Benefits of Using A Buck-Boost Converter As A Pre-regulator For LDOs
Intersil published a new white paper titled “Preventing Subsystem Brownouts in Mobile Devices“. This white paper demonstrates the benefit of using a buck-boost converter as a pre-regulator, which leads to better overall system efficiency and enhanced battery life.
Systems powered from a battery may have voltage brownout when they are subjected to a burst current discharge, that is because of internal resistance of the battery. The internal resistance in Li-Ion battery varies according to its charging level. It can reach 200 Milli-ohm at the end of the discharge. Thus, a 4A burst current can cause an 800mV droop at the terminal, pushing the nominal 3.4V voltage to 2.6V, which is considered as a brownout voltage if the target LDO output is 2.85V. In this case boost converters prevent the momentary brownout.
Overall system efficiency is another useful aspect of using a buck-boost converter as a pre-regulator. The battery voltage is first converted to a voltage slightly higher than the highest LDO output voltage of the target LDOs, which is typically 3.3V. The buck-boost output is then set to 3.4V. So, the LDOs see a 3.4V input voltage, regardless of the battery voltage.
We can see, by numbers, the improvement of efficiency by comparing the two setups, with and without using the buck-boost converter.
The figure below shows the comparison of battery discharge with and without a pre-regulator, while running the same applications with the same battery. You can see 12% enhancement to battery life.