• AVR Freaks

Hot!TB3155 What is this symbol?

Author
acharnley
Super Member
  • Total Posts : 270
  • Reward points : 0
  • Joined: 2016/05/01 06:51:28
  • Location: 0
  • Status: offline
2019/04/18 06:59:55 (permalink)
0

TB3155 What is this symbol?

It looks like a current sense symbol.

I asked Microchip support and they say it's probably an error, should just be a direct connection "dot". This I think is unlikely - should the buck be working above the Pic's input voltage it's going to damage the Pic.

My solution is to use a 10k resistor (max impedance) with a 3v zener. My buck can potentially be switching 100v (at low current) so worst case is 10mA / 1W loss. The problem with the 10k is it's fighting the FET (100v Rgs) to detect a voltage delta, that's a 0.01 ratio and if the comparator has an input offset of 5mv then it needs 0.5v to detect. 100v really is worst case so I could drop it to 2k/50mA (pin max) for 0.1v resolution.

Or maybe that symbol represents something better!

Thoughts?

Attached Image(s)

#1

8 Replies Related Threads

    Bob White
    Super Member
    • Total Posts : 240
    • Reward points : 0
    • Joined: 2010/11/06 19:52:38
    • Location: Denver, Colorado
    • Status: offline
    Re: TB3155 What is this symbol? 2019/04/18 08:53:36 (permalink)
    0
    It is not a symbol for a specific component.  This is representing a current sense function, which could be a current sense transformer.  It is up to you to figure out how to converter the sensed current to an an appropriate voltage for the input the comparators in the PIC.
    #2
    acharnley
    Super Member
    • Total Posts : 270
    • Reward points : 0
    • Joined: 2016/05/01 06:51:28
    • Location: 0
    • Status: offline
    Re: TB3155 What is this symbol? 2019/04/18 11:09:55 (permalink)
    0
    Right, but basically the PIC can't do it by itself as the document proposes.

    My math above was incorrect. To get a 25mV change at the comparator gate with a 100mOhm FET and a 2k resistor (which at 100v gives the 50mA pin max) I'd need 500.025v.

    After spending so much time on it I might throw in the towel and fit a diode.


    #3
    PStechPaul
    Super Member
    • Total Posts : 2210
    • Reward points : 0
    • Joined: 2006/06/27 16:11:32
    • Location: Cockeysville, MD, USA
    • Status: offline
    Re: TB3155 What is this symbol? 2019/04/18 11:19:25 (permalink)
    0
    That's definitely a high side current sensor. But there seems to be an error in the circuit for the buck converter. The high side MOSFETs need gate drivers. There is no discussion of the buck converter - only the boost topology. Perhaps the bulletin was released before it was completed?
     
    http://ww1.microchip.com/...lation-DS90003155B.pdf
     
    A suitable high side current sensor would be LT6101:
     
    https://www.analog.com/me...ata-sheets/ltc6101.pdf
    post edited by PStechPaul - 2019/04/18 18:10:00

     
    #4
    acharnley
    Super Member
    • Total Posts : 270
    • Reward points : 0
    • Joined: 2016/05/01 06:51:28
    • Location: 0
    • Status: offline
    Re: TB3155 What is this symbol? 2019/04/18 11:42:01 (permalink)
    0
    It's not sensing current though but voltage. 
     
    Imagine a duty of 0% in DCM mode where the lower FET is now on (the problem), and a large charged capacitor on the output is trying to backflow. 

    If LT6101 detects a current flow (which would have to be only for reverse) the FET would turn off. This would result in no current flow so the FET would turn on = oscillation.
    #5
    PStechPaul
    Super Member
    • Total Posts : 2210
    • Reward points : 0
    • Joined: 2006/06/27 16:11:32
    • Location: Cockeysville, MD, USA
    • Status: offline
    Re: TB3155 What is this symbol? 2019/04/18 18:30:43 (permalink)
    0
    The buck converter senses voltage at the output using OPA1 and DAC1. Current in the inductors is sensed by the CMP1 and CMP2, which provide half-bridge PWM to the MOSFETs. Being a buck converter, the input voltage is always higher than the output voltage, so there would never be reverse current through the body diode of the series MOSFETs, except maybe if input power was removed.
     
    You are right, though, that a normal PWM with the lower MOSFET ON would backfeed through it. But the current sensor should turn it off when current stops flowing. It is being used as a synchronous rectifier to improve efficiency over using the body diode for commutation.

     
    #6
    acharnley
    Super Member
    • Total Posts : 270
    • Reward points : 0
    • Joined: 2016/05/01 06:51:28
    • Location: 0
    • Status: offline
    Re: TB3155 What is this symbol? 2019/04/19 00:50:49 (permalink)
    0
    That's understood Paul. The problem is when the buck runs at a higher voltage, a resistor to CMP1 has to be higher which lowers the resolution against the FET Rgs to such an extent it's impossible to utilise. For that reason I was wondering if there was another way to detect the voltage at this point.
    #7
    PStechPaul
    Super Member
    • Total Posts : 2210
    • Reward points : 0
    • Joined: 2006/06/27 16:11:32
    • Location: Cockeysville, MD, USA
    • Status: offline
    Re: TB3155 What is this symbol? 2019/04/19 11:22:11 (permalink)
    0
    What do you mean by FET Rgs? Do you mean Rds(on), or Vgs? Or are you trying to drive the MOSFET directly using a resistor divider to the gate? I can't tell where you are adding the 10k resistor and 3.3V zener.
     
    The circuit obviously needs additional components, such as a 5V power supply for the PIC, and gate drivers for the MOSFETs, which might require a higher voltage for the gates.
     
    Are you trying to implement a current sensor circuit like that which is suggested in the following?
    https://e2e.ti.com/blogs_...oltage-mode-controller
     

     
     

     
    #8
    acharnley
    Super Member
    • Total Posts : 270
    • Reward points : 0
    • Joined: 2016/05/01 06:51:28
    • Location: 0
    • Status: offline
    Re: TB3155 What is this symbol? 2019/04/23 08:02:51 (permalink)
    0
    Good find Paul. I wonder if the built in OPA has enough resolution to do it. Here's what Microchip say:
     
    "Hi Andrew, So I received a response from the authors regarding this. This is a generic current sense symbol. There should be a component there. A differential current sense amplifier would do the work or a current sense transformer. The MCP6C02 current sense amplifier can be used or a current sense transformer such as the one done on the CIP Hybrid Power Starter Kit. Regards, Susmitha"
    #9
    Jump to:
    © 2019 APG vNext Commercial Version 4.5