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PGFB pin connection at very low output voltages (-0.2V)

Category: Datasheet/Specs
Product Number: LT3093

Hi all,

my LT3093 LDO is configured for an output voltage of -0.24V (derived from an input voltage of -2.3V). Where should I connect the PGFB pin? To IN or to OUT?

The datasheet states:

"Please note that programmable power good and fast start-up capabilities are disabled for output voltages between 0V and –300mV."

and

Page 15: "If power good and fast start-up functionality are not needed, tie PGFB to IN."

Page 22: "If fast start-up capability is not used, tie PGFB to IN or to OUT (for output voltages more than –300mV)."

Is there any advantage connecting PGFB to OUT instead of IN?

Thank you!

  • Hi.  Normally the PGFB resistor divider is programmed so PGFB = 0.3V when OUT is close to 90% of the desired regulation voltage.  When PGFB reaches 0.3V, that is when fast start up shuts off and PG asserts.  If the desired regulation voltage for OUT is 0.33V, then you can connect OUT directly to PGFB.  You can actually connect OUT directly to PGFB if the desired regulation voltage for OUT is higher than 0.33V, but in those cases fast start current and PG assertion will happen early compared to happening at the 90% point I mentioned.  You can always connect PGFB to IN, but then PG won't indicate properly and you won't have fast start current.  Finally, it is easier to connect PGFB to OUT on the positive LT304X LDOs and it is easier to connect PGFB to IN on the negative LT3093 LDO, just because the exposed pad is IN on the LT3093.  There is a lot in the datasheet about PGFB, but the 90% and exposed pad copper considerations are not really mentioned.   

    LT3093 DS pin config_1.png

    LT3093 DS block_1.png

  • Thanks for your answer, this is all valuable information, but mainly for output voltages with (absolute) values of more than 0.3V.

    In my case I have an output voltage of only -0.2V. With this small voltage, the PG and fast start-up features are not available at all (acc. to the datasheet). Even if I tie the PGFB pin directly to OUT, the PG pin never de-asserts, because the needed -0.3V threshold is never reached.

    The datasheet explicitly allows connecting PGFB directly to OUT at low voltages between 0…-300mV, for me this sounds like there are some cases where this is preferable. But I cannot think of any advantage compared to a connection to IN, but maybe I'm missing something?