“Many automotive and industrial applications require efficient 12V or 24V step-down power conversion from mains to low point-of-load (POL) voltages not only at full load, but also require very low current consumption when the device is idle or off. To achieve such low currents, you can simply use a low dropout regulator (LDO) in parallel with a buck converter to draw minimal current from the battery when the system goes into light/no load conditions.
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Many automotive and industrial applications require efficient 12V or 24V step-down power conversion from mains to low point-of-load (POL) voltages not only at full load, but also require very low current consumption when the device is idle or off. To achieve such low currents, you can simply use a low dropout regulator (LDO) in parallel with a buck converter to draw minimal current from the battery when the system enters a light/no load state.
Ultimately, the ideal situation for extending battery life in a system would be to prohibit any possible device from using the input power. In some cases, however, certain components in the system still need to be provided with regulated voltages to enable communication with other system blocks during the off-state (i.e. CAN bus transceivers in automotive applications). DC/DC converters that are not specifically designed for light-load efficiency consume a few milliamps of current at no load. Additionally, converters that do exhibit high efficiency at high loads will employ frequency foldback mechanisms and discontinuous operation, resulting in noisy output voltages and excessive electromagnetic interference (EMI). LDOs are ideal for light load conditions because they can be designed to consume very low current while maintaining a low noise output voltage. The no-load current (also known as “ground current”) into the input can be on the order of a few microamps or less. Therefore, there are clear advantages to combining the performance of the converter and LDO.
If the designer can disable the DC/DC converter when the load is going to be minimal, an easy way to use the two in parallel is for the designer to control the load with a signal that enables/disables the converter. An example of this is shown in Figure 1:
Figure 1. Block diagram of a C DC/DC converter in parallel with an LDO
A generic example of a low-lq LDO efficiency curve is shown in Figure 2, where a DC/DC converter efficiency curve for higher voltage conversion (ie, 12V to 1V) is also plotted. At light loads, the LDO is more efficient. If the system is lightly loaded most of the time, using an LDO to regulate the voltage can greatly improve the overall system efficiency.
Implementing the circuit in Figure 1 requires that the converter output voltage be set higher than the maximum LDO output voltage. During normal operation, when the converter is enabled, the converter will regulate the output voltage and supply current to the load. Most LDOs cannot sink current and therefore need to rely on the load current from the pass device to regulate the output. Pulling the LDO’s output voltage above its nominal voltage will force the LDO into an unregulated state where current will not flow from the input to the output and the DC/DC converter will operate efficiently as if it were not regulated. Same as connecting LDO.
Once the DC/DC converter is disabled, it will stop switching and the output voltage will drop until the LDO starts regulating the output. When enabled again, the DC/DC converter will start up in a pre-biased condition (a positive voltage on the output at startup is called “pre-biased”). The converter will begin its startup process without sinking any current from the output node, eventually pulling the voltage on the output above the nominal LDO voltage and regaining control of the output.
Please take a closer look at the simple calculation method of the service life of a battery with a capacity of 1400mAh. Say the device is held in standby after the battery is fully charged, and the power supply connected to it is an LDO with 10uA quiescent current at no load, or a DC/DC converter with 200uA quiescent current at no load .
| battery capacity | Battery life with DC/DC converter (full first charge) | Battery life with LDO (full first charge) |
| 1400mAh | 1400 / 0.2 = 7000 hours | 1400 / 0.01 = 140000 hours |
Battery life can be extended 20 times! In the next blog post, we’ll discuss an example of how to do it with TI devices.
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