24V to 1.8V for CPLD

Question

Hi,

I am looking for ideas how to use an existing 24VDC power supply to power an Altera CPLD with a core voltage of 1.8V and IO Voltage of 2.5V - I'm thinking of something like a buck converter that feeds into a power conversion IC. Does anyone have examples, or suggestions on the best route to go?

Regards, oldmanraskers.

michaelkellett · Answer

Are you sure you can't use 3.3V for the IO - it will make interfacing to other stuff a whole lot easier - the ULN...... driver chip we talked about before won't be driven properly by 2.5V.

The way I would do this is to use switching regulator for the 24 -> 3.3 (prejudice showing there re. 3.3 or 2.5 )

Then use a linear regulator for the 3.3V -> 1.8V conversion.

You need to check the current drain but I'm going to assume 100mA max at each voltage so your 24 -> 3.3V converter must be able to supply 200mA.

Consider LM5008 for the switcher - not the cheapest but easy to use, adjustable output, can cope with up to 90V input, about 85% efficiency at 24V in and 200mA out.

There are loads of other parts you could use - some nice cheap ones from Vishay but limited max voltage, all sorts of nice and fancy ones from Linear Technology (but probably in hard to solder packages).

Just search Farnell for a low drop linear regulator that you fancy - probably best to go for SO8 or SOT223 package to cope with power dissipation. there are parts with fixed 1.8V output readily available. TLV1117-18CDCYR from TI is dirt cheap and OK for 3.3V as the higher supply - if you want the regulator to work with 2.5V input you'll need to use a more expensive part.

MK