The A7987 chip output terminal has high pulses

[linda-gu]

The output status of A7987 chips varies from batch to batch. We purchased materials from different suppliers, and one batch will have a high pulse of about 7V after recovering from a 5V short circuit, while the other batch will have a high pulse of about 13V after recovering from a 5V short circuit. The maximum withstand voltage of the rear chip is 7V, and this 13V pulse can cause damage to the rear chip.

Analysis of Failure Causes of CAN Transceiver Chip on HJ81 Circuit Board

• A7987 abnormal chip, the voltage surged to around 13V when powered on after a 5V short circuit, causing the backend chip to burn out。

 

 

 

• After replacing the chip with A7987 that tested normally, the voltage fluctuated to around 7V when powered on after a 5V short circuit, within the range that the CAN transceiver chip can withstand。

 

 

 

 

[Peter BENSCH]

Welcome @linda-gu, to the community!

I'm sure someone would be very happy to help you, but without knowledge of your system and its documentation (at least schematics) and the measurement conditions (measurement points, load parameters of the A7987) this is de facto impossible.

Regards
/Peter

[linda-gu]

Schematic diagram

There is no output when the 5V output is short-circuited. After the short-circuit condition disappears, the 5V output can return to normal. However, when the 5V output is restored, there will be a relatively high pulse, as illustrated in the previous picture.

The pulse monitoring points are 5V and GND. The current at the power supply terminal is 20mA.

During the output short-circuit test, the input terminal of the circuit board was continuously supplied with 24V voltage normally.

Thank you for your help

 

[Peter BENSCH]

If I briefly check your design with eDesignSuite and the values of your compensation network, I get a negative phase margin. I therefore assume that this is causing your observations.

Please check the compensation network yourself again, taking into account all parameters (input voltage range, output current range), for which eDesignSuite is very well suited.

Regards
/Peter

[linda-gu]

Following your advice, I used eDesignSuite and filled in the parameters based on the schematic. However, I found that the phase margin was negative. Upon reviewing the recommended circuit in the chip manual, I discovered that adjusting the Rc value of the compensation network to 1K could increase the phase margin to 56.3°. The comparison between the two is shown in the figure below:

 

The specific test confirmation is expected to be completed by February 6th. Thank you for your help.

[linda-gu]

I adjusted the value of R6 in the feedback circuit from 10K to 1K according to the circuit recommended in the manual. The test revealed that the surge was higher when the output short circuit returned to normal, around 21V. Please refer to the picture for details.