We have found that the LM3478 on our product is exhibiting some variation in startup time, seemingly related to the temperature of the LM3478.
In the following plots, channel 3 is the VIN pin and channel 4 is the DR pin. The first plot shows that at 25C, there is approximately a 2.5ms delay after the VIN pin goes high until the part starts switching on the DR pin. The second plot shows that at 85C, the DR pin begins switching almost immediately.
The next plot was taken at room temperature and the startup delay is present. Channel 1 is the VIN pin, channel 2 is the DR pin, channel 3 is at TP6419 (see schematics below), and channel 4 is at the FA/SD pin. Clearly, we have set the input to FA/SD low when VIN goes high, but the FA/SD pin appears to be sourcing approximately 10uA which is floating it up to ~900mV.
Can anyone explain why we are seeing this variation in startup time? Is this normal for this part?
Thanks,
Mike