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LM25148: Sink and Source current

Griffin Burns
Prodigy 20 points
Part Number: LM25148
Other Parts Discussed in Thread: LM5170, TPS552882-Q1, LM25149-Q1EVM-2100, LM5171

Tool/software:

I'm working on a design for a LiPo battery emulator and i had the idea of using a synchronous buck in a non-traditional arrangement. I'm wondering if this is possible.

As a battery emulator the buck needs to output positive voltage and can source and sinks current.

1) Quadrant 1: Regular will operate as a buck and source current to a charge controller on a device.

2) Quadrant 4: Device charge controller will attempt to charge the emulator by raising the voltage on Vo above the intended output voltage to create a constant charge current. When looking at the regulator circuit with Vo being the input, it seems like this should create a boost circuit. My goal is to connect an active load to Vin to sink the current when Vin gets boosted. I will have a diode connecting 12V to the regulator and the active load will turn on when it sees the cathode voltage higher than the anode.

Is quadrant 4 possible with this chip? It seems theoretically possible that a synchronous buck can operate as a boost, but i'm not sure if there is some detection logic in the chip that would disable the low side FET from acting as a boost converter and prevent reverse current?

  • 0
    TI__Mastermind 40120 points

    Hello Griffin,

    you maybe looing for a bi directional controler like the LM5170?  This device cannot work in this way.

    Hope this helps.

    David.

  • 0 in reply to David Baba
    Prodigy 20 points

    I don't think LM5170 would work for my application.  it seems like the right idea but the parameters are out of spec.  I need a controlled output voltage from 3.0V to 9.0V which allows +/- 8A.  I stumbled across TPS552882-Q1 which seems like it might work but i don't know if it allows for reverse current.

    BQ25758ARRVR seems like it could work but it requires a bit more intelligence.  I have an MCU in my system so I2C control is reasonable.

  • 0 in reply to Griffin Burns
    TI__Mastermind 40120 points

    Hello Griffin,

    Please open a new thread so this can be directed to the right Product Line, thanks.

    David.

  • 0 in reply to David Baba
    Prodigy 20 points

    Quick follow up, i purchased LM25149-Q1EVM-2100 and it does seem to function as a boost for reverse current.

    Test setup:
    12V power supply to anode of diode. Cathode connected to Vin

    Test cases:
    1) Connect Eload to Vout. Observe 5.0V on Vout with about 12mV ripple. Simulate transient by enabling eload at 1A. I don't see any droop and ripple only increases to about 55mV

    2) Connect 120ohm 1W resistor to cathode of input diode.  Remove eload from Vout and connect second power supply to Vout and set PSU to 5.01V with a current limit of 0.5A.  Turn on 12Vin and observe rail pull 133mA. Enable 5.01V PSU and observe 5.01V PSU source 0.5A. I can see 12V source supply drop to 0.0A. Scope shows Vin jumps to 15V indicating the reverse boost is working.

    Since i don't have an active load for test case 2, i can see Vin voltage increase depending on the current limit of the 5.1V PSU.

    This seems to be working as intended.  Can i ask why you said, "This device cannot work in this way".  I'm not sure if there is some edge case i'm missing.

  • 0 in reply to Griffin Burns
    TI__Mastermind 40120 points

    Hello Griffin,

    This is not he intended use for this device.  if you are asking can this device in FPWM generate a voltage at the input due to the output voltage increasing and boost from input to output.  the answer is, it can, but this is not the intended use for this device hence the response.

    Not exactly sure what you are trying to achieve here, but typically when customers ask about this function, they are usually looking for a bidirectional controller, which this is not, although it can sink current in the reverse direction.

    Hope this helps.

    David.

  • 0 in reply to David Baba
    TI__Mastermind 40120 points

    Hello Griffin,

    It is possoble, there is nothing stopping this from having current go in the reverse direction, but you need to understand this is not the intended use for this device, which is why I am recomended you use the LM5171/70.  please be cautioned of the following also, further supporting my decsion not to  use this part for this application.

    Hope this helps.

    David.