What effect does switching frequency of voltage converter have on my project?

Hi all,

I have an application where I need to use some kind of boost/buck IC for maintaining 9.6V to power a radio module from a 12V battery through it's discharge down to 7.6V

I'm on digikey looking at all of my options but I'm not sure what effect the switching frequency of such an IC would have on the electronics "downstream."

What factors should I be considering?

Thank you!

It shouldn’t have any effect since the output is DC (zero Hz).

It will affect the overall design and parts selection, particularly the inductor selection.

And, there’s the possibility of RF leakage or noise on the DC line which could be an issue in certain analog applications. If you have an audio application, you’ll want to be well above the audio range, and if it’s an RF circuit you want to be away from the transmit/receive frequency, etc.

Always use linear regulators for RF modules if possible. You can use a DC-DC converter for efficiency
and then a low-dropout linear regulator to cleanup the output of the converter (and the cost of about 1V).

Any noise or ripple on the supply voltage tends to strongly modulate RF amplifiers and is to be avoided.

You could use a capacitor in parallel on the output, polarized, maybe would be better. something like on the schematic from the link below, but check the values and the number of the IC and check the output signal on a oscilloscope for any noises or drop downs in the voltage.

Thanks guys, so the datasheet for the radio module provides a schematic for the power supply (9.6V and 5V) and uses these regulators here to achieve that which are not linear...

The problem with them is that they are only a step down, and they are $23 a piece!

So that being said, if the datasheet suggests using a variable regulator with this RF module, then I suppose it may still be possible to go without the linear reg? I'm just trying to cut down on the component count, but if it's highly advised against doing this then I can make it work with the combination of the two regulators.

You mention a datasheet for the radio module...

Yup... it's a whole technical document:

check out page 165 for the schematic on the step down power supply. Basically I would need something similar that steps down / up to 9.6V and I can just have a regular step down converter to 5V (since it is below the low battery voltage).

Whilst switched mode suppliers are seen by some a universal panecea for power supply applications, as has been mentioned there are some applications where linear supplies are far better.

One such application is in powering RF modules, receivers in particular, have a read of this;

The Perils of USB power banks

Note the very significant effect the switcher in the power bank has on the receivers RF sensitivity. I have seen the same effect using typical 'cheap' switchers that you will find on eBay.

Whilst its possible to design EMI quite switchers, all to often people are more concerned with 'cheap'.

Unless you are prepared to spend a while testing and analysing the effect of a switcher supply on an RF receiver, if you want 'cheap' then stick to linear supplies.

Hi,
What battery are you using for a power source?

Tom.. :slight_smile:

Thanks srnet. Cheap isn't critical, I'm willing to pay extra to make it work right. I just thought the $23 per IC that the data sheet calls out is steep. Also, as mentioned, they're no good anyways as they are only step down regs.

Hi Tom,

It is a 12V battery, Samsung INR18650-15M battery pack... 3 cells to make 12V. It is basically a Milwaukee tools M12 battery pack.

Hi,
I think for Li ion battery with 3.6V nominal terminal voltage should not be discharged below 3.0V

So your minimum voltage with 3 cells = 3 * 3.0 = 9V not 7.6V

So you have a charge/discharge control circuit?

Tom... :slight_smile:

Hi Tom,

For the link I posted to the battery spec sheet, it keeps mentioning standard discharge of 2.5V

3 of these cells combined, that would make 7.5V ?

The batteries are still inside of the Milwaukee M12 battery housing which also contains a balance board + some kind of circuit that I believe cuts off the battery at its safe discharge voltage. The Milwaukee charging station will charge the (removable) battery.

If your problem is only interference on the power supply caused by AC to DC converter/transformer than I suggest to use a capacitor otherwise stick with the original datasheet and schematic provided by the manufacturer etc.

Hi,

The batteries are still inside of the Milwaukee M12 battery housing which also contains a balance board + some kind of circuit that I believe cuts off the battery at its safe discharge voltage. The Milwaukee charging station will charge the (removable) battery.

Okay, so 7.5V might not occur. but the are protected.

Tom… :slight_smile:

Thanks Tom,

Yes, 7.5 might not occur but I would like my power supply circuit to be able to handle boosting the voltage from 7.5 to 9.6V in this scenario. The peak draw of the Rf module is 3A for transmit.

Can anyone suggest a schematic that might handle such a demand?

What is the application you are implementing, Tetra stuff is not cheap I thought ?

You're right, it is not cheap stuff. I'm helping some guys I know prototype a radio controlled lighting system. They have to use the Tetra modules because they are trying to integrate it into the existing Tetra network.

I'm clearly not an expert but I'm trying to help where I can :slight_smile: