Can the voltage regulator work stably under no-load condition?

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Voltage Regulator: High Efficiency, Ultra-Low EMI

Some older 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 minimum load current of 1.7 mA (maximum 5 mA).

Figure A. LM1117 minimum load current specifications.

Most modern devices can operate with no load, and there are very few exceptions to this rule. Design techniques that allow LDOs to remain stable with any output capacitor (especially low ESR capacitors) are also used to ensure stability with no load. For the few modern devices that require a load, this limitation is generally due to leakage current through the pass element, not stability reasons. So how can you tell? Look at the data sheet. If the device requires a minimum load, the data sheet will definitely provide some information.

The ADP1740 and other low-voltage, high-current LDOs fall into this category. The integrated power switch generates a worst-case leakage current of approximately 100 µA at 85°C and 500 µA at 125°C. Under no-load conditions, the leakage current charges the output capacitor until the VDS of the switch is low enough to reduce the leakage current to negligible levels while increasing the no-load output voltage. The data sheet states that a load of at least 500 µA is required, so a dummy load is recommended if the device is to operate at high temperatures. This load is less than the 2 A rating of the device. Figure B shows the minimum load current specification listed in the ADP1740 data sheet.

Figure B. ADP1740 minimum load current specification.

What if the minimum load is not clearly stated in the data sheet? In most cases, no minimum load is required. Although it may not sound convincing, if a minimum load is required, it will definitely be provided in the data sheet. However, confusion often ensue because data sheets often use graphs to show specifications for a certain operating range. Most of these graphs are in logarithmic form, which allows them to show load ranges of decades, however, the logarithmic scale cannot go to zero.

Figure C shows the output voltage of the ADM7160 over the range of 10 µA to 200 mA, along with the ground current and load current. Other graphs, such as ground current vs. input voltage, show measurements at multiple load currents but do not show data at zero current.

Figure C. ADM7160 output voltage along with ground and load current.

Additionally, parameters such as PSRR, line regulation, load regulation, noise, etc. are specified over a certain load current range that does not include zero, as shown in Figure D. However, this by no means implies that a minimum load is required.

Figure D. ADM7160 load regulation.

您如果使用具有省电模式 (PSM) 的开关稳压器，则往往会担心稳压器在轻负载时的工作情况，因为 PSM 会减少工作频率、跳脉、提供脉冲群或出现这些情况的某种组合。在轻负载的情况下，PSM 会减少功耗，提高效率。其缺点在于输出纹波会显著增加，但是，器件仍可保持稳定状态，并且可以在空载时轻松工作。

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

Figure E. ADP2370 load transient in power save mode.

Figure F shows the ripple voltage as a function of load current. In this example, the ripple voltage is shown going all the way down to zero, indicating that the load can be zero and the noise at no load is no worse than at 1 mA or 10 mA.

Figure F. ADP2370 output ripple vs. load current.