LTM4646 Schematic help review

Hi there,

we newly import LTM4646 into our project for the frst time. To be careful, we would like to ask for asisting review. Insight of LT/ADI expert will be appreciated. .

SCH as PDF attached, and Load conditions are text-noted nearby the output port of IC. Thanks very much!

I might post this threas again...guess the specific device is covered by different ADI personnels.


  • The schematic looks ok. Here are a couple suggestions:

    1. For the compensation circuit, if use internal compensation circuit of LTM4646, the compensation circuit at COMP1A and COMP2A doesn't need to be populated. If use external compensation circuit, then the zero ohm resistor between the COMP1A and COMP1B pins can be removed (same as zero ohm resistor between COMP2A and COMP2B). The current design has the internal compensation circuit in parallel with the external compensation.

    2. For the 1.5V output, the FB resistors at VFB2 pins should be 40.2kOhm (to remote sensed GND) and 24.3kOhm (local GND), not two 30kOhm.

  • Hi, yanliang

    1. Ok, we will reserve both internal and external compensation on board. Default use internal comp. So I will add an 0ohm (NC) between COMP1A pin and external RC networks. same for COMP2A pin.

    2. This have been my doubting before. from the reference sch on datasheet, we see that VOUTS2- pin is actually connected to GND. So RFB2 and RFB3 are actually in parallel. And the real referance voltage of VFB2 is 0.3V according to page4 Electrial Charecter table.

    So we have VFB2=0.3V, RFB_Top=60.4K(internal), RFB_bottom = (RFB2 || RFB3) =15K to generate VOUT2 = 0.3*(60.4+15)/15 = 15.508V. 

    Also, you could notice that the values in Table 1:  40.2K||24.3K = 15.1K, equal to two 30K in parallel. I think RFB2 and RFB3 could be other values as long as they have a equivalent parallel value of 15Kohm.

    What's bother me is ,why need TWO resistor RFB2 and RFB3? seems one resistor would be enough, just like RFB1 on the VOUT1 side.

    Is somewhere of my understading wrong? 

    3. Another question, We are layout now. To use the remote feedback, I think RFB1/2/3 should be placed nearby LTM4646, but the trace  VOUTS1/VOUTS1- and VOUTS2/VOUTS2- should be routed from remote LOAD points, and be routed as two differential pairs. How do you think? 

    Thank you very much !


  • Vouts2- pin is the Differential Output Sense Amplifier (–) Input of channel 2. This pin should be connected to the negative terminal of the output load capacitor of VOUT2. If the load is pretty far from the uModule. There will be a voltage offset between local ground and remote output ground. Only if the load is right next to the uModule and there is no voltage offset between load and remote ground, RFB2 and RFB3 are in parallel. Please follow the datasheet in choosing these feedback resistors.

    The three resistor feedback network has the potential to use an external reference voltage. Although this function is not enabled for LTM4646.

    Agree with you about the layout. Thanks!