Hello, and welcome back to the Troubleshooting Buck Design Issues video series. I'm Sam Jaffe, applications engineer at Texas Instruments in the Greater Than 30 Volt Buck Converters and Controllers Group. The last video gave an introduction to Bucks, when you will use them and how they work.

This video details the common issues with the schematic and component selection of a buck converter or controller. We will see what happens when certain component values are too large or too small and the effect it has on the system. Let's get started.

Here's a simplified schematic again. A real schematic has more components than this, and we must be able to pick the best converter/controller for application and use the accompanying components which optimize a system to work as desired. First we must know the requirements or the power solution. This gives us boundaries and goals for the operation of the finished system.

Here are some typical specs-- input voltage, min and max. An example of this is an automotive off battery application where the battery voltage can have transients lower than six volts and higher than 30 volts with a nominal voltage around 13 and 1/2 volts. We care about nominal input voltage, but might also have to consider min and max.

Output voltage, min and max. Some applications require dynamically changing output. Others will have a constant output, which would only care about nominal voltage.

Output current. We have to consider the rated current to ensure the part does not overheat. We often also have to consider current limit, reverse current limit, and if the part can achieve a constant current output if this functionality is required.

Switching frequency. Higher switching frequency typically means smaller components and a less expensive solution, but will have a lower efficiency. We can also consider applications which prohibit switching at particular frequencies.

For example, many automotive applications avoid switching in the AM radio band to avoid interference. Some applications require the frequency to sync to avoid beat frequencies. Some require a constant frequency, which can limit the control scheme and exclude light load operating modes, like PFM.

Quiescent current. Typically, lower is better, but it's often hard to tell what current you can expect for certain conditions. Carefully read to determine if the spec you're seeing is shutdown current, sleep current, non-switching current, active current, or something else.

Control mode. Voltage mode, current mode, and constant on time are common. Each has its own benefits and drawbacks to consider.

Under voltage lockout. Sometimes the internal UVLO is too high for the required input voltage. Sometimes an application requires external UVLO.

And more-- there are many specs to consider. This is not an exhaustive list of every spec or consideration. There are even more considerations in the specs listed above, but these are the main ones.

The system must also have good performance. Some of these performance specs are efficiency, load regulation, line regulation, load transient response, thermal performance, EMI performance, stability, and more. Again, this is not a complete list, but these are the common performance specs of interest.

So how do you find the right part? There are hundreds of ICs to choose from, so how do you find the best part for your design? On TI.com, you can visit the end-equipment reference diagrams. This will show a typical application and recommended parts to fill the needs of that end-equipment. This keeps the application in mind, which helps with all the considerations that the application may require.

You can use the parametric search on TI.com. This sorts all of the parts in each category by specification. This is the way to go if you know your specs and requirements as well as the considerations for your application.

Another way is to use a selection guide. This shows part options concisely and more intuitively. Another good way to find the right part is to find a TI design or PMP which closely resembles your application. This helps to do more of the heavy lifting. From there, you'll just need to modify a design, not start from scratch.

You may also use WEBENCH, our online designed tool. Just put in your specs and it will give you recommended parts with complete designs to fit your specs. Now that you have your part, how do you select the right components around it?

You can look at the evaluation module. The evaluation module will show a solution for a typical spec for part evaluation purposes. This may be close to your design, which can give you a good idea of what your solution will look like. You can also use WEBENCH for designs, which stray further from the EVM. Just put in your specs and it will recommend a complete solution with schematic bomb simulations and typical layout.

For specific questions, there is no better place than the datasheet. The datasheet is the best resource for part specific design questions. It was made specifically to answer the essential questions required to make the part work. The datasheet is always the last stop for design questions and should be used to check the final solution.

Let's talk about the common essential components and how to select for a buck. Let's start with the inductor. Inductor selection is difficult, because, like everything else, there are a few key specs and you can't have it all.

Here are the most popular specs. The rules of the game are that you can move the dots, but you can't add dots. For example, you can sacrifice size for inductance or cost for a higher saturation current.

You can have an inexpensive inductor with poor specs or an expensive inductor with better specs. The best course of action is to find the essential specs and go from there. Let's take a closer look at these specs and how they interact with the performance of the system.

Inductance. A high inductance will achieve a smaller inductor ripple current. A lower inductance achieves a higher ripple. Higher ripple leads to more DCR loss and the higher peak current more easily saturates. A high ripple current may also hit current limit before allowing the maximum required output current.

Current rating. A high current rating typically means the inductor will get less hot during normal operation. Lower current rating will get hotter, which will decrease efficiency and may heat up the IC as well, depending on the layout.

Saturation current. High saturation current gives normal operation. Saturation current too low and the inductor will start to saturate. This can lead to current runaway, which creates more loss and can cause stability issues. This may also hit current limit, which can limit the achievable output current.

DC resistance. Low DCR means less DCR loss. Higher DCR will have higher losses, which decreases efficiency. This increases heat, which may heat up the IC. This is related to current rating by package size.

Next, let's look at input and output capacitance. Input capacitors at the high frequency timescale. This has to do with the inputs response to each switch cycle.

High input capacitance is generally good for a normal operation, but costs more. Input capacitance too low and you will see a drop in voltage at the VIN pin at the IC at every switch cycle. This is often more related to the layout and ESR of the capacitors, but we will discuss this more in the next video. Input capacitance too small can cause stability issues and may cause undesired UVLO tripping from the excessive droop. To fix this, add more ceramic low inductance low ESR capacitance.

Input capacitance at the low frequency timescale. This has to do with the input's response to a load transient. High input capacitance is ideal, but expensive. Input capacitance too low and VIN can droop, causing stability issues and possibly UVLO tripping again. This requires more bulk capacitance, typically an electrolytic capacitor.

Output capacitance at the high frequency timescale. This relates to the output's response to each switch cycle. Higher is better, but more expensive again.

Less output capacitance will cause higher output voltage ripple. Too much ripple will exceed the spec, which depends on how sensitive your load is. Reduce the ripple by adding more ceramic low inductance low ESR capacitance.

Output capacitance at the low frequency timescale. This relates to the output's response to a load transient. Again, higher is better, but more expensive. Too low, and the output will droop when the load current jumps up. Fix excessive droop by adding more bulk capacitance, typically, electrolytic capacitors.

The next most common issue is stability and compensation. This depends on the control mode. So we'll go over three common modes, what to expect, and how to fix stability issues.

Voltage mode. Bad stability in voltage mode controller or converters will look like a fixed frequency at a jittering duty cycle. In very bad cases, the controller may even skip some pulses. This can be fixed with type two or type three compensation. Consult the datasheet, quick start calculator, WEBENCH, or our application notes on the topic to find the best values for the compensation resistors and capacitors.

Current mode. Bad stability in a current mode controller is similar to voltage mode with one addition. You may see a case that shows as a big pulse, little pulse, big pulse, little pulse.

This is subharmonic oscillation and it's an artifact of a current mode controller without enough ramp compensation. Fix these stability issues by adding a feed forward capacitor. Select this value by consulting the datasheet.

Constant on time. Bad stability in a COT can be seen as jittering frequency. Very bad stability can look like it's skipping pulses. This can often be fixed by evaluating different ripple injection-- type one, and type two, or type three. Again, consult the datasheet for the best values for your application.

There are many other components to consider. These can all be selected by consulting the datasheet. Many can also be found using WEBENCH, the quick start calculator for that part, or finding an application note on the topic.

For the FETs, if you are using a converter, then the FETs are built in and you don't have to find FETs. A controller requires external FET selection. Freewheeling or a catch diode. Nonsynchronous parts require a diode in place of the low-side FET.

Soft start capacitor. This reduces the in rush current at startup. See the datasheet for a min and max values.

Boot capacitor. This capacitor provides the voltage to turn on the high-side FET. This value is typically a standard value in the datasheet.

Boot resistor. This optional resistor slows the rise time of the high-side FET. This can help with EMI in the high frequency range. This resistor is typically in the few ohms range.

Snubber. This is also optional to improve EMI. Consult the datasheet or app notes to select the best values.

VCC or VDD caps. This is typically a recommended set capacitor value from the datasheet. RT resistor. This Resistor sets the switching frequency. See datasheet for which frequencies correspond to which resistor values.

ILIM. Some parts have adjustable current limit. Again, consult the datasheet.

PGOOD or RESET. This usually requires a resistor connected to a voltage to pull up when the output is properly regulated. And more-- see the datasheet for all the pins and all the features of your IC. The datasheet will give all the information necessary to get the design up and running, but feel free to also use WEBENCH, the quick start calculator, or application notes to achieve your power solution.

This concludes the schematic portion of the Troubleshooting Buck video series. Again, this is not an exhaustive list, but these are the most important and most common components and how they interact with the system. Stay tuned for the next video, which we'll talk about common layout issues and how to minimize parasitics and coupled noise to achieve a robust design. Thank you for watching.