The influence of the power chip EN pin on the motor control board

“Embedded hardware design will become the core technology of microelectronics in the 21st century. Three key technologies in system-on-chip (SoC) design and some research fields that integrate with each other are described in detail, and the challenges and development trends of SoC design are presented. Outlook was made.

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Embedded hardware design will become the core technology of microelectronics in the 21st century. Three key technologies in system-on-chip (SoC) design and some research fields that integrate with each other are described in detail, and the challenges and development trends of SoC design are presented. Outlook was made.

A motor control board has the function of power recovery. When there is no booster battery, the rotation of the motor can continue to supply power to the control board. The uneven rotation of the motor will generate a rapidly fluctuating voltage, which will cause the power supply chip to output an extremely unstable voltage, so that the power supply of the post-stage device is frequently powered on and off in a very short period of time, causing the Bluetooth module on the board to frequently lose firmware or even burn. bad, reducing product performance. Later, the problem was solved perfectly by adjusting the related configuration of the EN pin of the power chip. Want to know what “hands and feet” did to EN? What kind of great wisdom does a small EN contain?

I. Overview

EN means Enable, which means “enable”. Different chips are called differently, such as EA, RUN, etc. And their functions are basically the same, that is, only when the pin is activated, the chip or module can output normally. For this function, we can add some simple peripheral Circuits to achieve the function of stabilizing the chip or outputting power-on sequencing. The EN pin of some more advanced power chips usually also has hysteresis.

2. Application skills

1. Use the voltage dividing resistor skillfully to realize the stable output of the power chip

For the power supply chip, we usually use a voltage divider resistor to connect the EN signal to the input pin of the power supply to prevent the voltage of the EN terminal from exceeding its withstand voltage value. Under the condition that the withstand voltage value is satisfied, the voltage of the EN pin should be set in the “appropriate” range.

For example, as mentioned at the beginning of the article, the 24V power supply of a motor control board also supplies power to other circuits through the DC/DC: MP2451 output of 12V while supplying power to the motor. When there is no booster battery, the motor generates power to supply power to the control board, but the rotation of the motor is not uniform, resulting in a voltage with large fluctuations, as shown in Figure 1 below, the yellow line is the reverse power generation voltage of the motor, and the green line is the output of MP2451 voltage.

Figure 1 Motor power generation curve and DCDC output curve

As can be seen from Figure 1 above, the power generation voltage (DC/DC input voltage) VIN of the motor enables the DC/DC output at about 6.2V. At this time, the input voltage is less than the set 12V output voltage, making the DC/DC output voltage. The MOS tube inside the DC has been rapidly turned on and off due to the output feedback, forming an unstable output voltage whose noise envelope fluctuates with the input. When the generator voltage of the motor is greater than 12V, the DC/DC outputs a stable 12V voltage.

This is because the voltage divider resistor network in the Circuit is improperly set, and the threshold voltage of EN is reached when the input voltage is very low, resulting in the premature enablement of the power chip output. This is an example of only setting the EN pin voltage of the power chip below the withstand voltage value in the design process, but not setting the EN pin’s voltage divider network to a “suitable” range.

So where is the appropriate position for the voltage divider network of the EN pin to be set?

Figure 2EN enable output curve

l As shown in curve ①, when the input voltage is low, the enable threshold of VEN is reached, and the chip output is enabled. At this time, the output is affected by the input fluctuation and the power-on is slow, which affects the working stability of the subsequent circuit;

l As shown in curve ②, when the input voltage VIN rises to 70%~80%, VEN reaches the enable threshold. At this time, the chip output eliminates the unstable stage of the input power supply, and the power-on is fast, the output is stable, and the The effect of input voltage fluctuations;

l At the same time, a margin of 20%~30% is reserved to avoid the problem that the output is turned off due to the fluctuation of the input power supply;

l It can be seen that it is more reasonable to set the EN threshold voltage of the power supply chip at 70%~80%×VIN through the voltage divider network. The EN threshold value can be found in the chip manual. As shown in Figure 3 below, the appropriate voltage divider resistance ratio can be obtained according to the known EN threshold and input voltage.

Figure 3 Assigning network resistances based on known EN thresholds

Figure 4 is the output waveform after adjusting the resistance value of the voltage dividing resistor of the EN pin, and the output voltage fluctuation has been significantly improved. Continue to adjust the resistance value of the voltage divider resistor to obtain a more stable output waveform. This method simply and effectively solves the problem of unstable output mentioned above.

Figure 4 Voltage waveform after adjusting the voltage divider resistor

It can be seen that the small EN pin can also cause a lot of trouble if it is not set properly. Therefore, under the condition that meets the EN withstand voltage value, the input voltage of EN should be stabilized within the “appropriate” range according to the actual situation. is also very important. Have you learned this little trick?

2. Use the EN function skillfully to realize the power-on sequence

In circuit design, chips or Modules often require a variety of operating power supplies, and corresponding requirements are also put forward for the power-on sequence of these power supplies. Failure to meet these power-up sequence requirements may result in faults such as bus collisions, device latch-up, etc. For example, the working power supply on a system has three power supplies: VCC_Core, VCC_DDR, and VCC_DIO, which are controlled by a discrete power supply chip. At this time, the power-on sequence can be controlled by adjusting the RC loop of the EN pin of the power chip, namely R1 and C1 in the figure.

Figure 5 RC delay circuit

If the RC time constant is large, an action delay will also occur, that is, it will start to work later, and change different parameters to obtain different delay times, thereby controlling the power-on sequence of the discrete power supply chip. This method can also meet the needs of using one EN signal to control multiple power chips.

It should be noted that the resistance in the RC should not be too large to meet the current demand required by the EN pin. The following figure shows the EN input current condition in a power supply chip manual.

Figure 6 EN pin input current example

3. Summary

Through the control of EN, the corresponding functions can be realized, including the reasonable setting of the static operating point of EN, which can not only avoid turning on the power supply of the chip in the stage of unstable power supply voltage, but also avoid the unexpected shutdown of the system caused by the fluctuation of the power supply voltage during normal operation. Electricity. By controlling the logic sequence of EN, the power-on sequence control of multiple power supplies can be realized.

In addition, the hysteresis voltage of EN can be amplified by adding an appropriate control circuit to the EN terminal. This for battery-powered systems, when the battery is nearly depleted, can avoid the circuit repeated power cycle.

It can be seen that this seemingly simple EN pin needs more attention when using it. Through the introduction of this article, do you also think that this little EN contains great wisdom?