an Automatic Power-on Circuit for LattePanda Mu

Overview

In unattended scenarios, it is often required that the LattePanda board automatically powers on after being connected to power, and it must be 100% automatic.

Relying solely on setting ACPowerLoss to PowerON in the BIOS may not be sufficient, especially when there is a brief power outage after the power comes back on, which can lead to the failure of automatic power-on.

Therefore, it is officially recommended to handle this with hardware solutions, which typically include: 1. Using a microcontroller to detect and trigger, or 2. Using an independent hardware circuit to trigger.

This article shares a purely hardware circuit from the LattePanda Mu Full Function Evaluation Carrier Board, which may be helpful for your carrier board design.

Circuit Diagram

Principle Analysis 

When the power is first turned on, 3V3STY receives power. Capacitor C246 starts charging, and the voltage across the capacitor gradually increases, a process that will last for some time. Since the voltage has not reached the threshold voltage(Vgs) of Q5, Q5 remains off. Due to the pull-up effect of resistor R160, Q6 conducts, pulling PWR_SW low, equivalent to pressing the power button. 

As the capacitor C246 charges, the voltage across the capacitor gradually increases, which means the gate voltage of Q5 increases. After a period of time, Q5 turns on, causing the gate voltage of Q6 to decrease, which results in Q6 turning off. Consequently, PWR_SW becomes high due to the internal pull-up(in the LattePanda Mu module). 

Subsequently, resistor R155 provides a continuous gate voltage to Q5, keeping Q5 always on, thereby ensuring Q6 remains off, forming a stable state. 

As can be seen, the function of this circuit is equivalent to holding down the power button for a period of time when the LattePanda board is first powered on. 

The JP2 jumper is used to select whether automatic power-on is required. 

When the power is cut off, capacitor C246 rapidly discharges through diode D39, preventing residual voltage in C246 from causing PWR_SW to not be pulled low during the next power-on, thus ensuring the failure of automatic power-on does not occur.

Precautions

When selecting components, note that the Igss for Q5 and Q6 should not be too high, to prevent the capacitor from not fully charging, which could cause the PWR_SW to continuously output a low level, equivalent to continuously pressing the power button. The Igss for 2N7002 should be within 100nA.

After the power is cut off, the 3V3STY voltage must drop quickly. If it does not quickly drop, capacitor C246 cannot discharge rapidly. If the capacitor has not finished discharging by the next power-up, the PWR_SW might not be pulled low, resulting in the auto power-on function not working. A resistor (1K~10K) can be placed in parallel with the 3V3STY to solve this issue.

If testing shows a low probability of auto power-on failure, the value of capacitor C246 can be increased, such as to 22uF, to extend the time for pulling PWR_SW low.