To use LDO or Step-down DC-DC converter

[ericmar]

Hi,

Suppose I have an input voltage source of +6~9V and would need an output voltage of +3.3V and +1.8V, should I use a simple LDO to get the required output voltage or use step-down DC-DC converter to get the output voltage?

If there are more than one DC-DC switching circuits on one single board, would it cause a lot of noise?

Please advise. Thanks.

Regards,
Eric

 

[FvM]

The question may be answered different depending on your systems interference susceptibility and power consumption. Regarding total losses, a 3.3V buck converter is the more effective measure in most cases (unless 1.8V has an extraordinary current consumption), 1.8V can use a LDO post regulator more likely.

 

[haff99]

To use and LDO or switcher basically is going to boil down to efficiency and if you can handle the heat generated by the LDO. A 6V to 1.8V conversion is going to give you and efficiency of 33%. Depending on how much current you have to source, all that power is dissipiated as heat and the LDO is going to get HOT!. Depending on what switcher you choose your efficiency can be upwards of 80%-90%. You won't be generating too much heat in this case.

Multiple switchers can be noisy but it isn't uncommon to have multiple switchers on the same rail. THere are ways to minimizie noise for instance by synching the switchers to the same frequency.

 

[ericmar]

Thanks guys,

In fact, I need several output voltage supplies such as 12V@0.3A, 6V@2.5A including the 3.3V@0.35A and 1.8V@0.15A. Since my input voltage is about 6~9V, I will need to boost up the voltage for the 12V supply and step-down for 6V supply. In my current design, I use two LDOs to get the 3.3V and 1.8V as they dont draw much current.

One thing about using LDO is the extreme simple configuration and low cost. Suppose I were to design all with switching DC-DC topology, then I will probably need 1 step-up and 3 step-down for my application. Wont this be too complicated and noisy?

Thanks.

Regards,
Eric

 

[imar]

Hi ericmar!

Since my input voltage is about 6~9V, I use two LDOs to get the 3.3V and 1.8V as they dont draw much current.

One thing about using LDO is the extreme simple configuration and low cost.

you said that you are using LDO ( Low dropout voltage). but when i saw the dropout voltage in our case it cames to be at least 2.7V to go from 6V to 3.3V and 4.2V for 1.8V.

i think that these two LDOs that you are using didn't use PMOS as pass element but PNP transistor in the Darlington configuration.

so these LDOs are simply linear regulators without the specification that theu are LDO! isn't it?

 

[ppdr123]

if you want multiple outputs, then dc-dc converter will be the best option as you can draw multiple windings from conerter. but you have to design the converter accordingly. if you use LDO, you have to use as many LDOs as the no. of outputs you want.

 

[karesz]

Hi ericmar,
Its not so clear with your LDO`s! :-(
If your Uin=6~9V, how will you make with some LDO the DC12V/0.3A output pls?
Your case is clear to solute with a multioutput switcher, i.e. PI`s Tiny or TOPSwitch families, they are developed for some more output apps with ca. 20W **broken link removed**
K.

 

[arghpok]

You could use a boost converter for the 12V output and a SIMO buck converter for the remaining 3. This solution will give you the best efficiency with the less passive components, considering that you would be using just one inductor for the 3 step-down outputs. Too much noise ? too complicated ? It's all relative. Is your circuit noise sensitive ? Generally, digital circuitry is sourced by DC/DC converters as they have a lot of noise margin. Also, front end RF for better efficiency. If the case is biomedical or instrumentation applications (or other high sensitive analog), you should use voltage regulators regardless any other consideration. Now, why complicating your life using LDO topologies if you have at least 2.7V dropout ? You can use a typical n-based regulator and saving yourself the problem of stability that arises naturally with p-mos topologies. If you wanna keep efficiency high, a mixed solution of DC/DC converter and regulators should be used (and in fact, this is what is really done). In that case, low voltage reference can save you some passive elements if you design quite well your error amplifier (which gain doesnt need to be that high, just around 50dB).

 

[dick_freebird]

At only ~2W of waste heat I'd go linear regulators for
the lower voltages unless it was a portable app that
needs to squeeze battery life. At ~20W net power
dissipation I doubt that's the case, and expect that
a fan will be provided. May as well just go simple and
dump the heat.