We have been using these 5V switcher chips in large quantities for the last 2 1/2 years. Recently, we have become aware of a problem where some of the chips are putting out the wrong voltage (e.g. 4.6V). The data sheet specifies +/-3% which should be about 4.85V worst-case minimum. The circuit it is supplying pulls about 35mA for about 10 seconds (while it is booting up) and then about 70mA from then on.
On chips that are working properly, we have measured a switching frequency of roughly 170kHz while booting up and then 320kHz. This behavior matches the data sheet and seems to be correct for how the part should be working.
On chips that are putting out the wrong voltage the behavior varies: Most of the time, the chip starts up with a very high switching frequency (e.g. 2.4MHz) and the output voltage is 4.6V. Occasionally, the chip starts up with the correct frequency of about 170kHz. When this happens, the output voltage is correct (4.99V) and stays this way even after the bootup process is finished and the current increases to 70mA.
The high switching frequencies are consistent and repeatable from chip to chip. We are currently working with 3 samples of chips that exhibit this problem and the measured frequencies are 1.8, 2.4 and 2.7MHz (with the 35mA load current).
The switching waveforms look good/correct and appear to match the data sheet.
According to the data sheet, it doesn't seem like switching frequencies above 2.2MHz should even be possible?
Are you aware of any problems like this when using this switcher chip?
Can you please offer some guidance on what to do?
