DC/DC Converter

[somayeh]

Hello all,
I have to design 3 DC/DC converters for a FPGA board with 3 needed voltages,
1.2 V, 1.8V and 3.3 V with 5 volts input. I have selected LTM4623.

What bothers me about the LTM4623 is that it is limited to 0.5A input current.

This means that certain output currents are not possible at all. Could you please help me, how can come out with 5 V input voltage and 0,5A input current e.g. 1,2V output with 3 A output current?

Thank you very much
best regards
Somayeh

 

[AnalogKid]

You need to use a different chip. The part you have limit the total power to 2.5 W, but the load at 1.2 V is 3.6 W, and that does not include the loss in conversion efficiency, typically 20%.

Select a part rated for at least 1 A switch current. For better long-term reliability, the chip should be rated for at least twice the current you need, so a 2 A part would be better.

ak

 

[somayeh]

Thank you for your reply, but in the datasheet the efficiency is attached at 5V input at 3A output, in a diagram it means it could be working, and the datasheet as well says that.

Best Regards
Somayeh

 

[Harald Kapp]

What bothers me about the LTM4623 is that it is limited to 0.5A input current.

Where does it say so in the datasheet? The only info I can find is this.


Which essentially states that the typical input current from a 12 V supply is 0.5 A. This is no upper limit.

You need to use a different chip. The part you have limit the total power to 2.5 W, but the load at 1.2 V is 3.6 W, and that does not include the loss in conversion efficiency, typically 20%.

The datasheet rates the module for 3 A output current. At least theoretically the module should be able to work from a 5 V input supply with a 3 A output current:

Of course with reduced efficiency at lower output voltages. When used with 5 V input, 1.2 V output and 3 A at the output, the input current will be 1.2 V / 5 V × 3 A / 0.8 = 0.9 A.

The module will be operated at its limit. That is usually not a good design choice, Consider @AnalogKid 's tip to chose a more powerful part. If you want to stick to this module, you will require a heat sink to prevent overheating of the module, see last pages of the datasheet.

 

[somayeh]

Thank you for the advice.

If you want to stick to this module, you will require a heat sink to prevent overheating of the module, see last pages of the datasheet.

I couldn't find where in the datasheet it conveys that a heat sink is required for total power > 2.5W.

You need to use a different chip

Honesty, our team searched a lot for parts fitting the working condition of 4 MHz freq, 5V input, 1.2V, 3A output. However, we couldn't find any strong alternative. I'd really appreciate it if you could mention a couple of other alternatives to this specific part.

 

[Harald Kapp]

I couldn't find where in the datasheet it conveys that a heat sink is required for total power > 2.5W.

Page 19 shows the derating without heat sink. If you want to avoid the derating, you'll have to use a heat sink.

 

[kellys_eye]

Honesty, our team searched a lot for parts

(my bold)

Sack your 'team'. What do they do 'professionally'?

 

[AnalogKid]

fitting the working condition of 4 MHz freq,

Why?

 

[Harald Kapp]

Just a few examples from just one manufacturer:


Many more available from other manufacturers, too.

 

[somayeh]

Just a few examples from just one manufacturer:
View attachment 54779

Many more available from other manufacturers, too.

Thank you for your reply, however the problem is that I need a DC/DC converter that has the inductance included. Since I have a very small board that requires a very small footprint, our choices with inductance included are limited.
Best Regards

 

[Harald Kapp]

the problem is that I need a DC/DC converter that has the inductance included.

The problem is that you haven't stated the complete set of requirements. We can't supply useful suggestions when we don't know the requirements.

Just an example: Analog devices alone lists 55 modules with output current between 3 A and 5 A.

 

[somayeh]

The problem is that you haven't stated the complete set of requirements. We can't supply useful suggestions when we don't know the requirements.

Just an example: Analog devices alone lists 55 modules with output current between 3 A and 5 A.

Thank you for your reply, you are right, but to be honest I started my work about a month ago and in the process I chose LTM4623, but this one was also in this list . The requirement are 5 volt input 1.2, 1.8, 3.3V output and 4 MhHz switching frequency for an ultra thin camera boards. Those were my requirements. I hope it works well (I didn't find unfortunattly a better solution), and I have another challenge now. Currently I simulate 3 of them together with PHModes, CLKin, CLKout for 3 output voltages and for 120 phase shift but I don't know why it takes from input supply 9A current at once through CM even though I used application note 77 and phase shift, in order to not too much current (about 9A) in one time overlay . In the attachment is the results. beside that my Boss will to know, how is the Input current for every DC-DC Converter.

Thank you
best Regards
Somayeh