LMR36503-Q1: Vin lower than Vout - behavior

Josef Warta

Part Number: LMR36503-Q1

Hello Team,

customer wants to use the DCDC converter for a short time (<300ms) with Vin lower than Vout due to voltage drop specification in his application.

As we do not have a 100% duty cycle mode in this converter - is it possible to describe what happens with output voltage during this voltage drop at Vin?

Specification is:

Vin typical is 14V..36V (load dump 58V)

Vout is 7V

Current max. 100mA

there is a voltage drop downto 6V for 300ms specified. What happens now with the output voltage? An output voltage down to 3.8V would be ok.

Many Thanks

Josef

over 4 years ago

0 Frank De Stasi over 4 years ago

TI__Guru 71935 points

Hello

It is OK for the input voltage to dip below the output voltage.

The output voltage will follow the input voltage minus a certain drop.

The behavior is fully described in the data sheet.

See figure 7.4 and section 8.3.7.1 and section 8.4.3.5 in the data sheet.

Thanks

0 Karlheinz Fleder over 4 years ago

Prodigy 20 points

Hi Frank,

thank you for quick answering. The "certain drop" seems logical. But what is value? In our application it shall be not more than 6V-3.8V=2.2V. Is it so?

Regards Karlheinz

0 Frank De Stasi over 4 years ago in reply to Karlheinz Fleder

TI__Guru 71935 points

Hello

The exact amount would depend on the resistance of the inductor and the load current.

The below approximate equation can be used to calculate the output voltage for a given

input voltage and load current and inductor resistance.

If you need more details, please ask Josef to contact me through TI email and I will be happy

to provide more details and/or assistance.

Thanks

Vout=Vin*0.98-Iout*(RL+0.91)

RL=inductor resistance

0 Karlheinz Fleder over 4 years ago in reply to Frank De Stasi

Prodigy 20 points

Hello Frank,

thank you very much! That helps a lot!

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LMR36503-Q1: Vin lower than Vout - behavior