How does ZL6205 solve the slow power failure caused by energy storage capacitors?

Have you paid attention to the LDO power down process? Have you ever wondered why some LDOs are slow to power down? Did you notice the power-up requirement for the MCU? Today we measured the power-down process of an ordinary LDO and ZL6205 and told you how the ZL6205 ensures the reliable power-on of the MCU.

1. The role of capacitors in LDO circuits

For products that need power-down storage or power-down alarm function, it is often the choice of many engineers to use the energy storage function of large-capacity capacitors to provide time for data preservation and system shutdown. In a system that does not require power-off to save data, in order to suppress power supply ripple, power supply interference and load changes, a capacitor with an appropriate capacity will also be connected in parallel at the power supply end. Capacitors are a necessary device for the output of LDOs, so what effects might so many capacitors have on ordinary LDOs? And how does the ZL6205 with the power-down and fast-discharge function not only ensure that the capacitor can provide time for the MCU to save data, but also can quickly power down?

2. A common LDO application circuit

Figure 1 shows an ordinary LDO circuit in practical application. The input and output terminals of the circuit are connected with large capacitors. The LDO is an ordinary LDO, which can basically be regarded as the output following the input during the power-on and power-off process. When the LDO input is powered off and the residual charge of the output capacitor cannot be released quickly, it will cause the LDO output to lose power slowly.

How does ZL6205 solve the slow power failure caused by energy storage capacitors?

Figure 1 A common LDO application circuit

3. A common LDO power-down test

In order to see the power-down process of an ordinary LDO, based on the application circuit in Figure 1, the actual measurement of the power-down process is shown in Figure 2. The blue line (channel 1) in the figure is the input voltage waveform when the power is off, and the pink line (channel 2) is the output voltage waveform when the power is off. It can be seen from the figure that when the LDO is not assisted by other additional current bleeder circuits, the output voltage will slowly power down after 2s, and the output voltage value only drops to 300mV.

How does ZL6205 solve the slow power failure caused by energy storage capacitors?

Figure 2 Slow power-down process of an ordinary LDO

4. The effect of slow power down on MCU

Increasing the capacitance and using one of the above common LDOs can indeed provide enough time for the MCU to save data, but the MCU saves data in the early stage of power failure. In the later stage of the power failure, the low-voltage area is still slowly powering down. What will be the impact?

We all know that MCUs have certain power-on sequence requirements. For example, Figure 3 shows the power-on requirements of a certain MCU. According to the figure, the MCU’s power-on requirements are as follows:

1. The voltage VI before power-on needs to be lower than 200mV for at least 12us;

2. The power-on time tr cannot exceed 500ms.

How does ZL6205 solve the slow power failure caused by energy storage capacitors?

Figure 3 Power-on requirements for an MCU

When the above ordinary LDO supplies power to the MCU, if the power-off and fast power-on test is carried out, it can be seen from Figure 2 that the voltage of the LDO will remain at about 300mV after 2s of power-off, even if the MCU is connected as a load, However, when only a light load such as MCU is supplied with power, once the MCU enters the undervoltage area, it will enter a protection state internally, many peripherals will be turned off, and the current consumption will be very small, which has a very limited impact on the power-down process of the LDO. Therefore, when the ordinary LDO powers the MCU and needs to be restarted quickly after power-off, it will not meet the requirement 1: the voltage VI before power-on needs to be lower than 200mV for at least 12us. In this way, the MCU may “crash”.

5. ZL6205 power down test

Now use the pin-package-compatible ZL6205 to directly replace the normal LDO in Figure 1 above, and then perform a power-down test under the same power supply. Figure 4 measured the power-down process of ZL6205 under the same circuit. The blue line (channel 1) is the input voltage waveform when the power is off, and the pink line (channel 2) is the output voltage waveform when the power is off.

As mentioned above, an ordinary LDO can also provide time for the MCU to save data in a circuit with large capacitance, but after the MCU saves the data, the voltage of an ordinary LDO cannot drop to 0V for a long time, which is easy to cause the MCU to start again. dead machine”. It can be seen from the power-down curve of ZL6205 in Figure 4 that VOUT will follow the voltage of VIN before VIN>2.1V, which can provide time for MCU to save data in the early stage of power-down. It can be controlled by changing the size of the input capacitor. The length of the follow-up phase provides the time needed to save the data. After VIN≤2.1V, the voltage of ZL6205 will quickly drop to 0V, so that it can avoid the phenomenon of “crash” that does not meet the power-on requirements of the above MCU. It can be said that it not only provides the necessary time for the MCU to save data in the early stage of power failure, but also realizes a rapid power failure in the later stage of power failure, which provides a guarantee for the MCU to be powered on again.

How does ZL6205 solve the slow power failure caused by energy storage capacitors?

Figure 4 ZL6205 fast power down process

Why can ZL6205 power down quickly? Figure 5 is the block diagram of the ZL6205 structure. The ZL6205 will turn off the output when the input is under-voltage or EN is disabled, so that even if the input is powered off very slowly, it will not affect the fast power-off of the output, and the ZL6205 will immediately start the internal power-off after the output is turned off. The electric rapid discharge circuit can quickly release the residual charge of the output capacitor and accelerate the voltage drop.

How does ZL6205 solve the slow power failure caused by energy storage capacitors?

Figure 5 ZL6205 internal block diagram

The power-down waveform in Figure 4 above was tested using the circuit in Figure 1. Because the LDO enable pin EN in Figure 1 is directly connected to VIN, there will be a process in which the output follows the input voltage. This circuit is ideal for systems where the MCU needs time to save data when powered down, but also needs to discharge quickly.

For a power supply system that needs to be powered on and off in one step, a circuit like Figure 6 below can be used. Because the ZL6205 has an enable pin, it has a relatively stable enable voltage threshold. The power-on start-up voltage value of the chip and the power-down output shutdown voltage can be set through different ratios of voltage divider resistors.

How does ZL6205 solve the slow power failure caused by energy storage capacitors?

Figure 6 ZL6205 typical application circuit

If the EN pin is used to set the power-on enable voltage value using a resistor divider (as shown in Figure 6 above, see the ZL6205 data sheet for the specific EN actual enable voltage), the power-on and power-off process will be shown in Figure 7, the blue color is The input voltage of ZL6205, pink is the output voltage of ZL6205. When the input voltage drops to the set shutdown voltage value, the output of the ZL6205 quickly powers down. When the input voltage rises to the set voltage value, the ZL6205 starts up quickly. During the power-on and power-off process, the output does not follow the input stage, and the power-off and power-on are all in one step. This power-on and power-on speed fully meets the power-on requirements of the MCU.

How does ZL6205 solve the slow power failure caused by energy storage capacitors?

Figure 7 ZL6205 uses EN to set the closing and opening points

In addition to ZL6205, ZLG also has ZL6105 series LDO compatible with 1117 package, which also has the characteristics of fast power on and off like ZL6205, in addition, ZL6201 with ultra-low power consumption (quiescent current as low as 1.6μA), especially suitable for battery power supply occasions.

The Links:   TPS65266RHBR LM32P073

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