You purchase a small device, like a programming and debugging interface for a microcontroller, only to find that it requires a bulky AC wall adapter for power? The inconvenience escalates when you’re on the go and there’s no electrical outlet in sight. Fortunately, you can directly utilize the USB bus as a power source if the required voltage is 5 V. For higher voltages, the USB converter detailed here comes into play. This compact switch-mode step-up converter can produce an output voltage of up to 15 V with a maximum output current of 150 mA. The 1M3578 functions as a versatile switch-mode voltage converter, and its internal block diagram is depicted in Figure 1. In this context, we utilize it as a step-up converter, as illustrated in the circuit diagram presented in Figure 2.
Voltage Conversion Mechanism
Voltage conversion occurs by turning on the internal transistor until it’s switched off either by the comparator or the current-limiting circuit. The collector current travels through coil L1, storing energy in the form of a magnetic field. When the internal transistor switches off, the current continues its flow through L1 to the load via diode D1. However, the voltage across the coil reverses during this process, adding to the input voltage. Consequently, the output voltage is the sum of the input voltage and the induced voltage across the coil.
Output Voltage Regulation
The output voltage is influenced by the load current and the duty cycle of the internal transistor. A portion of the output voltage is fed back to the comparator in the IC through voltage divider R5/R6 to regulate the output voltage. The clock frequency, approximately 55 kHz, is determined by capacitor C5. Loop compensation is provided by network R4, capacitor C2, and resistor R0. The current-sense resistor for the current-limiting circuit is created by three 1Ω resistors in parallel (R1, R2, and R3), as it’s challenging to find SMD resistors with values below 1Ω.
Output Voltage Ripple and Filter Design
The output voltage ripple is determined by the values and internal resistances of capacitors C11, C8, C7, and C6. Utilizing multiple capacitors reduces the total effective resistance and maintains a low construction height of the board. Components L2, C1, R0, and C10 form an input filter to ensure a smooth input. It’s essential to verify that the DC resistance of coil L2 does not exceed 0.5Ω. For connection to the USB bus, a Type B PCB-mount USB connector is recommended. An output voltage connector can be facilitated using a terminal strip with a pitch of 5.08 mm. Alternatively, a cable can be directly soldered to the board, and there are specific holes provided on the circuit board for this purpose.
Input and Output Current Considerations
It’s crucial to acknowledge that no device can produce more energy than it consumes. Thus, the input current of the circuit is higher than the output current. Generally, the input current can be estimated as the product of the output current and the output voltage, divided by the input voltage, and further divided by 0.8. For instance, with an output current of 100 mA at 9 V, the input current drawn from the USB bus is approximately 225 mA. Refer to Figure 3 for a detailed PCB layout of the circuit, where all components except the connector and terminal strip are SMDs.