LMZ31710: Fsw 750kHz for LMZ31710 and LMZ31506

Michael T Schneider

Part Number: LMZ31710
Other Parts Discussed in Thread: LMZ31506,

We have a LMZ31710 for 12Vin to 5.1Vout conversion. Datasheet states 1MHz switching. We have an LMZ31506 for a 12Vin to 3.5Vout conversion. Datasheet states 630KHz switching for this device.

Can we run them both at 750kHz with one out of phase of the other to reduce the input ripple somewhat?

In table 7 of both datasheets it states how to sync both devices however the published datasheet online for the LMZ31710 has incomplete entries so I would like to confirm that the Vout of both differnt ICs would work at 750kHz switching frequencies?

over 5 years ago

0 Jimmy Hua over 5 years ago

TI__Mastermind 47060 points

Hi Michael,

Unfortunately you cannot parallel these two different power modules together. The internal compensation (error amp, gain, etc.) of the LMZ31710 is different from the LMZ31506. In order to properly parallel devices, the master and slave is recommended to have the same internal circuitry hence why it is recommended to parallel multiple of the same power module together.

Regards,
Jimmy

0 Michael T Schneider over 5 years ago in reply to Jimmy Hua

TI__Expert 3780 points

Im sorry, I don’t want to parallel them at all. I just want to run them with an external clock at 750kHz. The 710 will produce 5.1V, the 506 will produce 3.5V.

The issue is that the datasheet recommends running the 5.1V at 1MHz, while datasheet recommends running the 3.5V a 630kHz. What potential problems will I run into if they are switching at 750kHz?

0 Jimmy Hua over 5 years ago in reply to Michael T Schneider

TI__Mastermind 47060 points

Hi Michael,

I must have misunderstood your initial question. My apologies.

Since both devices have adjustable switching frequency selection features, there should not be any issue running it at an external frequency of 750kHz. Running at a lower frequency would actually help with efficiency since there will be less switching losses.

Regards,
Jimmy

0 Michael T Schneider over 5 years ago in reply to Jimmy Hua

TI__Expert 3780 points

Thanks Jimmy,

Would we run into any danger of saturating the internal inductor if you run the Fsw at 750kHz instead of the recommended 1MHz?

0 Jimmy Hua over 5 years ago in reply to Michael T Schneider

TI__Mastermind 47060 points

Michael,

I don't suspect that will be an issue since both parts allow full adjustable switching frequency option. Table 7 of both datasheets provide guidance on "Synchronization Frequency vs Output Voltage". Though the table doesn't provide the maximum output voltage value above 600Hz since it is listed as "tbd", I think it would be okay to operate the LMZ31710 at 750kHz for 5V output. I am currently in contact with the apps engineer that wrote this datasheet and will get back to you when i get more information on this.

Regards,
Jimmy

0 Jimmy Hua over 5 years ago in reply to Jimmy Hua

TI__Mastermind 47060 points

Hi Michael,

There shouldn't be any issues with running both power modules with an external 750kHz frequency signal.

Regards,
Jimmy

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LMZ31710: Fsw 750kHz for LMZ31710 and LMZ31506