The reference circuit on p. 1 of the LT3094 datasheet seems to work fine for -3.3 V.
However, changing the 33.2 K resistor to 24.9K to get -2.5V doesn't work.
Even the LTspice reference circuit has this same problem.
Both only allow the voltage to go to about -3V.
However, if the 50K between PGFB and GND is moved to OUT and GND, the circuit
seems to work as expected. i.e. It will go to -2.5 V.
What am I missing?
LTspice ref circuit for -2.5 V (not working)
LTspice datasheet circuit for -2.5 V (not working)
LTspice datasheet circuit with modification for -2.5 V (working)
The answer might be too late for Tim, but others might face a similar problem.
If you change the output voltage by changing RSET, you also have to adjust the voltage divider from OUT to PGFB. If you…
If you change the output voltage by changing RSET, you also have to adjust the voltage divider from OUT to PGFB. If you don't do that, the startup circuit may try to quickly reach about 3V, which would be the target for a 3.3V output. Then the startup ISET would be switched off, but kick in as soon as the PGFB voltage sinks by 7mV, so you always have more than 2.5V.
The threshold for switching from 2mA startup SET current to operational 100µA SET current must always be lower than 100µA*RSET.