LM311: How to calculate power dissipation of voltage comparator?

Chadchayut Pipatkajornsak

Prodigy 50 points

Part Number: LM311

Hello,

I have a questions about power dissipation of voltage comparator include LM311 and LM319.

I've already checked both models in datasheet and my understanding, that is permissible power dissipation

which is maximum power that can be dissipated at maximum junction temperature.

But I need to know actual power dissipation, so I can find actual junction temperature that both models

are suitable to my circuit or not (current models, I use NJM311 and NJM319 which has lower permissible power dissipation)

Thank you in advance

over 5 years ago

0 Jonathan Nguyen over 5 years ago

TI__Expert 4555 points

For Max power dissipation, take (max) positive supply current and multiply by your Vcc.

The absolute max is capped at 500mW for the part.

Regards,
Jonny

0 Chadchayut Pipatkajornsak over 5 years ago in reply to Jonathan Nguyen

Prodigy 50 points

Thank you for your reply, Jonathan.

Does it mean power dissipation is calculated by only input?

For my understanding, power dissipation is loss power which converts to heat

and basic formula is power dissipation(Pd) = (Vin-Vout)Iout

By this formula, I can calculate Pd of voltage regulator because

it has only 1 input and 1 output.

But voltage comparator(LM311 and LM319), they have Vin(+,-) and Vs(+,-)

That's why I confuse to calculate Pd.

And I think your calculation is only input power which is power consumption.

0 Jonathan Nguyen over 5 years ago in reply to Chadchayut Pipatkajornsak

TI__Expert 4555 points

Chadchayut,

Yes, I misinterpreted your initial question but your understanding about power dissipation as power lost via heat.

The input voltages won't have a large effect on the power dissipation since the bias and offset current are in the nA levels.

Figure 5 in the LM311 datasheet shows the relationship of power dissipation as a function of Vout level. To calculate specific values, take your Voh and Vol levels and multiply by short circuit current.

Regards,
Jonny

0 Chadchayut Pipatkajornsak over 5 years ago in reply to Jonathan Nguyen

Prodigy 50 points

Thank you for your reply, Jonathan.

I need to confirm Figure 36 in LM311 datasheet which shows output limiting characteristics.

Am I correct to use this Figure? And can I refer this Figure to calculate power dissipation at any kind of circuit?

0 kai klaas69 over 5 years ago in reply to Chadchayut Pipatkajornsak

Guru 140200 points

Hi Chadchayut,

are you short circuiting the output of LM311-N at all?

Can you show a schematic?

Kai

0 Chadchayut Pipatkajornsak over 5 years ago in reply to kai klaas69

Prodigy 50 points

Dear Kai,

This is my circuits which use with NJM311 and NJM319

that I will change to LM311 and LM319 respectively in the future.

0 kai klaas69 over 5 years ago in reply to Chadchayut Pipatkajornsak

Guru 140200 points

Hi Chadchayut,

figures 33 and 37 of datasheet of LM311-N are relevant for you. Positive supply pin will draw 5mA and 4mA supply current will flow through the negative supply pin. Makes 5mA x 15V + 4mA x 15V = 135mW. And when the output of LM311-N is pulled-down about 12mA will flow into the output of LM311-N causing a voltage drop of about 0.3V. Makes 12mA x 0.3V = 4mW. Gives about 140mW in total. This is the typical power dissipation of LM311-N in your circuit. Take the maximum current specifications of supply currents (see page 5 of datasheet of LM311-N) to calculate the maximum power dissipation.

During the edges of output signal the maximum power dissipation of output transistor is about 7.5V^2 / 1200 = 47mW. But this is only for a very brief period.

Kai

0 Chadchayut Pipatkajornsak over 5 years ago in reply to kai klaas69

Prodigy 50 points

Thank you for you help, Kai

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LM311: How to calculate power dissipation of voltage comparator?