In Part 2 (Integrating the Trimming & Satisfying the Customer’s Needs, Part 2), we looked at the method to use fused-links programming to trim a voltage regulator's voltage reference. This trimming resulted in changes to the UVLO (under voltage lockout) and ENABLE thresholds of the power supply IC. Due to time constraints, I verified that only some of the logic sequences worked well. As my available time expired, I decided to fix this bug during the starting phase of the mass production of the unit at my company's power supply facility.
I decided to interface my testing machine with an automatic handler in order to gather statistical information on a high number of samples. The data that I gathered was related to the effect of each trimming sequence on the parameter that I wanted to control (i.e., the internal reference voltage). This resulted in the trimming table below that shows the real results:
After implementing this fine tuning, all the trimming sequences were effective, but this was true on a population of 100 units per sequence, i.e., on 1,500 devices. Then the mass production started and I observed a 20 percent yield loss due to the trimming procedure. To solve this issue, I switched the testing mode from automatic to manual.
I noticed that the problem disappeared, so I concluded that the problem was in the handler test board when running the automatic testing. My guess was that this was probably due to an increased resistive path between the equipment generating the logical trimming sequence situated in the ATE (Automatic Test Equipment) and the device under test. This increased resistance due to a longer path (test board ATE + automatic handler interfacing board) modified the voltage levels of the pulse sequence utilized to force open the fuse-links as part of the bit-trimming sequence.
Hence the voltage level utilized to supply a bit “1” was modified from the desired pulse value of 16V to ~15.2V. This difference was apparently enough to cause the trouble. See Figure 1:
I solved the problem by soldering a capacitor and a resistor near to the device to be programmed. See Figure 2. This R-C filter was generally helpful: It held the supply voltage up high enough until it was needed for the programming action. It guaranteed the right voltage levels of each trimming sequence.
The final result of this action was winning the business: The customer requirements have been satisfied and the client obtained the requested product.
Have you ever experienced a similar situation? As an engineer have you ever modified your hardware in real time or on the fly to perform a task? What strategies do you follow for your trials, by adapting to the particular conditions you have to face?
- Integrating the Trimming & Satisfying the Customer’s Needs, Part 2
- Integrating the Trimming & Satisfying the Customer’s Needs, Part 1
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