Suitable switching/buck regulator - Vin = 24V and Vout = 5V and max current = 0.4A

Hi all,

Due to a sudden change in design requirements with regards my circuit supply voltage I'm looking to change my voltage regulation on my circuit from the classic LM7805 linear voltage regulator to a switching/buck regulator.

This is my first time attempting to use switching regulator so apologies if some bits of my post sound trivial. In saying this I've gone through this for the theoretical aspects and understood the basics of how the switching regulator works.

My circuit

My new supply voltage requirement is +24V from +6V and I need a Vout of 5V to be supplied across the circuit. My max current is 0.4Amps. I can't utilise my LM7805 as the heat dissipation is just too much. See calculations based on the calculations below using help from this Power and Thermal Dissipation Tutorial by SparkFun.

Power dissipation = V × I = (24V − 5V) × 0.4A = 19 × 0.4 = 7.6W

7.6W is too much heat.

Hence my question: which switching regulator (possibly available in the eagle libraries) would be suitable to take in a Vin of 24V or below and deliver a Vout of 5.0V?

Here's my schematic:



Any help is appreciated. Thanks.

Hi,

first: don´t be afraid of the switcihng technique. It works well. Most important is the layout design. short and wide traces, good power plane, multiple vias to gnd, use ceramic capacitors and good quality inductors.


there are ready to use step down switching modules as pin compatible replacement for 78xx.

If you want to build your own, then linear.com is a good site for information and they have a interactive selction guide to choose the right IC.
Just input your parameters and get a list of ICs.


Hope that helps
Klaus

Hi all,

I've looked further into the use of the switching regulator specifically the LM2575-N to deal with the heat dissipation problem as outlined in my first post. Please see my proposed approach here (Apologies if bits of the diagram are not that clear. I drew my approach by hand before committing time into finding the components in Cadsoft Eagle and subsequently updating my schematic).

**broken link removed**

I also took the approach as seen here in order to supply +3.3V to the Zigbee module as seen in my schematic in my first post

The LM2575 on the LHS is for the stepping down of +24V to 5V and the Vout from this LM2575 is to supply +5V to the second LM2575 for stepping down to +3.3V which is supplied to the zigbee module. The +5V from the first LM2575 is also supplied to the rest of the circuit (as seen in my circuit diagram in my earlier post).

Does anyone foresee any problems with the approach e.g. with the selection of the input capacitor, inductor, output capacitor and catch diode? I came up with the approach by following the design procedure as per pages 12, 13, 14 and 16 of the LM2575-N datasheet.

Thanks,
TokTok.