There are various methods for converting DC voltages from one domain to another and there are benefits and drawbacks to each method. I will go over some of the approaches to stepping down voltages in this post and cover some of the factors involved in determining efficiency. We’ll be covering:
- Low Dropout Regulators (LDO / Linear Regulators)
- Buck Converters (Magnetic conversion)
- Charge Pumps (Capacitive conversion)
A linear regulator or LDO is by far the simplest of the options available. Many parts come in a 3-pin package and simply expose an input, output and ground pin. These parts function by dissipating the voltage differential at the load current as thermal energy. The term low-dropout derives from their ability to function with a low differential between input and output voltages.
The LM317 was originally developed in the 1970’s and is a typical part used in this arena. Modern implementations (like the TI part linked above) require a minimum of passive components to accompany the regulator and have some great characteristics such as low ripple output and a high ripple rejection ratio.
For low power applications we are most interested in the efficiency and thermal dissipation of the part. The calculation proceeds as follows: Eff% = 100% * Io*Vo / [(Io + Iq) * Vi] where Io is output current, Iq is quiescent current, Vo is output voltage and Vi is input voltage. For example, with the LM317 linked above, the quiescent current is typically 50uA. Therefore in order to convert from 3.3V to 1.8V at an output current of 25mA our efficiency would be:
100% * .025 * 1.8 / [.02505 * 3.3] = 54.4%
Thus, the selected package must be able to dissipate (3.3-1.8)*.02505 = 37.575mW
Let’s evaluate another one of Texas Instruments newest buck regulators, the TPS62736. This part comes in a 14-pin QFN package with a large grounding block in the center. This feature is especially useful for maximizing thermal conductivity and therefore increasing the maximum power dissipation of the part. The typical application circuit from the datasheet:
As we can see, the part requires at a minimum 7 passive components, one of which is a notoriously large and noisy inductor. This inductor is the primary drawback when choosing to use a Buck regulator. This part also features a digital interface to a microcontroller for enabling and disabling sleep modes. The advantage to using a switching converter is that the efficiency of the conversion is no longer directly proportional to voltage differential. This characteristic allows the circuit to operate more efficiently under a wide range of varying loads.
Switching regulators can take on many different mode and topologies, but most of the efficiency characteristics are similar to the following graph.
While the graph is typical, this part is unique in its ability to maintain high efficiency across many orders of magnitude of load current. Most switching regulators cannot provide an efficiency greater than 50% for loads lower than 1mA. We can see here that the TPS62736 provides 90% efficiency all the way down to 15uA. For this reason it becomes very attractive for use in the low-power / energy harvesting space.
Any switching power supply at its core is simply providing injecting charge into a circuit in order to keep the output voltage as close to a given reference as possible. Buck converters accomplish this function by storing charge in an inductor (an inductor naturally resists change in voltage: V=L*di/dt). A charge pump accomplishes the task by switching capacitors into the circuit to maintain regulation.
The Microchip MCP1252 is a typical example of a charge pump. It provides both Buck and Boost capabilities. We can note the switchover point from Boost to Buck at some voltage slightly above Vout. Thus we can see that a charge pumps efficiency is more dependent on the voltage differential than the output current.Yet varying output current does have a different effect!
Note that the ripple voltage increases dramatically as load current increases. Therefore this type of power supply should not be used on analog or RF circuits. Even digital ones will need to be well within the switching thresholds to function properly.
When choosing a regulator to power your device or peripherals many different factors come in to play. An LDO can fit perfectly for low voltage differentials and low output currents. A switching regulator can help to compensate for a wide range of loads. A charge pump can provide both Boost and Buck capabilities without the need for an inductor.
There are drawbacks to each type as well. A designer needs to be especially concerned with the efficiency of an LDO for the predicted loads. A buck regulator can be noise, expensive and less efficient than an LDO for light loads. A charge pump can be unusable for certain applications requiring clean (low-ripple) power.
As always in engineering, there is no free lunch and each of these design factors need to be considered in order to arrive at the optimal design.