On this weeks agenda : Buck-Boost controller!

Literature Buck-Boost vs Linear Tech Buck-Boost

First of all I have to define what a Buck-Boost is and how it is defined in different sources. In the common literature you get at University for example a Buck-Boost converter is defined as a inverting converter. It converts a positive input voltage into a negative input voltage. This negative output voltage can be either a bigger or a smaller negative representation of the input voltage (for example 5V in and -3V or -10V output voltage with the same circuit, just a different duty cycle). At Linear Tech they do have such converters, but they name it inverters, which is some kind of more intuitive because they invert the voltage.

A good representation of such a inverter is the LTC3896 which is shown below.

LTC3896 – sync inverting circuit

Linear Tech has a different definition of Buck-Boost Converters (and some other semiconductor mfg are following) because when you think of a Buck and a Boost you think of first a step down converter and secondly a step up converter. So a Buck-Boost from Linear Tech does both. In cases where the input voltage is lower then your desired output voltage it boosts the input voltage and in cases where the input voltage is higher than the output voltage it steps down the input voltage. This brings some degrees of freedom to your design.

Where is this of usage?

First of all in all areas where the power supply for your design is of very poor quality. Its often the case for large industrial installations where you have long lines on 24V rails or even 12V rails. In such cases the voltages can be very unstable.

Second for battery application where the battery chemistry allows to be discharged to a very low level. In such applications it comes in very hand that the input voltage doesn’t matter that much and you can use more of your battery.

Third, when you don’t need isolation in your design and you don’t want to care about the topology on the input. If you have for example a 12V output and its possible that your design sees an input voltage from 5V to 48V on the input.
In such cases I recommend to spend some extra attention to possible input over voltages and the EMC filter.

The LT8390

Now to the LT8330. This component as in the picture drawn below can be used for a wide variety of input/output voltage combinations. In the case of this application its 4V to 56V input to a stable 12V output.

LT8390 – Buck-Boost controller for 4V to 60V input

How the circuit works

The experienced power designer will spot in the schematic above a H-bridge design around the inductor (above the IC with 6µH). With this 4 power switches it is possible to create either a buck or a boost topology. For a buck topology the power switches on the left hand side of the IC’s are used to step the voltage down with the inductor. For the power switches on the right hand side this operation means a steady state operation. The lower power switch is in permanent off state and the higher power switch is permanently on. For the boost topology its mirrored. The power switches on the right hand side are switched for the boost operation and on the left hand side the lower power switch is in permanent off state and the upper one is in permanent on state.

Here is a question for the circuit designers: Will both topology settings work if the gate signals on the switches with the permanent state are failing (means ground) ? (Hint: Body diodes 😉 )

Spread Spectrum – your helping hand in EMC

Buck converters can often be crucial when it comes to EMC – noise on the input. With wide input voltage ranges it can happen that the converter switches from boost to buck mode during operation at some time and your conducted EMC worsens immediately on the input (but it gets more calm on the output). To get a helping hand Linear Tech built spread spectrum in this IC. Spread sprectrum modulates the switching frequency in a certain way that it distributes the energy of the noise peak to flatten it and make it lower.

You can also find some other hints to EMC and buck converters at my blog post: 7 ways to improve your buck converter EMC

Demo Boards

As usual there are demo boards available – just contact your Arrow account manager of FAE for that and they will get you one!

LTSpice model is still in the make and will be on the website as soon as possible.