We are building a product using TPS61081 to boost 5V to 12V. The schematic is identical to Figure 1 on page 1 of the datasheet except FSW is tied to ground (600 KHz selected). Downstream we are driving qty 4 .1uF ceramics and qty 1 10 uF tantalum all in parallel. The 5 caps are located in close proximity to a TI DAD1000 and are located several inches away from the PS circuit on the board. On a recent batch of boards we are experiencing startup problems with the 12V TPS61081 PS. Testing shows that the PS shuts down after about 2 msec, this is consistent with the VIN-Vsc shutdown described in the START Up section of the DS. The failure happens on 100% of cold starts, however if input power is cycled on-off-on quickly then the PS starts normally with the correct soft start slope. We have looked at load current and do not see anything unusual compared to a good board, the load current has a couple of 100 mA fast peaks before shutdown. The 5V input voltage is low and noisy during the 2 ms timeframe but never drops below 4.5V. When the load is disconnected the PS starts up normally.
What is the charging current available when the Vgs clamp is active ? What effect does the load have on this startup operation ? Do we need to isolate the load during startup, if so what do you recommend ?
We have built several hundered boards with this circuit shich operate correctly. We cannot find any component value or PN errors on the failing boards. The input / output caps are x7R ceramic.
