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LT1054: Positive Voltage Doubler with Regulation

Mark Bennett1
Prodigy 40 points
Part Number: LT1054
Other Parts Discussed in Thread: LT1013, , LM358

Hi there,

I am trying to produce a regulated 15V output voltage from an input voltage of 9V. There is a design on the Datasheet of the LT1054 (pg. 22, figure 28) titled "Positive Doubler with Regulation (5-V to 8-V converter)" that appears to be what i'm looking for. However, as this design is specific to a conversion from 5V to 8V, i'm unsure how to adapt the design for my specific needs - no details about how the resistance values were calculated, or why specifically the LT1013 op amp was chosen.

Could you please shed some light on the operation of this circuit, and if possible how to modify it to suit my design? 

Thanks!

  • 0
    TI__Mastermind 31315 points

    Hi Mark,

    The engineer responsible for this part will respond next week.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    TI__Guru* 78675 points

    Hi Mark,

    Thank you for considering the LT1054.  Figure 28 uses two 1N5817 diodes to replace the internal output stage switch legs in order to reverse the charge pump output polarity, and LT1013 is used for output error amplifier.  By re-selecting the resistor divider at the non-inverting pin of LT1013, namely replacing one of the 5.5k and 2k resistor  branch in Figure 28, you can adjust the output voltage.  The preferred selection is to keep the high side 5.5k untouched, but change the bottom resistor (was 2k), such that the divider will produce 2.5V at the non-inverting pin of LT1013.    The series 10k at the non-inverting and the 0.03uF+10k basically forms the loop compensation, which should stay as is.

    Specifically, your new resistor value of the original 2k resistor in Figure 28 should satisfy the following equation:

     Vonew x Rnew / (Rnew +5k) = 2.5V

    Hope this clarifies.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 40 points

    Hi Youhao,

    Thank you so much for your quick response. Altering the resistor divider on the output was my initial thought, but I didn't realize that the 5.5k resistor needed to remain unchanged. What is the reason for that? is there some sort of minimum current that must pass through the divider network? 

    Also, You didn't comment on the choice of Op amp for the error amplifier. Was the LT1013 chosen arbitrarily, or was it selected for a specific characteristic, such as its ability to handle inputs/outputs at ground off a single supply?

    Further, are the 10k resistor connected between the LT1013 and the fdbk pin, and the 50k resistor connected between Vin and the fdbk pin arbitrarily selected values, or do they play a roll in the regulation? 

    Thanks again,

    Mark Bennett

  • 0 in reply to Mark Bennett1
    TI__Guru* 78675 points

    Hi Mark,

    The reason that I suggest the change of 2.5k-resistor is preferred because you will need to reduce it to get 15V output.  If you change the high side, you will need to increase the 5.5k-resistor, which would increase the impedance seen by the non-inverting pin, and the offset current would cause more error.  For this design it is not very significant but it is a good practice to follow.  

    LT1013 is kind of arbitrarily chosen.  I believe the engineer who wrote the datasheet chose it for its good precision and low power consumption. Other Op-amps having small offset voltage and current, and low supply current, can also do the work.

    Regarding the 50k and and 10k at the FB pin, these seems being placed to for current limit purpose because FB pin also control the internal Ref block.  The 50k looks like a pull up resistor for the FB pin to enable the device. It may not be necessary to have the 50k because the 10k at the FB pin already has the pull-up function from the LT1013.   

    Thanks,

    Youhao 

  • 0 in reply to Youhao Xi
    Prodigy 40 points

    Hi Youhao, 

    Thank you once again for your quick reply. This has fully answered all of my questions, thank you so much!

  • 0 in reply to Youhao Xi
    Prodigy 40 points

    Hello, Just wanted to follow up on this post a final time for the information of anyone else who may wish to use this circuit in the future. As an extension of this pedal, i've been able to generated a regulated 30V output from a 9V input by using two addition charge pump stages and the appropriate resistor divider. I used a dirt-cheap LM358 op amp, and it works perfectly! 

    The issue I ended up running into, however, was noise. I'm using this circuit in an audio amplifier, and at first I was getting a large hum coming from the ground plane. The fix ended up being the 50k pull-up resistor, which I had initially left out as Youhao suspected that it wasn't necessary. Putting it back in solved my noise problem, making the circuit dead quiet. 

    Thanks again for the quick response and solving this issue for me! Cheers :)