-nents.
Voltage conversion is achieved by switching on the internal transistor until it is switched off by the comparator or the current-limiting circuit. The collector cur-rent flows through coil 11, which stores energy in the form of a magnetic field. When the internal transistor is switched off, the current continues flowing through 11 to the load via diode Dl. However, the voltage across the coil reverses when this happens, so it is added to the input volt-age. The resulting output voltage thus con-sists of the sum of the input voltage and the induced voltage across the coil. The output voltage depends on the load cur-rent and the duty cycle of the intemal transistor. Voltage divider R5/R6 feeds back a portion of the output voltage to the com-parator in the IC in order to regulate the output voltage. C5 determines the clock frequency, which is approximately 55 kHz. Network R4, C2 and 0 provides loop compensation. The current-sense resistor for the current-limiting circuit is formed by three 141 resistors in parallel (R1, R2 and R3), since SMD resistors with values less than 1 fl are hard to find.
The output volt-age ripple is determined by the values and intemal resistances of capacitors C11, C8, C7 and C6. The total effective resistance is reduced by using several capacitors, and this also keeps the construction height of the board low. L2, C1, 0 and C10 act as an input filter. Ensure that the DC resist-ance of coil L2 is no more than 0.5 Cl. Use a Type B PCB-mount USB connector for connection to the USB bus. A terminal strip with a pitch of 5.08 mm can be used for the output voltage connector. Of course, you can also solder a cable directly to the board. Two additional holes are provided in the circuit board for this purpose. As we haven’t been able to invent a device that produces more energy than it con-sumes, you should bear in mind that the input current of the circuit is higher than the output current. As a general rule, you can assume that the input current is equal to the product of the output current and the output voltage divided by the input voltage and divided again by 0.8. Specifically, with an output current of 100 mA at 9 V, the input current on the USB bus is approximately 225 mA. Finally, Figure 3 shows a small PCB layout for the circuit. All of the components except the connector and the terminal strip are SMDs.
