Building a Buck Converter Using the LT1170

[sodoyle]

TL;DR

I am trying to change from 25-50 Vin to 5 Vout

Hi, I am trying to use an LT1170 to convert between 25-50 Vin to 5 Vout. Actually, I would like to keep a constant 5 V out regardless of the input. To do that I know the input would have to be 5 V plus any voltage drops with a 100% duty cycle but it's going to be low current so large duty cycles aren't really an issue (I don't think).

I made an LTspice model using the LT1170 from the built in library. I connected everything the same way as shown in the datasheet for a "positive buck converter" but do not get 5 V out. I will mention that a few differences I have are the zener diode and output resistor. Their circuit doesn't define the output resistor since it should regulate the voltage but will just output a higher current. I used 1 Ohm to make things simple. I'm not sure what voltage the zener diode should be rated for because I'm not really sure what it's even doing.

I understand if I can't really get help with LTspice here but even just getting help understanding what the components from the schematic do would be really helpful. Listing all of the components, there is what I think. I tried listing these starting at the left of the circuit and moving right in the order of top to bottom. C5 -- input capacitor to save the source from hard switching.
D3 -- not sure
C3 -- not sure
D1 -- normal buck converter diode (I'm not sure why they show a zener diode though).
C1 -- not sure
R2 -- not sure
R1 -- not sure
C2 -- not sure
D2 -- not sure
L1 -- normal buck converter output filter inductor
C4 -- normal buck converter output filter capacitor
R Unidentified below R4 -- Normal buck converter load resistor
R4 -- not sure

I've attached a screenshot of my LTspice schematic and the output current and voltage waveform at "out" and through "Rload". The link to the datasheet is below. I've also attached a screenshot from page 15 of the "positive buck converter" that I'm trying to build.

https://www.analog.com/media/en/technical-documentation/data-sheets/117012fi.pdf

 

[Baluncore]

D1 is not a Zener, it is a Schottky diode. Notice the S shaped cathode symbol.

Rather than a screen image of your circuit, please post the schematic.asc file which is a text file. To do that here on PF, change it's name by adding another extension to make filename.asc.txt
You can do the same with your filename.plt.txt
Then we can load run and view the plots.

 

[sodoyle]

I didn't realize Schottky diodes used that symbol. Thanks for that. Here is the asc file. It said the file was too large when trying to include the plot data so I will try to post it separately in another message.

 

[sodoyle]

Ok here is the plot data from the LTspice export option.

 

[sodoyle]

Here is the raw file.

 

[Baluncore]

With your LT1170HV_Example.asc file there will also be a plot file called LT1170HV_Example.plt Those are the only two files that need to be copied using the .txt extension. Those two files are the smallest possible way of communicationg the project.
Since I now have your .asc file I can generate the plot file.
I will take a look at your model later.

 

[Baluncore]

Two problems.
FB resistors R3, R4 were incorrect values for 5V output. Use 1k24 & 3k74 ohm.
C6 was insufficient to quieten the output noise. Use 1000uF as per example.

 

[Baluncore]

On page 1 of the LT1170 data sheet, in the fine print;
USER NOTE:
This data sheet is only intended to provide specifications, graphs, and a general functional description of the LT1170/LT1171/LT1172. Application circuits are included to show the capability of the LT1170/LT1171/LT1172. A complete design manual (AN19) should be obtained to assist in developing new designs. This manual contains a comprehensive discussion of both the LT1070 and the external components used with it, as well as complete formulas for calculating the values of these components. The manual can also be used for the LT1170/LT1171/LT1172 by factoring in the higher frequency.

Application note AN 19 is available here;
https://www.analog.com/media/en/technical-documentation/application-notes/an19fc.pdf
See pages 15 and 16 for a good description of the circuit and component selection.

 

[sodoyle]

Application note AN 19 is available here;
https://www.analog.com/media/en/technical-documentation/application-notes/an19fc.pdf
See pages 15 and 16 for a good description of the circuit and component selection.

The app note was helpful. One thing that I noticed is on page 27 it states that the minimum recommended load current is 100 mA. My load is only approximately 20 mA so I guess I can add a "dummy" load to force it to meet that minimum current requirement? I know efficiency will be significantly lower since it will be a total waste of power.

 

[Tom.G]

There is a simpler way, use a Zener regulator circuit. It takes a 750 Ohm, 5Watt resistor and a 5.1V 1Watt Zener. You will get about 0.25V volt variation of the 5V over the 20V-50V input range.Cheers,
Tom

 

[sodoyle]

There is a simpler way, use a Zener regulator circuit. It takes a 750 Ohm, 5Watt resistor and a 5.1V 1Watt Zener. You will get about 0.25V volt variation of the 5V over the 20V-50V input range.

Thanks Tom. I keep seeing the Zener used as a regulator and now it makes more sense. I have a couple of series capacitors with a parallel string of series balancing resistors. I thought it would be simple to "tap" off of one of the 20ish V capacitors and drop it down to 5 V to run my my low voltage circuits. It turns out it's harder than I though. I tried connecting this circuit to it and have the same problem...Voltage drops to only a few volts.

I'll make sure the Zener regulator in mind for future applications where I can use it though. Thanks for the help with this.