How to Improve Buck Converter Light Load Efficiency with an LDO




Designing a DC/DC converter that only consumes micro amps of current at no load can be compared to fueling a muscle car with lighter fluid – you may get it to work, but it won’t be easy. High efficiency at full load currents is commonplace in most modern DC/DC converters; however, achieving high efficiency when the load is disabled or disconnected is still a difficult and/or expensive task.

How to Improve Buck Converter Light Load Efficiency with an LDO




Many automotive and industrial applications require efficient 12 V or 24 V buck power conversions from the main supply down to the point-of-load (POL) voltage at full load, but also need very low current consumption when the device is in an idle or shutdown state. In order to achieve such low current, you can easily use a Low Dropout Regulator (LDO) in parallel with a buck converter to achieve minimal current draw from the battery when the system enters a light-load/no-load state.

Ultimately, the ideal situation to prolong battery life in the system would be to disable every possible device from the input supply. In some cases though, a sub-regulated voltage is still necessary for certain components within the system to allow communication with other system blocks during the shutdown state (i.e. a CAN bus transceiver in an automotive application). DC/DC converters that are not specifically designed for light-load efficiency can consume several milliamps with no load. Additionally, converters that do exhibit high light-load efficiency will utilize frequency-foldback schemes and discontinuous mode operation, resulting in noisy output voltages and excessive EMI emissions. An LDO is ideal for light load situations because they can be designed to consume very low currents while maintaining a low noise output voltage. The no-load current into the input (also called “ground current”) can be on the order of a few micro amps or lower. Combining the performance of both converter and LDO, therefore, has obvious benefits.

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A simple method of using both in parallel is possible if the designer has the ability to disable the DC/DC converter when the loads will be minimal, i.e. using the same enable/disable signal for the converter that is used for the load. An example of this can be seen in Figure 1.

Figure 2 gives a generic example of an efficiency curve of a low-IQ LDO plotted with an efficiency curve of a DC/DC converter for a higher voltage conversion (i.e. 12 V to 1 V). At light loads, the LDO is more efficient. If the system spends a significant amount of time at light loads, using the LDO to regulate the voltage can drastically improve the total system efficiency.

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