RT8059 step-down DC/DC converter - can't get it to be stable, annyone with experience?

[Andy_c_]

Hi all,

I have an RT8059 step-down DC/DC converter. I want to use it to regulate the voltage from a li-ion battery (seeing as the battery voltage drops from 4.2V to ~3.4V as its charge level decreases). I've constructed the circuit from the IC's datasheet, using suggested component values and component types. However, the circuit doesn't provide a stable output - the output voltage wanders and doesn't stay at a fixed level. I've tried several identical IC's, different components around the IC, and tried constructing the circuit on a breadboard, and copper stripboard, but still no stability.

Does anyone have any experience of this regulator, or any suggestions for how to get it to perform in a stable fashion?

Many thanks.

 

[(*steve*)]

Perhaps you can post the schematic, links to the datasheet, and give us some idea of the way the output voltage varies (and especially the timescale).

It would be useful to know how you are measuring the output voltage -- a multimeter, or an oscilloscope?

 

[Andy_c_]

Hi Steve, thanks for your reply.

Here's the link to the RT8059 datasheet
http://www.richtek.com/assets/product_file/RT8059/DS8059-05.pdf

Here's the schematic of the constructed circuit


Here's a picture of the breadboard circuit


Here's a picture of the copper stripboard circuit


The circuit feedback network resistors are R1 = 1M ohm; and R2 = 200k ohm.
From the formula in the datasheet, this should give an output voltage of 3.6V.
I have an LED and 100 ohm resistor in series as the load (basic 5mm indicator LED, forward voltage ~2.4V)

The circuit is powered by an 18650 li-ion battery, which I've measured regularly and it has a stable output.
I've been using a digital multimeter to measure the output voltage from the circuit. The output voltage typically starts at 1.4V and drops steadily at a rate of approximately 100mV per second. The voltage sporadically rises and falls, without settling at a stable level. I've noticed that touching the ground wire with my finger causes the output voltage to rise significantly.

I suspect that the type of components and their layout are the important factor for stability - the datasheet discusses this, however, I don't have the ability to make a PCB with the appropriate layout! Any suggestions about how to construct the circuit with the components in the required layout but without a custom made PCB?

Thanks again for your help.

 

[(*steve*)]

My first guess is that R1 and R2 are waaaaay too high in value. Start by reducing them by a factor of 100, and if problems persist, by another factor of 10.

Also, please post images directly here. On a phone, pastebin is a pain and it's just lucky that your image was simple enough that I could read it.

You'll probably find that the datasheet gives suggested max values for R1&R2, but it can be somewhat hidden unless you know what to look for.

 

[(*steve*)]

Having looked at the datasheet and your construction, it is pretty clear you didn't comply with the section of the datasheet about layout. On the other hand, the voltage divider may simply require the values to be halved.

it looks like you've set the output voltage to 3.6V. that will require a minimum input of 3.8V, when your battery discharges below this, the output voltage will fall.

To comply with the layout requirements you really need to make a PCB. If you can't do that then you must keep all wiring as short as possible around the output, the inductor, the output capacitor, and ground.

ok, you see slow changes in voltage using a multimeter.

 

[Alec_t]

I don't have the ability to make a PCB with the appropriate layout!

You'd be surprised what you can do with copper-clad board and simple hand tools, such as a drill, needle file, and a robust craft knife (or even a length of broken hacksaw blade). A Dremel or similar tool would be an advantage. Fortunately the recommended layout (Fig 3 of the datasheet) is fairly simple.

 

[BobK]

At 50nA max current into FB, the large resistors should not be a problem.

I also don't think layout would cause it to be off by that much. After checking the connections over very carefully, I would start trying to substitute parts, starting with the IC if you have another one.

Bob

 

[Jordan Dawes]

I would certainly reduce the resistor values alot, as pickup off the tracks and noise can cause issues in the feedback at such low currents. I dont want to repeat what has already been said but i agree with (*steve*)