LMR33620: About the max duty cycle.

Kengo.Y

Part Number: LMR33620

Hi team.

I'd like to know about the max duty cycle.
According to the datasheet, it is written 98%(TYP).
However, the minimum switch off-time does not fit the calculation.

I want to use B version(1.4MHz) and RNX PKG.
My condition is bellow.
Vin=5.5V, Vout=5V/0.5A

Duty is 91% in my condition.(=5V/5.5V)
So, On-time is 65uS and Off-time is 64.9nS.

The minimum switch off-time need 70nS(MAX) from the datasheet.
Is it not possible to use under these conditions?

Sincerely.
Kengo.

over 5 years ago

0 Daniel Li14 over 5 years ago

TI__Mastermind 27470 points

HI Kengo

the frequency will be different by the inductor you choose, you can download the load transient calculator tools for frequency esimation. LMR33620 can be used in this condition.

www.ti.com/.../snvc234

Thanks

0 Kengo.Y over 5 years ago in reply to Daniel Li14

Guru 11255 points

Hi Daniel.

Thank you for your quick response.

Can I use my condition when Fsw is 1.4MHz if I set 1uH?
Do I have to keep the minimum switch off-time when Fsw is 1.4MHz?

Sincerely.
Kengo.

0 Daniel Li14 over 5 years ago in reply to Kengo.Y

TI__Mastermind 27470 points

Hi Kengo:

i put your conditon into excel, it shows the frequency will go down to 764Khz, it will not go to min off time condition. the frequency drop is due to the vallely current command,

but you need check in normal working condition, whether the 1uH inductor can support the large current ripple.

0 Kengo.Y over 5 years ago in reply to Daniel Li14

Guru 11255 points

Hi Daniel.

Thank you for your reply and calculate.

The output current may change up to 1A.
At this time, it becomes 1.4MHz.
Should I care about off-time ?

Sincerely.
Kengo.

0 Daniel Li14 over 5 years ago in reply to Kengo.Y

TI__Mastermind 27470 points

Hi Kengo:

it is very close to min off time, when the temp go higher or some voltage drop on the line, it will touch the min off time.

the device can continue deliver 5V 1A at this condition, but frequency will decrease to make ton longer.

Thanks

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LMR33620: About the max duty cycle.