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LM10011: Compensation components for dc-dc regulator

Kiran Kerai
Prodigy 110 points
Part Number: LM10011
Other Parts Discussed in Thread: TPS544C25,

I am using a TPS544C25 regulator with LM10011 dac providing adjustable output voltage as required by our processor. 
I have designed the TPS544C25 using webench and compensation components have been chosen to meet design requirements l, phase margin etc.

My webench design doesn’t include the LM10011

My question is whether the LM10011 will affect the compensation components chosen by webench. If it does affect my design, how can I ensure the compensation network is designed correctly?

  • +1
    TI__Guru 50145 points

     

    Since the LM10011 is a high-impedance current source, it will not, by itself, affect the compensation network, however the compensation network - and specifically the DIFFO to FB resistor, will affect the bit-weight of the LM10011 and how it translates to the output voltage step-size of 1 LSB from the LM10011.

    In 4-bit mode, each LSB of the LM10011 DAC input will source 3.76μA of current into the FB net, which will increase the regulated output voltage by Rfb x 3.76μA where Rfb is the resistor between FB and DIFFO.

    If your current TPS544C25 compensation uses the correct Rfb resistor for the LSB step-size you want, great, you're done.

    If, however, you need to resize Rfb to get the LSB step-size you want from the LM10011, you will need to scale all of the compensation components equally.

    Scale all resistors to a value of Rnew = Roriginal x (Rfb_new / Rfb_original)

    Scale all capacitors to a value of Cnew = Coriginal x (Rfb_original / Rfb_new)

    For example, if you original compensation used a 20k DIFFO to FB resistor, but you need an LSB step-size of 10mV, you would need to change Rfb to 2.67kΩ (0.010V x 3.76μA = 2,659Ω)

    You would need to divide all of the resistor values by 3.76 and multiply all of the capacitor values by 3.76.

    For Resistors, which set gain factors, try to get as close as possible.  For capacitors, which set frequencies, it is generally best to pick the next larger standard available size except for the smaller FB to COMP resistor, where it's best to select the next available smaller size