Article Technical Rating: 8 out of 10
When designing electronic circuits you will find yourself spending an enormous amount of time reviewing datasheets for all of the various electronic components. Learning how to read datasheets is a critical skill for anyone wanting to design electronic circuits.
In this first datasheet review (blog + video) I’m going to review the datasheet for a relatively simple electronic component: a linear voltage regulator.
As simple as it may sound, the datasheet for a voltage regulator can still be quite complex. The datasheet I will review in this video (the TPS799 from Texas Instruments) is 42 pages long!
As a component becomes more complex, like a microcontroller or a microprocessor, the data sheet may be hundreds of pages.
The TPS799 is a 200mA, low-quiescent current, ultra-low noise, high-PSRR, low dropout linear voltage regulator. That’s definitely a mouthful, and in this review I’ll break down exactly what this all means.
Let’s start by breaking down the TPS799 datasheet title “TPS799, 200-mA, Low-Quiescent Current, Ultralow Noise, High-PSRR Low-Dropout Linear Regulator”.
The 200-mA in the title refers to how much load current the regulator can supply. It is guaranteed to be able to supply this amount of load current under all conditions (temperature, input voltage, part-to-part variation, etc.).
As do most all linear regulators, the TPS799 embeds a current limit circuit that will cause the output voltage to drop if this limit is hit.
Quiescent current is the current that the component itself uses. You’ll see this current specified as quiescent current, ground current, or supply current. All these are basically the amount of current that the component itself will use in performing the intended function.
On the first page of the datasheet you can see that this regulator has a typical quiescent current of only 40uA, which is quite low for a linear regulator.
There are linear regulators that use much lower current, even less than a microamp of quiescent current. But they’re not going to have near the performance of the TPS799.
Next in the headline is “ultra-low noise”. Noise, in electrical terms, is different than what a layperson would think of as audible noise, which is just vibrations of air.
Electrical noise can mean a lot of different things depending on the type of circuit and application that you’re talking about. For a linear regulator it refers to physical noise. It’s noise at a fundamental, physical level that is generated by various components and circuits.
For instance, a resistor is a passive component but it has a noise associated with it, that is called thermal noise. The higher the temperature, the more noise that is generated by a resistor. The 799 is a linear regulator that focuses on generating low amounts of noise on the output.
PSRR stands for Power Supply Rejection Ratio. High PSRR means that the regulator has a high rejection of any unintended AC ripple on the input.
If a high-frequency AC signal is unintentionally coupling on to the input voltage, a high-PSRR regulator will reject it and keep the output voltage clean and stable.
PSRR is similar to another specification called a line transient. A line transient is when you put a step on the input voltage. A “step” refers to a fast transition from one voltage to another. For example, if the input voltage where to quickly go from 4V to 5V, that would be a 1V step.
For an ideal regulator, this would have zero impact on the output voltage as long as that low point (4V above) of the step remains above the minimum input voltage required by the regulator.
No regulator is perfect, and they will pass some of this transient onto the output, but it will be greatly attenuated. In almost all cases a high-PSRR regulator will also exhibit good line transient characteristics.
Next is low-dropout. Dropout for a regulator is how close the input and the output can be in order to still have the parts regulate the voltage.
If you have a five-volt input and a 3.3-volt output, then you have a voltage drop across the regulator of 1.7 volts. That’s plenty of voltage for most regulators to work.
Let’s say all of a sudden you have a 3.7-volt input from a lithium-ion battery, and you want to output it as a regulated 3.3. You’ve got 3.7 on the input, 3.3 on the output. That’s only a difference of 400 mV between the input and the output.
If that difference is below the specified dropout voltage for the regulator, then the regulator no longer acts like a regulator. It essentially becomes a small resistor between the input and the output.
A low-dropout regulator allows you to use the regulator in applications where the input and the output are really close together. In most cases, this is the best case scenario for using a linear regulator.
The other type of voltage regulator is called a switching regulator. Switching regulators are much more complex to use, so I will cover those in a future tutorial.
Typical Application Circuit
At the bottom of the front page you’ll see two circuits. The first is the typical application circuit for fixed output voltage versions. The second circuit is the circuit for the adjustable voltage version.
Figure 1 – Typical application circuit for fixed voltage versions and the adjustable version.
Let’s start with the fixed version since it’s the simplest. You have the three primary pins required for a regulator: input, output, and ground. You need a capacitor on the output, and ideally one on the input as well.
Then you have the enable pin, which in this case is active high. That means if the enable pin is high, the part’s on. If it’s low, the part’s off. Finally, there is the NR pin which allows the use of an additional capacitor.
Although NR stands for Noise Reduction, adding this capacitor provides many advantages including: reduces noise, improves PSRR, reduces inrush current, and eliminates any transient output voltage overshoot at startup.
On the adjustable voltage version you pretty much have the same thing except now the NR pin is gone and is replaced by a feedback pin. This allows the use of an external resistor divider (R1 and R2) to set the output voltage.
The CFB capacitor shown across R1 is for compensation of the feedback loop. For the fixed versions, R1, R2, and CFB are all internal.
A downside of the TPS799 adjustable version is you lose the NR pin. So the adjustable version ends up having significantly lower PSRR and higher output noise compared to the fixed versions.
If high PSRR and low noise are really critical for your application, then you definitely want to go with the fixed version.
I’ll now review the TPS799 features highlighted on the first page.
Figure 2 – Primary features of the TPS799 linear regulator.
200 mA Low-Dropout Regulator with EN means it has an enable pin, so you can turn the part on and off.
Multiple Output Voltage Versions Available: Most regulators will tend to come in two types: fixed output voltage versions, and an adjustable output voltage version.
By using two external resistors and you can program the regulator to output any voltage within its specified range. For this product, it can go as low as 1.2 volts and as high as 6.5 volts. These resistors are simply internal on the fixed voltage versions.
The reason that 1.2V is the lower end of the output voltage range is because 1.2V is the voltage of the regulator’s internal voltage reference. This voltage then gets multiplied by the feedback loop to provide the intended output voltage.
For example, if the output voltage is set to 2.4V, then the output gain is exactly 2x so the 1.2V internal voltage reference is doubled at the output.
Inside of any regulator is a voltage reference, which is also called a bandgap reference. The bandgap voltage is a physical characteristic of silicon itself, so the reference will always be 1.2V for silicon based chip designs.
The key characteristic of the bandgap reference is that it’s voltage can’t vary much at all with temperature. This is important because most things in an integrated circuit are drastically impacted by temperature changes.
Inrush Current Protection with EN Toggle: Inrush current is when you first power up a regulator, such as a linear regulator, and there can be a huge amount of current that rushes into the part to charge the output capacitor. You have to keep in mind that a capacitor, when it’s not charged, essentially looks like a short to ground.
When you first power on the part, the output capacitor is not charged. You apply the input voltage and all of a sudden the regulator thinks, “Oh, I should have 3.3V on the output but I’ve got 0 volts.” So the regulator tries to quickly charge the output capacitor by supplying as much current as possible.
As the output capacitor charges up and gets to the target output voltage, then that current will disappear. You can have a huge inrush current – a momentary spike in current, when you first turn on a regulator as it is charging the output capacitor.
The TPS799 only has inrush current protection if you power up the part first and then toggle the enable pin.
It is able to provide this feature by using an external capacitor that is connected to the regulator’s internal reference. This capacitor causes the regulator’s output voltage to rise up more slowly. The bigger the capacitor, the slower it goes.
This capacitor actually serves three purposes – it eliminates inrush current, it provides a smooth voltage ramp-up, and it reduces noise.
Sometimes when you do just a full turn on really fast, a regulator will overshoot a little bit and then eventually settle, in microseconds, back down to the intended voltage. But having this inrush current protection allows you to have a much smoother output voltage ramped up without any type of overshoot.
Low IQ: 40 μA. This is the typical quiescent current consumed by the TPS799. Just keep in mind this is a typical specification and the quiescent current will be higher at hotter temperatures. Nonetheless, 40uA is really quite low for the level of performance the TPS799 provides.
Figure 3 – Ground Current vs Temperature
The TPS799 is built on a BiCMOS process. BiCMOS is a semiconductor fabrication process that incorporates two types of transistors: Bipolar transistors and CMOS transistors.
You’ll see some older regulators that are built using only bipolar transistors. But, in general, I avoid bipolar-only regulators because the big downside is that the quiescent current increases as the load current increases.
A BiCMOS regulator, however, will have a quiescent current that doesn’t really change much with load current. With a bipolar-only design, though, a percentage of the load current is simply wasted as additional ground current.
High PSRR: 66dB at 1kHz – Power supply rejection varies over frequency. All regulators are much better at rejecting ripple at lower frequencies than higher frequencies.
Figure 4 – PSRR versus frequency for the TPS799285 (Vin – Vout =1V)
I don’t want to get too off topic, but a decibel is a logarithmic scale. 66 dB is roughly 2,000. This means if you put a 1-volt 1kHz sine wave on the input, then you’ll see it on the output but it will be 2,000 times smaller (with an amplitude of only half a millivolt).
The requirement for high PSRR and ultralow noise becomes especially critical for Radio-Frequency (RF) circuits, so that’s why the TPS799 is commonly used in cellphones, wireless LAN, Bluetooth, RF, etc.
Stable with a Low-ESR, 2-μF Typical Output Capacitance: Let’s start with what it means by stable. A linear regulator or any regulator has a feedback loop.
Basically, feedback is what allows the linear regulator to produce an accurate output that remains constant even as the input voltage and load current vary.
The device is constantly measuring the output and then feeding that signal back into the control circuit so the circuit can make any necessary adjustments so as to maintain the desired output voltage.
Every day you use similar feedback from your eyes, ears and other senses to make ongoing adjustments to whatever task you are performing.
The issue with a feedback loop is it has to be a negative feedback loop. Let’s say the output tries to go high, you want it to feed back through the feedback loop so the regulator says, “The output is going high, I need to make the output do just the opposite to keep it on target. I need to make the output go low to compensate”. That’s negative feedback.
Whereas, if you have positive feedback, then it can be just a runaway condition. Let’s say the output goes up, then the regulator says, “I need to make the output go higher.” So the output goes higher, and then that feeds back through again, and the control loop says to go even higher. You will have this runaway condition.
Essentially, what you will get in a regulator if the feedback becomes positive is an oscillation. All of a sudden, your nice, clean, stable linear regulator now becomes an oscillator. In 99.9% of the cases, you don’t want that.
Obviously, there are cases where you want an oscillator, but not on your power supply line. A regulator has to be stable during operation. The way you do that is a really complex subject called compensation.
There are capacitors and resistors internal to the device, usually that help to make sure that this feedback loop never becomes positive feedback. Part of the compensation that’s required is the output capacitor. Most regulators, you have to have a capacitor on the output, which does several things.
It acts like a small battery and stores charge. Let’s assume, all of a sudden, your system requires a quick spike in current faster than the regulator can supply or respond to. Then, the capacitor will supply that load.
That capacitor also serves to stabilize the feedback loop of the regulator.
You have to be very careful and follow the manufacturer’s recommendations on the capacitor that you want to use.
In this case the TPS799 datasheet says it’s stable with a low ESR capacitor. ESR stands for Equivalent Series Resistance. Any capacitor has an equivalent resistor in series that is internal to the capacitor itself.
A lot of older regulators would require capacitors (like tantalum capacitors) that have a higher series resistance. They actually use this ESR to compensate the feedback loop.
The TPS799 works with low ESR which allows you to use ceramic capacitors. Low ESR capacitors will also give you a better transient response. But you just have to make sure the regulator will be stable with one.
The 2 μF is the minimum output capacitor. Some products will require a 4.7 μF or a 10 μF. This one works for the 2 μF. There are some that will go down 1 μF or even less than a microfarad of output capacitor, but you almost always need some type of capacitor on the output.
Excellent Load and Line Transient Response: I’ve already talked about line transient. That’s if you quickly change the input voltage, what affect does it have on the output voltage?
Figure 5 – Line transient response
Load transient response is whenever you do a step on the load. Let’s say the regulator is sitting there supplying only one milliamp of load current. But all of sudden something turns on in the system and the regulator has to almost instantaneously supply 200mA. That’s a load transient.
Figure 6 – Load transient response
Ideally, when you go from 1 mA up to 200 mA, the output voltage will stay perfectly flat at the regulated voltage. But, nothing is ever perfect.
What will happen in reality is that there will be a momentary pull down on the output voltage until the regulator’s feedback loop has sufficient time to respond.
How much that dips down when you increase the load, or how much it overshoots if you quickly decrease the load, is a measure of the load transient response of the product.
2% Overall Accuracy (Load, Line, and Temperature): This says that regardless of the load current, input supply voltage, or temperature the output voltage is always guaranteed to be within 2% of the target voltage.
The TPS799 is fully specified over the temperature range of -40 C to 125 C and is offered in BGA, SOT and SON packages.
Very Low Dropout: 100 mV: I’ve already discussed dropout as part of the datasheet title. This says the regulator has a very low dropout of only 100 mV (which is extremely low).
So as long as the input voltage is at least 100 mV higher than the output voltage (and higher than the minimum allowable input voltage), then the regulator will not be in dropout mode. It will continue to operate as a linear regulator.
Figure 7 – Dropout voltage versus load current at several temperatures for the TPS799285.
Low drop-out (LDO) linear regulators are so common now that you’ll actually hear people referring to them as an LDO regulator or just sometimes just an LDO.
Remember, if the input and output are too close together then the linear regulator goes into dropout, which means the entire regulator at that point can be simplified to just a really small resistor between the input and the output.
At that point the output will track the input. That means your power supply rejection ratio will be nearly zero!
In dropout, if you wiggle the input, the output is going to wiggle just as much because there’s essentially just a resistor between the two.
If you do a line transient on the input voltage while in dropout, you’re going to see that same line transient on the output voltage because, once again, there’s just a small resistor between them and no rejection at all.
When designing any power circuit it is critical that you consider the thermal dissipation, especially for linear regulators. Linear regulators are horribly inefficient in many applications. This means the chip can get incredibly hot.
You have to be careful with a linear regulator, especially if you’re operating it with an input voltage that is much higher than the output voltage.
For instance, let’s say you have the TPS799 setup with a 6.5V input voltage and a 1.2V output voltage that needs to supply 200mA. To calculate the power dissipation for a linear regulator you simply multiply the output current by the difference between the input and output voltage.
PD = (Vin – Vout) x Ioutput
So for this example, that means the power dissipated by the regulator is (6.5V – 1.2V) * 0.2A = 1.06 Watts.
Now you need to calculate how much the regulator will heat up when dissipating this amount of power. If the power dissipation causes the regulator’s temperature to exceed the maximum specified temperature of 125°C then it’s too much power for that regulator. If it gets hot enough, usually around 150°C, there’s a thermal shut down circuit inside the chip that will turn it off.
Figure 8 – Thermal information for the TPS799.
To do this calculation you need to find the thermal information section of the datasheet and look up the rating called the Junction-to-ambient thermal resistance or RØJA (pronounced R-theta-JA) which is specified in °C/W.
This thermal resistance is determined by the package for the regulator. The TPS799 is available in three packages so the thermal resistance for each is shown. You can see the SON package is much better than the BGA in regards to power dissipation. Let’s assume we’re using the SON package.
The thermal resistance for the SON package is 74.2 °C/W. This means when dissipating 1.06 Watts, the SON package will heat up about 79 °C. But then you have to add the ambient air temperature.
If you assume the ambient temperature is room temperature then the regulator’s temperature will be 25 °C + 79 °C = 104 °C. This is below the maximum specified temperature of 125 °C so this is acceptable.
However, if you wanted to use the design in an ambient temperature of 50 °C, the regulator’s temperature will be 50 °C + 79 °C = 129 °C. This exceeds the maximum specified temperature. When calculating the maximum allowable thermal dissipation it’s essential that you use the maximum ambient temperature.
The TPS799 linear regulator from Texas Instruments is an ideal choice for applications requiring an exceptionally clean voltage supply such as radio frequency applications.
It offers high-PSRR and ultralow noise while consuming very little quiescent current. The low supply current makes the TPS799 an ideal regulator for mobile RF applications such as cell phones.
The fact that the TPS799 also provides exceptionally low dropout voltage makes it ideal for use in applications where the output voltage is close to the input voltage. For example, in applications where the power source is a 3.7V rechargeable lithium battery and the output voltage is 3.3V.
For applications where there is a much larger difference between the input and output voltages, it may be necessary to use a more efficient switching regulator in front of the TPS799. The switching regulator will efficiently step down the high input voltage before feeding it to the TPS799.