Can the regulator work stably under no-load conditions?

Some old power devices require a minimum load to ensure stability, because one of the electrodes that must be compensated is affected by the effective load resistance. For example, Figure A shows that the LM1117 requires a load current of at least 1.7 mA (maximum 5 mA).

Some old power devices require a minimum load to ensure stability, because one of the electrodes that must be compensated is affected by the effective load resistance. For example, Figure A shows that the LM1117 requires a load current of at least 1.7 mA (maximum 5 mA).

Can the regulator work stably under no-load conditions?
Figure A. LM1117 minimum load current specifications.

Most new devices can work without load, and there are very few exceptions to this rule. Some design techniques enable the LDO to maintain a stable state when any output capacitor (especially low ESR capacitor) is used. They are also used to ensure the stability of the device under no-load conditions. For a small number of modern devices that require a load, this limitation is generally caused by the leakage current of the bypass component, rather than due to stability. So, how do you tell? Please refer to the data sheet. If the device requires a minimum load, the data sheet must provide some information.

The ADP1740 and other low-voltage, high-current LDOs fall into this category. In the worst case, the leakage current generated by the integrated power switch is approximately 100 µA (85°C) and 500 µA (125°C). Under no-load conditions, the leakage current will charge the output capacitor until the switch’s VDS is low enough to reduce the leakage current to a negligible level, while increasing the no-load output voltage. The data sheet states that a load of at least 500 µA is required, so if the device is to be operated at high temperatures, it is recommended to use an emulated load. The load is less than the rated value of the device 2 A. Figure B shows the minimum load current specifications listed in the ADP1740 data sheet.

Can the regulator work stably under no-load conditions?
Figure B. ADP1740 minimum load current specifications

What should I do if the minimum load is not clearly indicated in the data sheet? In most cases, no minimum load is required. Although it may not sound convincing, if a minimum load is required, such information will definitely be provided in the data sheet. However, confusion often ensues, because graphs are often used in data manuals to show the specifications of a certain working range. Most of these graphs are in logarithmic form, which allows them to Display load ranges over decades, but the logarithmic scale cannot go to zero.

Figure C shows the output voltage, ground current and load current of the ADM7160 in the range of 10 µA to 200 mA. Other graphs, such as ground current and input voltage, show the measurement results for multiple load currents, but do not show the data when the current is zero.

Can the regulator work stably under no-load conditions?

Can the regulator work stably under no-load conditions?
Figure C. ADM7160 output voltage and ground current and load current.

In addition, parameters such as PSRR, power supply voltage regulation, load regulation, and noise specify a load current range that does not include zero, as shown in Figure D. However, this by no means means that a minimum load is required.

Can the regulator work stably under no-load conditions?
Figure D. ADM7160 load adjustment.

If you use a switching regulator with a power saving mode (PSM), you will often worry about the regulator’s working conditions at light loads, because PSM will reduce the operating frequency, skip pulses, provide pulse trains, or some of these conditions. Kind of combination. Under light load conditions, PSM will reduce power consumption and improve efficiency. The disadvantage is that the output ripple will increase significantly, but the device can still maintain a stable state and can work easily when there is no load.

As shown in Figure E, when the load is switched between 800 mA and 1 mA, the ADP2370 high voltage, low quiescent current buck regulator generates more ripple due to PSM operation. The fact that the test is done at 1 mA does not mean that 1 mA is the minimum load.

Can the regulator work stably under no-load conditions?
Figure E. ADP2370 load transient in power saving mode.

Figure F shows the ripple voltage as a function of load current. In this example, the ripple voltage shown in the figure has dropped to zero, indicating that the load can be zero, and the noise at no load will not be worse than the noise when the current is 1 mA or 10 mA.

Can the regulator work stably under no-load conditions?
Figure F. ADP2370 output ripple and load current.

in conclusion

Most modern regulators can work stably with zero load current. If in doubt, please refer to the data sheet. Nevertheless, it is still necessary to pay attention. The logarithmic graph cannot be returned to zero, and the test is not always performed at zero load current. Therefore, although no-load data is not displayed, you should not infer that the regulator cannot work normally under no-load conditions. When using a switching regulator, it is normal for ripples to appear in the power-saving mode, and it does not mean that it is unstable.

The Links:   LM64P70 LM150X05-A3

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