Question about LTC7851

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,

Mike

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,

Mike