Question about LTC7851

We are trying to use LTC7851 as the controller of a 4-phase buck converter. However the input voltage rating and the output voltage rating exceed the sense voltage range of both the feed-forward control and current sharing control pins. Before we try to put voltage dividers to scale down the pin voltage, we need to figure out the internal structure, or at least the input impedance of these control blocks. However, they are unclear in the datasheet.
1. About the current sharing. On page 27, it says that the current sharing loop is internally compensated but it isn't indicated on the block diagram on page 11 that how the internal compensation network is connected to the current sense amplifier. I need to know what type of the compensation it is and how it is connected because I need to connect some resistor voltage divider to the current sensing pin and do not want to interrupt the stability of the current sharing loop.
2. About the line voltage feed forward. Similar as the first question, may I know the feedforward circuit which is not indicated in the block diagram on page 11? I also need    to connect some resistor voltage divider to this pin and do not want to interrupt the stability of the feedforward loop.
Thanks a lot for your help!
  • Hello,


    The current sense inputs of the LTC7851 will need to be divided down to keep the voltage under their maximum rating. This may require precision resistors with a tolerance of 0.1% or better to minimize the common mode error due to resistor mis-match. In addition, the current sensing and sharing will be less accurate since the signal is being divided down. The VINSNS pin will also need to be divided down. The gain from the COMP pin to Vout will be higher since the PWM ramp will have a more shallow slope. The line feed forward circuit will otherwise not be affected. These effects can be modeled in LTspice.


    For a simpler solution, here are several dual phase controllers with higher Vout and Vin ratings to consider:


    LTC3857/58: Vin(max)=38V, Vo(max)=24V

    LTC3890: Vin(max)=60V, Vo(max)=24V

    LTC3892: Vin(max)=60V, Vo(max)=99%*Vin

    LTC7810: Vin(max)=150V, Vo(max)=60V


    No current sense dividers will be required if Vout is kept below the maximum rating. Since these are current mode controllers they do not have a VINSNS pin for the line feed forward circuit. The LTC7851 is a voltage mode controller.


    To parallel the phases for the LTC38xx devices mentioned above, tie their ITH,VFB, RUN and TK/SS pins together. Examples can be found in the LTC3892 data sheet. To parallel the phases of the LTC7810, tie VFB2 to INTVCC. The ITH, VFB and TK/SS pins will then be internally tied together. To parallel 3 or more phases, externally tie the ITH, VFB and TK/SS pins from phase 1 of each controller together.


    For solutions with 3 or more phases, the controllers will need to be driven by a multi-phase clock. For 4 phases, drive each controller 90 degrees apart. This will provide phases of 0, 180, 90 and 270 degrees. For 6 phases, drive the controllers 60 degrees apart for phases of 0, 180, 60, 240, 120 and 300 degrees.


    The current mode controllers mentioned above all have gate drivers for driving MOSFETs whereas the LTC7851 provides three-state compatibles PWM outputs for DrMOS or gate drivers. The data sheet applications will provide guidance on which MOSFETs to use.


    Refer to the data sheet for further details. Contact your local ADI sales office for more assistance.


    Best regards,






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