We have an old and new version of a small PCB using the TPS61221DCKR. The old board works fine. The new board we tried to add the Enable line, but then decided against it and just shorted EN to VIN. But on all of the new boards with this modification, the regulator works fine up to a voltage of about 1.5V. Above 1.5 it begins to lose current and by 2.5V it is limiting current to the point our LEDs begin to drop out. We have checked that all components on old and new boards are the same. We have tried shorting out traces to see if anything in the board itself could be contributing. We need to figure out what's causing this failure before we build the next rev of the board.
The capacitors are
| C1,C2,C3,C4,C5,C6,C7 | CC0805ZKY5V6BB106 | CAP CER 10UF 10V Y5V 0805 | Yageo |
| L1 | LBC3225T4R7MR | INDUCTOR WOUND 4.7UH 680MA 1210 | Taiyo Yuden |
Schematic and layouts are in attached zip file
Two scope captures ds01 and ds00.bmp -- 00 shows the old board's inductor (L pin) operating at 2.5V. The other shows the same point on the new board at 2.5v.
daver
