TPS7B4250-Q1 transient thermal resistance

Yamauchi-san,

We don’t have transient thermal resistance data. As under different work condition, devices’ junction temperatures are totally different. It is difficult to get so much data, and cost too much.

May I know why does customer want to try to work around 150 degree C of junction temperature? If you could help some more information, I can help estimate the pulse width.

What is the ambient temperature and what is the load condition customer using? Does customer use single layer, double layer or multi-layer PCB? And what is the thickness of the copper? What is the thermal pad size in PCB layout and how many via if using multi-layer PCB?

Best regards,

Jason Liu

Dear Jason-san,

I have confirmed some conditions.

- Ambient temperature -30 to 80degreeC, Battery voltage max 16V and Vout -GND short (usually about 20mA, they need the check in the case Vout-GND short by accident.)

- 4 layer 1.6mm thickness 190mm*141mm, copper thickness 35um, GND size for the surface: 1mm*0.5mm, Inside GND size 100mm*100mm, via 1pc

For this IC, it does not have thermal pad. So, I confirmed the surface GND size connected GND pin of IC.

Could you please estimate the pulse width?

 And they want to know whether the junction temperature reaches 150 degreeC for 50msec or not?

Best regards,

Atsushi Yamauchi

Dear Yamauchi-san,

If customers use TPS7B4250-Q1 with 20mA load, it is safe for this device and will not reach 150 degreeC even with 16V input voltage and 80 degreeC ambient temperature.

You could find the equation in datasheet page 10.

P_D = I_O * (V_I-V_O) + I_Q*V_I. I_Q is far smaller than I_O here and can be ignored.

T_J = T_A + thetaJA * P_D = 80 + 171.7 * 0.02 * (16-5) = 80 + 37.8 = 117.8 < 150

But if VOUT short to GND, current will rise to the limitation value quickly. This current limitation is about 100 to 500mA which could be found in datasheet page 3. Different devices have different current limitation, and it is very hard to estimate the pulse width. Usually it would be shorter than 50ms when current was larger than 200mA (can be easily reached) when ambient temperature is 80 degreeC.

So we suggest customer should prevent VOUT short to GND, or thermal shutdown will happened to protect the device.

And I am still concerned about customers’ application, do they want to turn off the power supply of TPS7B4250-Q1 when VOUT short to GND or protect the load?

May I know the customers’ application?

Thanks and best regards,

Jason Liu

Dear Jason-san,

Thank you for your reply.

The application is "power tail gate" (tail gate with opening automatically).

I don't know why this IC stops in the case Vout short GND. I will confirm it to the customer.

And I got the condition of the substrate. Could you please estimate the time until approaching junction

temperature 150 degreeC?

I understood the time is smaller than 50ms but they need the time.

Best regards,

Atsushi Yamauchi 

Dear Jason-san,

I have the additional question.

In the thermal shutdown, the junction temperature is 175 degreeC.
And the absolute maximum rating is 150 degreeC max.
I think the chip is dameged in the temperature 175 degreeC (note (1) in the ABSOLUTE
MAXIMUM RATING, in this case, the temperature affects the device reliability.).

So, they want to stop it under 150degreeC.
Are there any impacts for the reliability of the device with thermal shutdowns (more than once)?

Could you please reply this question and the requests in the previous mail?

Best regards,
Atsushi Yamauchi

Dear Yamauchi-san,

What does "IC stops in the case Vout short GND" mean? Does that mean the TPS7B4250-Q1 device cannot recover after VOUT short to GND condition is removed? Or device cannot regulate when VOUT short to GND?

If the formor one, could you please send details to jason-liu@ti.com? If latter one, that is because Output Short-Circuit Proof is trigged, to protect power supply and our device, and device will recover after the condition is removed.

I am sorry I cannot estimate the time as short-circuit will result to large current. Usually there is a short reponse time before current limit is effective. The reponse time is short, but during the reponse period, the current rise rapidly to a very large value, and generate lots of heat. It is hard for us to calculate the time as is totally different from normal operation.

For the absolute maximum rating and the thermal shutdown temperature, we can promise that our device could work with normal function without any problem under 150C temperature, and our device will shutdown for the heat but is not demage under 175C temperature.

We have done plenty of validation and test work to guarantee this spec.

If customers still concern about this and find device is broken, please send me details to jason-liu@ti.com. I can support you directly offline.

Best regards,

Jason Liu

Dear Jason-san,

They want to stop IC function before reaching junction temperature 150 degreeC in the condition VOUT shorts to

GND.

The means is "latter".

I understood IC is not damaged in the thermal shutdown, but I think the reliability of IC is impacted by the

shutdown, because there is note (1) in the datasheet at "ABSOLUTE MAXIMUM RATINGS". So, they want to stop it

before 150 degreeC.

Aren't there any impacts for the reliability by the thermal shutdown?

And I got the information of the substrate, because you requested it to estimate pulse width. So I don't understand

you can not do it. Can you estimate pulse width (your first reply)?

Best regards,

Atsushi Yamauchi

Dear Yamauchi-san,

I thought TPS7B4250-Q1 was working in normal status (Iout<=50mA), but not in short to GND condition(Iout>>50mA). As I said, the current though TPS7B4250-Q1 is so large when VOUT is short to GND, and different devices have different maximum current value during the loop response period (a short time before output current limitation). The time is short but the engry is too large, and that makes the temperature rise rapidly.

Before thermal shutdown, TPS7B4250-Q1 could no doubt regulate with normal function, but we don't promise when VOUT is short to GND, as it is in fault condition.

Customers should prevent "Vout-short-to-GND" from happening. But if VOUT short toGND happened, don't worry, TPS7B4250-Q1 would close the internal FET to prtect our device, and do not regulate any more before Vout-short-to-GND condition removing. After Vout-short-to-GND condition removing, TPS7B4250-Q1 would recover from the fault confition.

So you could tell customers, our device TPS7B4250-Q1 do have Overtemperature Protection and Output Short-Circuit to Ground Protection. The pulse width is no need, as TPS7B4250-Q1 has self-protection function inside.

Best regards,

Jason LIU