Attention to Details
The world’s IC manufacturers have produced a rather astonishing super-abundance of linear voltage regulators—a quick Digi-Key search indicates something on the order of ten thousand different parts. This vast array of options could be somewhat problematic for anyone who is subject to bouts of indecision; overall, though, this situation is beneficial because it allows us to find an LDO that is just about perfect for a particular application.
However, you can’t thoroughly fine-tune your regulator circuit without understanding some of the less-prominent details of LDO performance. I readily admit that many designs will be fully functional if you choose a part based merely on input-voltage range, output voltage, and maximum load current. But your selection process needs to be a little more involved if your application requires, for example, low power consumption or high precision.
Before we get started, a note about terminology: The proper way to refer to the type of device discussed in this article is “linear voltage regulator.” This is a rather cumbersome term, though, so it would be nice to have recourse to the corresponding initialism, i.e., LVR. Unfortunately, this initialism is never used. For better or worse, the de facto abbreviation for “linear voltage regulator” is LDO, which stands for “low dropout,” as in “a low-dropout linear voltage regulator.” This sort of terminology lacks the precision that one would expect from engineers, but at least the abbreviation is rendered more appropriate by the fact that nowadays virtually all linear voltage regulators exhibit dropout voltage that can be considered “low.”
Doesn’t All Current Flow to Ground?
In this age of tiny electronic widgets that are expected to run for months or even years on a single battery, power consumption is serious business. Thus, it’s important to understand that your LDO consumes some current in the process of converting the input voltage into a regulated output voltage. This current is often referred to as “ground current”—a term that I consider fabulously vague, considering that current of all kinds has a tendency to return to the ground node. A better choice is “ground-pin current,” i.e., current that flows directly from the input terminal back to the power supply through the ground terminal.
In any event, you have to factor the LDO’s ground-pin current into your power budget, but this is not particularly simple because ground-pin current is influenced by the input voltage and the load current. Here are the ground-pin current specs for part number ADP3339 from Analog Devices:
Take note of three things here:
- There is an approximately factor-of-three discrepancy between the “typical” and “maximum” specs, and you need to keep that in mind when planning for “expected” vs. “worst-case” power consumption.
- Ground-pin current increases significantly with load current; you need to take this into account if your design incorporates a low-power mode in which the regulator supplies much less current than it does during normal operation.
- Operating the LDO in dropout can lead to increased ground-pin current.
Here is another example of ground-pin-current specs, in graphical form this time. This is taken from the datasheet for the LT3007 series from Linear Tech.
Notice how higher input voltage reduces the ground-pin current.
A distinct but related specification is “quiescent current.” This term, in contrast to “ground current,” is quite informative—“quiescent” recalls the word “quiet” and refers to a state of inactivity, and thus quiescent current is the current consumed by the regulator when it is not supplying load current.
Here are quiescent-current specs for the LT3007 series.
Note that the quiescent current increases with temperature:
Some writers use “quiescent current” as another term for “ground current,” but I think it’s a good idea to maintain the distinction used in this article: “quiescent” refers only to ground-pin current consumed by the regulator when it is not supplying load current.
Line and Load
I can’t readily think of a situation in which you would need an extremely precise voltage merely for power-supply purposes; integrated circuits are tolerant of variations in supply voltage. However, there are certainly times when you want to reduce cost or component area by using an existing linear regulator as a reference voltage for a data converter. When this is the case, you need to carefully consider the various factors that can cause the regulator’s actual output voltage to deviate from the expected output voltage.
It’s no surprise that one source of inaccuracy is the initial difference between the regulator’s nominal VOUT and actual VOUT—for example, the actual output of a “2.5 V” regulator at a particular combination of input voltage and load current might be anywhere from 2.45 V to 2.55 V. To minimize this source of error, you can measure the actual output voltage and modify hardware or firmware accordingly, or you can simply choose a highly accurate LDO (I’ve seen parts with initial accuracy as good as 0.5%).
In addition to initial accuracy, though, you have to account for line regulation and load regulation. Line regulation refers to how much the output voltage changes as a result of variations in the input voltage, and load regulation refers to how much the output voltage changes as a result of variations in the load current. As shown in the following specs for the ADP3339, line regulation can be expressed in units of mV/V (i.e., millivolts of output change per volt of input change), and load regulation can be expressed in units of mV/A (i.e., millivolts of output change per ampere of load-current change).
If you have at least a general idea of your load current and input voltage, you can use the line and load regulation specs to more accurately predict the regulator’s output voltage. Also, you could compensate for changes in load current by accounting for load regulation in your firmware. For example, if you have a microcontroller that knows when your board is in a low-power state, it can modify the calculation applied to an analog-to-digital conversion result based on the expected current consumption in that particular state.
Don’t Burn Your LDO
Some other important yet easily overlooked specs in a linear regulator’s datasheet are those related to temperature limits and thermal resistance:
Designing a circuit in which an LDO can overheat is easier than you might think. This topic is covered in Thermal Design with Linear Voltage Regulators.
We’ve covered some of the finer points of linear-regulator operation. Remember to look at ground current and quiescent current when you need to minimize power consumption. If your system calls for a high-precision voltage, you should consider initial accuracy, line regulation, and load regulation.