There are applications where you may need a specific voltage, but you only have a slighter higher voltage to work with. For instance, perhaps you'd like to power a 3.3V microcontroller with a 3.7V LiPo battery – which may be closer to 4V, or even higher, when fully charged. Rather than just plugging it in to see what happens – generally a bad idea – a low dropout regulator (LDO) produces a consistent output voltage through a variable input voltage range.
Low dropout regulator tutorial
What makes LDOs special? Well, standard voltage regulators can require as much as two volts of difference between the input voltage and the lower-regulated output voltage. However, LDOs can operate with voltages much closer to each other. This important difference is known as headroom, or dropout. Essentially, dropout is the minimum voltage required across the regulator to maintain consistent voltage regulation. The maximum dropout voltage, therefore, is one of the most important specs for such a device.
LDO battery example
As an example, the MCP1700-3302E/TO device found here, has a maximum dropout voltage of .35V at its maximum 250mA output current. This LDO is set to a fixed 3.3V output, with an input voltage range of 2.3V to 6V. Consider if you need to power a 3.3V circuit with a 3.7V LiPo battery voltage. This gives you 3.7V – 3.3V output = .4V of headroom. The .35V dropout voltage rating will be sufficient for this application.
If, however, the battery voltage drops below 3.65V, offering less than the required .35V of space between input and output, the LDO may no longer be able to maintain a 3.3V output level. At this point, it enters dropout regulation, where:
VOUT(dropout) = VIN - VDO
So, if the battery output was 3.6V with a .35V dropout, then the output voltage would be 3.25V, not the 3.3V that you'd expect. Here, you'd need an LDO with a smaller dropout rating. Fortunately, LDOs can be spec'd in the 100mV range, or even lower.
3.3V low dropout regulator: maintaining voltage
The example above is actually the worst-case scenario. Why? The .35V dropout is spec'd at the maximum current level of 250mA, and this value decreases with the current level. So, if the current is low enough, this LDO may still operate correctly. Take a typical performance, found on page 7, figure 2-12 of the MPC1700 datasheet. When operated at 125mA at 25ºC (77ºF), the usual dropout is less than .1V, well within the dropout level needed to maintain a 3.3V voltage.
Note that temperature, as well as input voltage, can affect the minimum dropout resistance. In LDO operation, input voltages produce a lower dropout, and higher temperatures correspond to a higher dropout.
LDO Power Efficiency
When using LDOs, one final consideration is power efficiency. Here, the Power loss (Ploss) is expelled as heat. It's expressed by the equation:
Ploss = I x (Vin – Vout)
So, regardless of the dropout rating, when using an LDO, loss is proportional to the difference between the input and output voltage. Because of this, these devices are most appropriate where the difference between the input and output voltages are small, and/or the total power is relatively insignificant.
In other situations, you may want to consider using a buck converter, or another method for dropping the voltage level. A second option would be to use a Zener diode, as discussed here.
LDO Circuit Setup
As outlined in the MCP1700 datasheet linked earlier, this series of LDOs has three pins: VIN, VOUT, and GND. While one can simply hook the input voltage to VIN and ground to GND, and expect the correct voltage at VOUT, the typical application circuit recommends a 1µF capacitor for stability at both the input and output voltage pins. The negative ends of both are tied to GND. The circuit in action is shown below, with an MCP1700-3302E/TO LDO, which outputs 3.0 VDC:
When the circuit's ground is measured with respect to the rightmost (VOUT) leg, it produces 3.0 volts, as expected. This could, for example, be appropriate, if you wanted to replace a circuit that normally takes input from two 1.5V AAA cells with a LiPo, and can be utilized in a wide variety of applications.
Versions of this particular LDO are available in a wide range of fixed levels, from 1.2 to 5V, and can even be customized if needed. The “TO” lettering signifies the TO-92 through-hole package, but they're also available in DFN-6, SOT-89, and SOT-23.
Of course, the MCP1700 LDO is just one of the many low dropout, fixed regulators that Arrow stocks. In fact, over 27,000 parts are listed on the product page. In addition to fixed regulators, adjustable regulators are also available. These can be set to a specific output level, as needed. Whatever your project, you can almost certainly find something that suits your needs!
