LM5121: Design Requirements Question

James Su5

Part Number: LM5121

For spec page 33 8.2.1 Design Requirements ,all parameter is bring TYP 9V for Calculation, If CMTR TYP spec is 6V, bring TYP 6V for all parameter Calculation , The Calculation rule can support Vin 3~12V workable right ?

over 8 years ago

0 EricLee over 8 years ago

TI__Guru 58301 points

Hello
I think so. Important spec is 3V Vin(min)
Thanks

0 Vental Mao over 8 years ago

TI__Expert 5745 points

Hi for the boost circuit, the peak current is appeared at Vin-min, so is your input voltage is in a range, please use the Vin-min

0 James Su5 over 8 years ago in reply to EricLee

Expert 1340 points

Hi Eric,
Do you mean typ with 3V to calculate?

0 James Su5 over 8 years ago in reply to Vental Mao

Expert 1340 points

Hi Vental,
Do you mean typ with 3V to calculate?

0 Garrett Roecker over 8 years ago in reply to James Su5

TI__Mastermind 21720 points

Hi James,

As Eric mentioned 3V is the value that you want to use in the calculation. This is because the in a boost DC/DC converter the worst case condition is when VIN is at its minimum and a maximum load current. Doing the calculations here should allow for the device to work at all other operating points.

Thanks,

Garrett

0 James Su5 over 8 years ago in reply to Garrett Roecker

Expert 1340 points

Hi Garrett
Why the reference design to use Typ 9V to calculate,
Why not use Vin min 3V ?

0 Garrett Roecker over 8 years ago in reply to James Su5

TI__Mastermind 21720 points

James,

The calculation for the inductor value can be done with either input voltage values. Using the minimum value ensure that you select an inductor that will not saturate at full load. If the typical value is used the inductor is optimized for the normal operating point but the selected inductor will need to checked to make sure it doesn't saturate at minimum VIN.

Thanks,

Garrett

0 James Su5 over 8 years ago in reply to Garrett Roecker

Expert 1340 points

Hi Garrett

For Frequency compensation, it is correct to use Typ 9V or Vin min to calculate ?

0 PYTsai over 8 years ago in reply to James Su5

TI__Genius 11525 points

James,

I thought you need to use 9V for VIN.

Garrett, Eric,

Would you confirm this? thanks!

0 Garrett Roecker over 8 years ago in reply to PYTsai

TI__Mastermind 21720 points

James,

Please uses the minimum input voltage to calculate compensation values. This is due to the minimum input voltage being the worst case operating condition.

Thanks,

Garrett

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LM5121: Design Requirements Question