TPS546D24A: Inquiry regarding short occurrence between SW pin and PGND pin when TPS546D24A low current load

Hello Grady,

Due to US holiday please allow us until next week to provide a response.

In the meantime, do you have SW and VOUT waveforms that you can share of the intermittent short? Does short recover after some time or only with higher load?

Best regards,

Britton

Hi, Britton

The reply was late because I had to check with the customer.

I checked and found that I had contacted E2E about the same issue a year ago.

(https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1190323/tps546d24a-short-issue-between-sw-and-gnd)

(https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1202207/tps546d24a-question-about-tps546d24a-low-fet-damage/4535784?tisearch=e2e-sitesearch&keymatch=TPS546D24A%25252526%25252526boot#4535784)

Please note that data such as waveforms, schematics, and PCBs were uploaded at the time.

To sum up, there was a lot of advice at the time, but the problem of the IC dying due to the SW pin and PGND pin being shorted still occurs intermittently.

The customer conducted a reproduction test to improve the problem, but the problem was not reproduced, so there is no improvement yet and the exact cause is unknown.

Please check whether the questions asked above in Q1~Q3 may have an effect on the defect phenomenon, and is it possible to check if there is a solution to the problem?

Thanks.

Grady Lee 

I want to make sure I am understanding the issue you are describing.  You say 

Grady Lee said:

Intermittent short circuit

Intermittent means "comes and goes"  But the text appears to describe a low failure rate device damage resulting in a permanent SW to PGND short rather than an intermittent one, is that correct?

Is the only damage a short from SW to PGND, or are you also seeing PVIN to SW shorts?

Regarding your questions:

Grady Lee said:

Q1) On the datasheet, “If AVIN is connected to the same supply as PVIN or VDD5, TI recommends a minimum 10-µs R-C filter with a 1-Ω to 10-Ω resistor and AVIN bypass capacitor between AVIN and PVIN to reduce PVIN switching noise on the AVIN input." It says.
If there is no R-C filter configuration as in the circuit below, will this be the cause of the current problem (short between sw pin and PGND pin)? (U2 is TPS546D24A)

It is unlikely that failing to filter the PVIN to AVIN connection would result in damage to the low-side MOSFET and the SW to PGND node.  The primary purpose of this resistor is to limit switching frequency noise on PVIN from affecting the analog control circuitry powered from AVIN.

Grady Lee said:

If the resistance of the Bootstrap value between SW and BOOT is 7.5 ohm as shown in the circuit below, will this be the cause of the current problem (short between sw pin and PGND pin)?

A boot-strap resistor of 7.5V is large, and could be resulting in a long turn-on time or an extended turn-off time.  If the high BOOT resistor is causing the BOOT to SW capacitor voltage to be low, it could result in an extended high-side driver turn-off, which could result in cross conduction and low-side MOSFET failure.

Monitor the SW and BOOT voltage with respect to the same ground point with the same vertical resolution at 1 or 2μs per division to check the BOOT to SW voltage and the falling edge of SW at 100ns or less per division to look for evidence of cross-conduction.

Grady Lee said:

Q3) Additionally, is there a root cause of the short between SW pin and PGND pin? Can you confirm what methods are available to solve the problem?

Using an oscilloscope probe with bandwidth limiting off, monitor the SW voltage on rising and falling edges to look for evidence of the SW voltage exceeding the Absolute Maximum ratings.

If the SW node or the PVIN - SW differentials are exceeding their absolute maximum values, we can look into ways to reduce the resonance and ringing that would be causing it to do so.

Hi, Peter

There was some confusion, so I'd like to explain again.

When about 1,000 units are produced, about 4 to 5 defects occur in the field.

SW and PGND shrink, causing IC to die permanently.

Only SW and PGND cause shrot. (There is no short for PVIN and SW.)

Thanks.