TPS65094: Low temperature issue

Sakai-san,

Can you please use a PCB that is failing and swap out the IC with a TPS650947 sample unit? If you re-test with TPS650942 removed and TPS650947 installed on the board, it is easy way to determine if fPWM on BUCK3-5 will resolve your issue.

As you can see in Table 3-1 on page 5 of the TPS65094 datasheet, this is the only performance-related difference between the -942 and -947 product variants.

Sakai-san,

Auto-mode is beneficial for saving power, and it is commonly referred to as power-save mode instead of the technical name which is Auto switch fPWM/PSM (pulse-skip mode).

If your system will run off of a battery, Auto-mode is very important to extend battery life.

fPWM is preferable for multiple reasons: more reliable power-on, less EMI (more predictable switching frequency), and many others. In my opinion, even if you are running off of a battery it is easier to power-up in fPWM mode and then change to Auto-mode after the system is up-and-running.

You can use TPS650947, power-on in fPWM, then change all rails to Auto-mode by I2C at a later time to save power.

Sakai-san,

Yuto Sakai said:

Are there any errata or application notes on this low temperature problem?

There is no errata because errata are no longer permitted at TI for analog components. The issue also depends on PCB layout and the cannot be reproduced in the TI labs using bench boards with ideal layout. If the switch node is noisy the low temp will cause instability during start which causes BUCK3/4/5 to overshoot the target voltage or act in an otherwise erratic manner. Because this is a combination of an application issue and a device issue, it is problematic to modify the datasheet and capture all use cases.

Yuto Sakai said:

Is TPS650947 recommended if low-temperature operation is expected?

The TPS650947 is recommended in most cases, unless PSM (pulse-skip mode) is critical for saving power during start-up. Power-savings during start-up is not usually needed if you can start in fPWM then switch to Auto-mode. Alternatively, if you can use TPS650942 and hold PMICEN low until you send I2C write to change BUCK3/4/5 to fPWM mode, then set PMICEN high, this will also work.

Sakai-san,

This is good news. You can continue to use TPS650942, power up the PMIC normally and set BUCKx_MODE (for fPWM mode) whenever the software is up and running. Or, you can plan to set BUCKx_MODE (for fPWM mode) only when you detect temp < 5 C.

It is not necessary to switch to TPS650947.

Sakai-san,

Yuto Sakai said:

Does that issue occur when the temperature is below 5 ℃?

In the previous customer's inquiry, the issue occur between -10 ℃ and +5℃. To be more conservative, I would say there is risk with TA < +10 ℃.

Yuto Sakai said:

Are TPS650945 and TPS650947 with fPWM set by default designed to solve this problem?

I cannot say if the TPS650945 and TPS650947 with fPWM set by default were created specifically to solve this problem, but I can say with certainty that using these variants of the device solves the issue. Using TPS650942 with BUCKx_MODE = 1b (fPWM enabled) also resolves the issue.

The release of new product variants is not usually driven by customer issues. If a customer issue is determined to be a flaw in the device, it is our policy to modify the datasheet for the devices that are affected by the issue. At this time, issues with TPS650942 in Auto-mode (PFM) at < +5 ℃ have not been shown to be an issue with the device and in the one case we investigated previously it was also due to environmental factors such as non-ideal layout and noise.

Although we can do further analysis on your issue, it would require reviewing schematic, layout, and oscilloscope captures of the issue occurring. It would take a long time and considerable effort from the customer and TI. For this discussion, it is sufficient to say that we know:

Using fPWM will solve the issue due to the similarities between your case and the case that was investigated previously. Writing I2C to TPS650942 so that it switches in fPWM mode or using TPS650947 device (with fPWM by default) are both approved solutions.