MCU power circuit and component selection considerations

芯情观察猿

MCU power circuit and component selection considerations [Copy link]

MCU generally requires core power supply, reference power supply, and universal power supply. The performance parameters of each power supply are different. In order to operate stably, MCU requires these power supplies to meet three conditions: low load transient fluctuation, high ripple rejection ratio, and low power consumption.

Figure 1. MCU power supply

The core power supply of the MCU is the most important. At present, the power supply voltage of the MCU core is about 1.0-1.2V. This value tends to decrease continuously and is expected to reach about 0.8-1.2V in the future to meet the needs of the ever-increasing miniaturization, lightweight and low energy consumption of smart terminals such as smartphones and tablets.

1. Design requirements

The MCU core power supply is the load current of several hundred mA, or the instantaneous rise or fall of the peak current when the CPU handles a large load. If the peak voltage fluctuates greatly with the sudden increase and decrease of the load current, the control logic function may fail, causing the entire setup to fail due to MCU failure. Therefore, the transient response performance of the load is very important. In order to avoid logic failure caused by noise, the core power supply must provide a stable low voltage, must be able to eliminate switching noise from devices such as PMIC and DC-DC converters, and have a high ripple rejection ratio. Since the MCU itself will heat up, it is necessary to reduce power consumption to reduce the impact of the MCU on the surrounding.

Figure 2. MCU power circuit using a small form factor, low power LDO regulator

Among various voltage regulation devices, LDO regulators are widely used in smart terminals such as smartphones and tablets, which can continuously meet the miniaturization, lightweight and low energy consumption requirements of these devices. In particular, the use of lower voltages of around 1V is becoming increasingly popular in the power supply of MCUs used in mobile devices or imaging and audio-visual equipment, and the requirements for LDO are becoming increasingly stringent.

2. Component selection

The above requirements can be met by using a small-size, low-power LDO regulator. VBIAS is the power supply pin for the entire circuit, with an input voltage of 2.5V or above, and VOUT of 1.4V or above. The power supply of the external input voltage here must be as stable as possible, and its noise will significantly affect the output voltage of the LDO drive circuit. The startup sequence of the external power supply should be "VBIAS → VIN → CONTROL."

Figure 3. A simplified MCU power circuit BOM

(1) LDO

The new TCR5BM/8BM series of LDOs use low on-resistance N-channel MOSFETs manufactured using the latest generation of processes and an external bias voltage to provide VOUT as low as 0.8V or as high as 3.6V, and reduce the voltage drop, the culprit for power loss, to the lowest voltage drop level in the industry. The TCR5BM series supports a voltage drop as low as 100mV and an output current of up to 500mA, while the TCR8BM series supports a voltage drop as low as 170mV and an output current of up to 800mA. These small surface-mount LDO regulators have a 98dB (typical) ripple rejection ratio for stable operation, suppressing high-frequency noise from the external environment and DC-DC converters that can cause malfunctions. They are also able to provide fast load transient response to avoid malfunctions caused by rapid switching of IC operating modes.

(2) Capacitor

It is recommended to select ceramic capacitors with ESR no greater than 1.0Ω for the capacitors in the circuit, and the working environment should be considered when selecting. For stable operation, a 1μF or larger capacitor should be connected to the VIN pin, a capacitor no less than 0.1μF should be connected to the VBIASui pin, and a capacitor no less than 2.2μF should be connected to the VOUT pin. Although some possible oscillation problems have been considered in the design, such as the built-in phase compensation capacitor, the capacitance and inductance generated by the wiring may still cause oscillation, which depends on the PCB pattern and the use environment.

Figure 4. MCU power supply layout considerations

There are also some general precautions. The two pins of VIN and GND should not form a loop, and the routing width should be as large as possible to reduce the wiring impedance. In addition, special attention should be paid to the routing path to ensure that these impedances do not affect the internal circuit of the LDO. The routing of VBIAS should not be too long, otherwise it is easy to cause noise.