TPS22810: TPS28810 Rise Time accuracy?

Bob Lee62837

Mastermind 7230 points

Part Number: TPS22810

Dear Support Member,

My customer used TPS28810.

I have a question.

Data sheet 9.3.4 Adjustable Rise Time (CT) SR = 46.62 / Ct is described.
How much is the accuracy of 46.62?

Best regard.
Bob Lee.

over 7 years ago

0 Aleksandras_Kaknevicius over 7 years ago

TI__Genius 15780 points

Hi Bob,

In general, the timing of our load switches has some variation due to process, temperature, load capacitance, load current, and input voltage. In fact, we have an app note that describes the variation here:

I would say that if you want to figure out your expected rise time for an application, it would be worth ordering an EVM online and testing your specific conditions.

Thanks,

Alek Kaknevicius

0 Bob Lee62837 over 7 years ago in reply to Aleksandras_Kaknevicius

Mastermind 7230 points

Dear Alek Kaknevicius,

Thank you very much for reply.

I send my customer your answer.

I will feedback it.

Best regard.
Bob Lee.

0 Bob Lee62837 over 7 years ago in reply to Bob Lee62837

Mastermind 7230 points

Dear Alek Kaknevicius,

I replied my customer.

I have question about QOD resistar.

Load capacitor is 470uF.

In Data sheet 10.5.2.1, I calculate the external resistance value with the item of shutdown.

In calculate the resistance value,
Could you teach what constraints need to be fulfilled by the thermal point of view of the IC?

Best regard.
Bob Lee.

0 Aleksandras_Kaknevicius over 7 years ago in reply to Bob Lee62837

TI__Genius 15780 points

Hi Bob,

The internal QOD resistance varies across temperature, as shown in the graph in figure 6. This will affect the strength of the pulldown and therefore the rate at which the 470uF capacitor is discharged.

Thanks,

Alek Kaknevicius

0 Bob Lee62837 over 7 years ago in reply to Aleksandras_Kaknevicius

Mastermind 7230 points

Dear Alek Kaknevicius89,

Thank you very much for reply.

I misstaked question.

Not the temperature change of the internal resistance,
Do I have to think the temperature rise (heat loss) of the IC when calculating the QOD resistance?

Best regard.
Bob Lee.

0 Aleksandras_Kaknevicius over 7 years ago in reply to Bob Lee62837

TI__Genius 15780 points

Hi Bob,

I'm not sure I understand the question. Are you asking if you need to consider the self heating of the device when choosing a QOD resistance for your application? Please take a look at section 9.3.2.2 for more information on the limits of the QOD resistor.

Thanks,

Alek Kaknevicius

0 Bob Lee62837 over 7 years ago in reply to Aleksandras_Kaknevicius

Mastermind 7230 points

Dear Alek Kaknevicius89,

Thank you very much for reply.

I was asking it.

9.3.2.2 description
『Care must be used to ensure that excessive current does not flow through RPD during discharge so that the maximum TJ of 125°C is not exceeded.
An external resistor, REXT, must be used to ensure the amount of current flowing through RPD is properly limited and the maximum TJ is not exceeded. 』

Since 9.3.2.2 description, there is no calculation method of the resistance value that does not exceed the maximum TJ of 125 ° C,
I can not calculating.

How do you calculating about resistance and TJ 125 ° C?

Best regard.
Bob Lee.

0 Aleksandras_Kaknevicius over 7 years ago in reply to Bob Lee62837

TI__Genius 15780 points

Hi Bob,

The calculation depends on the thetaJA value of the package, so you first need to determine what that number is from table 7.4 in the datasheet. From there, you need to figure out the total amount of current flowing into the QOD pin. This is the output voltage divided by the internal QOD resistance + external QOD resistance. You then take the squared current, multiply it by the internal QOD resistance, and multiply that by the thetaJA number. This will give you the expected temperature rise in the device.

As an example, if the output voltage is 12V, there is a 300Ω external QOD resistance, and the DBV package is being used, then the total current into the QOD pin is:

QOD Current = VOUT / (External QOD + Internal QOD)

QOD Current = 12V / (300Ω + 265Ω)

QOD Current = 21.2mA

The total power dissipated in the device will be:

Power = I^2 x R

Power = (21.2mA)^2 x 265Ω

Power = 119mW

This will give the following temperature rise in the device:

Temperature Rise = Power x ThetaJA

Temperature Rise = 119mW x 182°C/W

Temperature Rise = 21.7°C

Therefore, as long as the ambient temperature is below 103.3°C, the device will not exceed the junction temperature of 125°C.

Thanks,

Alek Kaknevicius

0 Bob Lee62837 over 7 years ago in reply to Aleksandras_Kaknevicius

Mastermind 7230 points

Dear Alek Kaknevicius

Thank you very much for your kindness answer.

I understand.

I will reply my customer your answer.

Best regard.
Bob Lee.

0 Bob Lee62837 over 7 years ago in reply to Bob Lee62837

Mastermind 7230 points

Dear Alek Kaknevicius,

I feedback my customer reply.

The customer was calculating, but calculation is stopped because the minimum value of internal QOD is unknown.

Could you tell me the minimum internal QOD value?

Best regard.
Bob Lee.

0 Aleksandras_Kaknevicius over 7 years ago in reply to Bob Lee62837

TI__Genius 15780 points

Hi Bob,

The lowest value we measured in testing across process and temperature was 212Ω. We do not guarantee this number as a minimum, but it should be a reasonable estimate for minimum value.

Thanks,

Alek Kaknevicius

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TPS22810: TPS28810 Rise Time accuracy?