LM3940: LM3940IMP-3.3/NOPB Ig calculation for Heat sink.

Sathya Priya

Intellectual 260 points

Part Number: LM3940

Hi Team,

We are using LM3940IMP-3.3/NOPB this LDO in our design.

Our requirement: 5V to 3.3V @1A

We need to know how to decide Heatsink need to be added or not?

We seen the formula in datasheet. But we don't get the value of Ig (ground current)?? in datasheet for calculation.

Also, ambient temp choosing?

It would be helpful if you given the example for calculation according to our requirement.!!!

Waiting for the response.

Regards,

Sathya Priya N.

over 1 year ago

0 Sathya Priya over 1 year ago

Intellectual 260 points

Hi,

Gentle Remainder!!

0 Juliette over 1 year ago in reply to Sathya Priya

TI__Mastermind 18466 points

Hello Sathya,

This plot is mislabeled and should be Ground Current vs. Load.

PD = (VIN - VOUT)(IOUT) + (VIN)(IGND)

PD = (5 - 3.3)(1)W + (5)(.110)W = 2.25W

TJ = TA + (PD)(Theta-JA)

TJ = TA + (2.25)(59.3)C = TA + 133C

The recommended junction temperature is 125C, which means your will need a heatsink.

Best,

Juliette

0 Sathya Priya over 1 year ago in reply to Juliette

Intellectual 260 points

Hi juli,

Thanks for the response.

Can you help out us ,to get any LDO without heatsink for our requirement in SOT23 package.

0 Sathya Priya over 1 year ago in reply to Sathya Priya

Intellectual 260 points

Team,

can we use this part for the above requirement.???

TPS73701DRVR

0 Juliette over 1 year ago in reply to Sathya Priya

TI__Mastermind 18466 points

Hey Sathya,

TPS73701DRVR:

PD = (VIN - VOUT)(IOUT)

PD = (5 - 3.3)(1)W = 1.7W

TJ = TA + (PD)(Theta-JA)

TJ = TA + (1.7)(53.1)C = TA + 90.27C

What is the ambient temperature of your application?

This white paper will be helpful for your layout portion of design:

An empirical analysis of the impact of board layout on LDO thermal performance

Best,

Juliette

Power management

Power management forum

LM3940: LM3940IMP-3.3/NOPB Ig calculation for Heat sink.