Most modern LDOs and switching regulators operate reliably under no-load conditions, and some older power devices require a minimum load to ensure stability, because one of the electrodes that needs to be compensated is affected by the payload resistance. For example, the LM1117 requires a load current of at least 1.7 mA (5 mA max).
Most new equipment is capable of no-load operation, a rule that is rarely the exception. Some planning techniques make LDOs stable even with any output capacitance, especially low ESR capacitance, and they are also used to ensure the stability of the device under no load. For some modern devices that require loads, this limitation is generally caused by the leakage current of the bypass element, rather than the stability. If the equipment requires a minimum load, it is necessary for the data sheet to provide some information.
ADP1740 and other low-voltage, high-current LDOs fall into this category. In the worst-case scenario, the leakage current of the integrated power switch is around 100 A (85°C) and 500 A (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 an overnegligent level while improving the no-load output voltage. According to the data sheet, a load of at least 500 A is required, so if the equipment is to be operated at high temperatures, it is advisable to use a simulated load. The load is less than the rated value of the equipment 2 A.. Figure B shows the minimum load current specifications listed in the ADP1740 data sheet.
What if the minimum load is not clearly indicated in the datasheet? In most cases, a minimum load is not required. While it may not sound convincing, this information will certainly be available in the data sheet if the demand load is minimal. However, confusion often ensues, as charts are often used in data tables to show specifications for a particular job size. Most of these charts are in logarithmic form, which allows them to show the size of the load over decades. However, the logarithmic scale cannot be changed to zero.
The ADM7160 are available in output voltages, ground currents, and load currents ranging from 10 A to 200 mA. The ground current and input voltage show the measurement results of multiple load currents, but do not show the data when the current is zero.
In addition, PSRR, supply voltage regulation, load regulation, noise, and other parameters rule a specific load current scale that does not include a zero point, however, this does not mean that a minimum load is required.
If you use a switching regulator with a power saving type (PSM), you will often worry about the regulator's operation at light loads, as the PSM will drop the operating frequency, pulse hopping, supply pulse bursts, or some combination of these conditions. At light loads, PSM will reduce power consumption and improve efficiency. The disadvantage is that the output ripple will increase significantly, but the equipment can still be kept in a stable state, and it can be easily operated without load.
When the load is switched between 800 mA and 1 mA, ADP2370 high voltage, low quiescent current buck regulator will experience greater ripple due to PSM operation. The fact that the test finishes at 1 mA does not mean that 1 mA is the minimum load.
Bottom line: Most modern voltage regulators operate reliably at zero load currents. If in doubt, please refer to the datasheet. It is still important to pay attention. The logarithm cannot be zeroed, and the test is not always done at zero load current. So, although the no-load data is displayed, you should not infer that the regulator will not work properly in the no-load situation. When using a switching regulator, ripple in the form of power saving is normal and does not mean that it is unstable.
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