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LP3878-ADJ external Vout control

david mcclure1
Prodigy 50 points
Other Parts Discussed in Thread: LP3878-ADJ, LM10011, LM10010, LM317, LM117

What is the recommended method for implementing external adjustment control for variable voltage output of the LP3878-ADJ?

  • 0
    TI__Mastermind 22610 points

    Is this a different question from your other post "LP3878-ADJ value for Vadj "?

    If yes, then I would suggest LM10010 or LM10011 "VID Programmable Current DAC for Point of Load Regulators".

    http://www.ti.com/lit/gpn/lm10010

    http://www.ti.com/lit/gpn/lm10011

    The data sheets are written with a focus on adjustable SMPS, but the technology works the same with adjustable LDO's.

    If this suggestion is off the mark, then please advise.

  • 0 in reply to Donald Jones
    Prodigy 50 points

    Hi Donald.  Thanks for your reply for both of my questions re LP3878-adj. Can the LP3878-adj be adjusted using an external offset voltage connected to the ADJ input?   Using the LM1001x appears more complex than my perceived requirement.   I need a simple way of controlling the output of 3ea parallel LDOs (LP3878-adj) using a single control panel potentiometer to adjust Vo between 3.6 to 5.0 VDC, approx. 300 mA maxcurr. Precise control is not a requirement.  It is for switch indicator illumination where the panel pot is adjusted for desired illumination brightness.

    Dave

  • 0 in reply to david mcclure1
    TI__Mastermind 22610 points

    Yes, with your description here the VID devices would probably be too much.

    Need to get a crystal clear understanding of the goal...

    One control point (single potentiometer), three parallel adjustable LDO's,  ... three separate outputs? all three outputs at the same voltage (or close), 100mA from each separate output (for 300mA total)?

    Or, all three LDO outputs are tied together to make one output with each LDO sharing the 300mA output current (~100ma each)?

    Output voltage range is 3.6V to 5.0V, what is the input voltage range?

    Did you pick the LP3878-ADJ for any specific reason? It's a nice part, but it might not be appropriate for what you're trying to do (if I understand correctly).

     

  • 0 in reply to Donald Jones
    Prodigy 50 points

    I picked the LP3878-adj because of adjustability, LDO, 800 mA, simple small footprint design. I was thinking of connecting the pot to a LM317.  The LM317 would then control the three LP3878-adj.   REQUIREMENTS: one single pot control point, three parallel adj LDOs, three separate outputs, all three outputs at same approximate Vout, 300mA max worst case from each output, desired output voltage range = 3.6 to 4.8VDC, Vin = 5.0VDC

  • 0 in reply to david mcclure1
    TI__Mastermind 22610 points

    The Vin=5.0V to Vout=4.8V does not leave much operating headroom.

    I was going to propose LM117/LM317 since it's really insensitive to output capacitor type, and fairly bullet-proof :

    http://www.ti.com/lit/gpn/lm317-n

    Figure 55. Adjusting Multiple On-Card Regulators with Single Control

    which would be too simple, but you will need Vin >=8V (Vout + 3V) for this scheme to work. So, never mind.

    It may be possible to simulate what the VID devices offer which is a controlled current into the ADJ node, but I'll need to do some work with LP3878-ADJ first to see if it stays happy.

     

  • 0 in reply to Donald Jones
    Intellectual 680 points

    This discussion very much parallels my own question, hence my "butting in".

    I require a LDO to supply 1600mA over an adjustable output voltage of 2 to 8V. I intend supplying the input to the LDO from a voltage source (DC/DC) that is always about at least 0.5V higher than desired LDO output but never more than about 2V higher (don't want high dissipation in LDO).

    The LDO output voltage should be finely controllable from a DAC driven in OPEN LOOP by an external controller.

    I thought of driving the normally Ground connected point of the resistor feedback network from the DAC.

    Can you suggest a suitable LDO and also comment on this control scheme.

    Thanks