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# TPS54394: Inductor peak current

Part Number: TPS54394
Other Parts Discussed in Thread: TPS62130, TPS62903

Hi,

Looking at page 14 of the datasheet the peak inductor current is calculated at 3.46A.

Trying to replicate the design in webench, I get a value L Ipp of 2.08A.

Can you advise why there is a difference here?

I don't need that exact design of supply, I was just trying to get a baseline in Webench. What I'm ultimately interested in is how to spec the inductor current wise for the two following output voltages if you can help there?

Vin 5V -> 5.5V

Vout 1 -> 1.5V 3A

Vout 2 -> 1.1V 3A

Many thanks,

G

• Hi Graham,

I created a 5V-5.5V design for the 1.5V/3A case to try out. The design shows a 1.5uH inductor but you can change it up to 2.2uH if you want to reduce ripple further. I observe a L Ipp of 1.47A which is the inductor peak to peak ripple. The peak current is the variable "IC Ipk" which is 3.74A in this case. Both look about right. Differences from the equation in the datasheet are due to the non-ideal lossy duty cycle term (datasheet just uses a simplified Vout/VinMax for this) and also slightly lower switching frequency (FSW term in the equation). So, you can use the WEBENCH numbers for your inductor selection.

Here is the 1.5V/3A design I am checking: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=583F8DF26D147AF5

And here is the 1.1V/3A design: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=A80D775DF109F777

In addition to the datasheet considerations, I would add that in a situation where the device enters current limit due to output short or high current demand especially during startup, the device will run into overcurrent protection scheme where the maximum inductor current can be as high as 6.5A. So, would recommend picking an inductor with saturation current higher than 6.5A. WEBENCH seems to be picking 7.8A rated inductor which should work but you can pick any other inductor that has saturation current > 6.5A.

Hope this helps.

Thanks,

Amod

• Hi,

Is the relatively high "IC Ipk' value due to the relatively low switching frequency of this part? Would higher frequency parts have a lower 'IC Ipk'?

If an inductor was used with a saturation current less than 6.5A what would happen during a short or high current demand situation?

Thanks,

G

• Hi Graham,

IC Ipk depends on the ripple current plus the load current itself. Since the ripple current depends on duty cycle, Vin, Vout, inductor choice and also switching frequency, IC Ipk will depend on all these parameters too. As I mentioned earlier, you can increase the inductor to 2.2uH to reduce the ripple current which will also help with the reduction of IC Ipk.

Operating beyond saturation current can cause the inductive impedance to drop and result in uncontrolled rise in current which can damage the inductor and also the IC itself. If picking an inductor with saturation current below 6.5A, I would recommend picking inductors with soft saturation characteristics where in the drop in inductance is gradual with increasing currents. You should be able to check saturation current characteristic in the inductor datasheet curves. You will want to avoid ferrite drum core inductors as they tend to have "hard" drops in inductance value after saturation current.

Hope this helps.

Thanks,
Amod

• Hi Amod,