USB Converter

Does this sound familiar: you buy a small piece of equipment, such as a programming & debugging interface for a microcontroller, and you have to use a clunky 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 generate an output voltage of up to 15 V with a maximum output current of 150 mA.

 

The LM3578 is a general-purpose switchmode voltage converter. Figure 1 shows its internal block diagram. Here we use it as a step-up converter. The circuit diagram in Figure 2 shows the necessary components. 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 current flows through coil L1, which stores energy in the form of a magnetic field. When the internal transistor is switched off, the current continues flowing through L1 to the load via diode D1. However, the voltage across the coil reverses when this happens, so it is added to the input voltage. The resulting output voltage thus consists of the sum of the input voltage and the induced voltage across the coil.

 USB Converter Circuits Diagram 1

The output voltage depends on the load current and the duty cycle of the internal transistor. Voltage divider R5/R6 feeds back a portion of the output voltage to the comparator in the IC in order to regulate the output voltage. C5 determines the clock frequency, which is approximately 55 kHz. Network R4, C2 and C3 provides loop compensation. The current-sense resistor for the current-limiting circuit is formed by three 1-Ω resistors in parallel (R1, R2 and R3), since SMD resistors with values less than 1 Ω are hard to find. The output voltage ripple is determined by the values and internal resistances of capacitors C11, C8, C7 and C6.

 
 USB Converter Circuits Diagram 2

 

The total effective resistance is reduced by using several capacitors, and this also keeps the construction height of the board low. L2, C1, C9 and C10 act as an input filter. Ensure that the DC resistance of coil L2 is no more than 0.5 Ω. 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 consumes, 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 R5 and R6 for other output voltages:

6V:	R5 = 47k, R6 = 9,1k
12V:	R5 = 110k, R6 = 10k
15V:	R5 = 130k, R6 = 9,1k

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.

Parts List:

(for UO = 9 V)

Resistors

R1,R2,R3 = 1Ω

R4 = 220kΩ

R5 = 82kΩ

R6 = 10kΩ

Capacitors

(SMD 1206)

C1 = 100nF

C2 = 2nF2

C3 = 22pF

C4 = 100nF

C5 = 1nF5

(tantalum SMD 7343)

C6 = 68μF 20V

C7 = 68μF 20V

C8 = 68μF 20V

C9 = 47μF 16V

C10 = 47μF 16V

C11 = 68μF 20V

Inductors

L1 = 820μH (SMD CD105)

L2 = 47μH (SMD 2220)

Semiconductors

D1 = SK34SMD (Schottky)

IC1 = LM3578AM (SMD SO8)

Miscellaneous

K1 = 2-way PCB terminal block, lead pitch 5mm

(optional)

K2 = USB-B connector

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