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# TPS7A78EVM-011: TPS7A78EVM-011, wattage spec for R5, R6, and R7

Part Number: TPS7A78EVM-011
Other Parts Discussed in Thread: TIDA-010036, TPS7A78

Hello,

The EVM user guide (www.ti.com/.../sbvu048.pdf) states that the R5, R6, and R7 are rated at 0.25W and that the EVM can supply 55mA in the full-bridge configuration.

0.25/(280)^2 = 29mA. That will mean that the board can not source more than 29mA in both configurations (half of full-bridge). Is that right?

The user guide however states that the board can operate at full-bridge at 55mA.

Therefore the resistors should be rated at 1W given that .055^2 * 280 = 0.847W.

Thank.

Sana

• Hi Sana,

Yes that is correct, the EVM is designed to operate for 30 mA full load in terms of continuous power supply. As for higher output currents like 55 mA, your right as well, the surge resistors must be rated for continuous power.

BR,

Haidar

• Hi Haidar,

Thanks, how would I rate higher currents? Can I use the same formula? This is an LDO so the input current is not equal to the output current. What would be the formula for that?

As a matter of fact, I measured the input current with HB and FB and in both cases at max load (27mA and 55mA respectively), the input current was less than 20mA.

My main concern is why the resistor R4 in TIDA-010036 is rated at 5W. The easy answer is 470* (100mA)^2 = 4.7W, even that the max load of the board is 50mA. Is that how we should rate these resistors?

www.ti.com/.../TIDA-010036

Thanks.

Sana

• Hi Sana,

My apology for the late answer as it is very crucial in the design process of TPS7A78 device in any system. Also similar detail explanation will be available in the device datasheet in 2 – 3 months as it is fully released. Now Let me break your question into the following;

1. The device output current, (ILoad) , sourced from LDO_OUT pin is determined by the following;
1. Bridge configuration, either FB or HB
2. VAC supply voltage and its frequency.
3. Size of cap-drop Cs
4. Size of CSCIN capacitor
5. Using the minimum or larger than the minimum capacitors for other pins.

As specified in section 8.1.4 and Table 2 on pages 16-17 and recommended operating conditions of the datasheet.

1. Input current to the device supplied by the cap-drop Cs into AC+/AC- pins is composed of two components;
1. VAC(rms) current which is calculated by VAC(rms) / XCs, where XCs is the impedance of your cap-drop capacitor Cs. For the EVM setup, the input rms current with 120VAC supply with 60Hz is 120V / (2*pi* 60*470 *1E-9) = 21.3 mA (rms)

1. The shunt current when the device active bridge control, section 7.3.1 in the datasheet, shunts (connects) AC+/AC- pins to the device GND to maintain CSCIN capacitor voltage at its operation level. Shunt current is calculated by VSCIN / Rs, for the EVM setup shunt current is 16V/ (280+280+280) = 19 mA. The value of Rs in the EVM was calculated based off the TVS maximum clamping voltage and VSCIN voltage is calculated by 4* (VLDO_OUT (nom) +0.6V) but for 3.3V output voltages and below is kept at 16V.

The attached screen capture 1 shows the input current with its two components, VSCIN voltage and AC+/AC- pins shunting activity for HB configuration specifically for the EVM default configuration with 27 mA of ILoad current. AC- pin is not shunting since it is tied to device GND for HB configuration.

The attached screen capture 2 shows the input current with its two components, VSCIN voltage and AC+/AC- pins shunting activity for FB configuration of the EVM with 55 mA of ILoad current. AC- pin is shunting with FB configuration.

You can see that in both cases, VAC(rms) current component is the dominate current component with ~ 27 mA (rms) current with occasional appearance of shunt current ~ 40 mA in 100 ms or ~ 6 VAC cycles.

The input current with its two components will be passing through the sure resistors Rs, so this resistor power rating must be able to handle input current. For the EVM surge resistors, I choose ~ 27 mA of input current for calculating the power rating to be ~ 0.25W.

In other cases the shunt current is the dominate current component of the input current where many shunting events happens in one cycle like what is shown screen capture 3 and the reason for that is when a larger Cs capacitor is used to power a small load current. This case can be seen in application where a small load is powered by large Cs capacitor to get a fast startup time.

As for TIDA-010036 design, I am not sure why they used 5W Rs resistor, but it is seems that they took a very conservative approach since their VAC(rms) current ~ 35 mA and their shunt current ~ 35 mA.

Please let me know should you have further questions.

BR,

Haidar

• Hi Haidar,

Thank you for the detailed response; it is in line with what I'm thinking.

Will the parts be available in 2 months time frame?

Do you have a brochure for this type of products?

Thank you.

Sana

• Hi Sana,

TPS7A78 product folder is available online and you can get 3.3V output samples now, as for evaluation module (EVM), unfortunately TI store run-out of stock but they should replenish stock within 2 months. The EVM online users's guide and the device online datasheet provide the starting point to built a prototype for your end application. The online product folder also has a ink to a TI reference designs that use TPS7A78 device.

The device unique features enables it to be used in many application such as electricity meters, factory automation, white goods, ...etc.

Please let me know should you have further questions.

BR,

Haidar

• Hello,

The samples are preproduction and it seems that all of your distributors are out of stock for production parts.

Is that right? I don't know if these questions are to be asked in e2e, if not, then would you please direct them to the right channels?

I would like to know when the production parts will be available?

Thanks.

Sana

• Hi Haidar,

I have contacted the chat line and they said:

"Sorry you were provided a generic response at e2e
I checked internal tools to see the availability of the parts you mentioned.
There's no plan to replenish the stocks for TPS7A7833PWPR and TPS7A7833PWPT"
Is that right?
also, would you please let me know the difference between TPS7A7833PWPR and TPS7A7833PWPT
Thanks. Sana
• Hi Sana,

My apology for any inconvenience, but please use a computer to go to ti.com rather that using a smartphone, however some smartphones allow for a desktop view. Once you are in a desktop view you can order a limited number of free samples maybe 50 or less if I remember correctly.

Yes the device is not yet fully released to the market and that's why it says pre-production, as for the difference between TPS7A7833PWPR and TPS7A7833PWPT; they are the same part but the difference is in the number of parts in the reel when you make a big parts purchase.

As for other timing questions regarding the part full release date and the EVM stock replenish time line, I have pinged our marketing team and you should get an answer soon.

BR,

Haidar

• Hi Haidar

I've never used a phone to answer you or to get to TI.com, I've always used a computer. I have no idea why you are saying that. I've already located the sample order. Thanks though.

The chat line said that the other parts are obsolete, but I will take that matter with the marketing team.

I'm simply trying to get a firm confirmation of the parts' availability, as I'm designing in this parts we speak and I would like to make sure that I don't get stuck afterwards.

Thanks.

Sana

• Hi Sana,

Full release to market of the TPS7A7833PWPR is set for later this year, around August-September time frame. Today the PTPS7A7833PWPT is being leveraged as an advanced release product. You can order units from this exact part number until we have full release of the TPS7A7833PWPR. Both device orderable numbers will be available together for a short period of time, until we transition to TPS7A7833PWPR

Once we have full release we will support the TPS7A7833PWPR for larger volume orders and our availability will be published through our ordering table.

Hope this helps!

• Hi Jose,

It does. Thanks a lot.

Case closed.

Sana