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CSD18510KCS: What is the linear region condition and which gate driver is suitable for this device?

Venkatesh P17
Intellectual 895 points
Part Number: CSD18510KCS
Other Parts Discussed in Thread: TLV431, , OPA277, LM7171, LM224

Hi TI Team,

Please help me to understand the linear and saturation region of this Mosfet and the supply conditions. I am trying to form a series regulator using this part. My Vds is 6V, my required output is 4V, 3A, I can drive the Vgs = 0V to 12V using an error amplifier. So that I need this linear region.

What is the charging and discharging current of the Gate? Is there any circuit combination to quick charging and discharging of the gate? Please share the Tina Model for this Mosfet or suggest equivalent part

which Gate driver is suitable for this Mosfet? is it advisable to use Gate driver for this application? Is the gate driver will drive the Mosfet in linear region?

Thanks,

  • 0
    TI__Mastermind 39530 points

    Hello Venkatesh,

    Thank you for your interest in TI MOSFET products. Please see definitions and curves below. For a series regulator application, the FET can operate in either the linear or saturation region. There is a lot of literature available on using a N channel MOSFET as a pass element for a series regulator. You can use a shunt regulator such as a TLV431 to create a series pass regulator and drive the gate of the the FET. A FET driver is not required. An example is shown below as well. The PSpice model for this device is available by clicking on the Tools & software tab in the product folder on ti.com.

    – linear region, where Vds << Vgs − Vth leading to Ids ∼ (Vgs − Vth)Vds

    – saturation region, where Vds > Vgs − Vth leading to Ids∼ (Vgs − Vth)2

  • 0 in reply to John Wallace1
    TI__Mastermind 39530 points

    series regulator.pdfHi Venkatesh,

    It looks like the pictures I tried to paste did not come thru. They are included in the attachment.

  • 0 in reply to John Wallace1
    Intellectual 895 points

    Hi John Wallace,

    Thanks for your suggestions... It's really helpful.

    The part you mentioned is adjustable from vref(1.24V) to 6V, but I need to adjust the output from 0V.

    My application model is working good using CSD18510KCS. My output is not come to stable in quick while add/remove load, it takes some long time so I suspect the charging and discharging current to the gate may insufficient. This is the reason I asked about gate driver.

    What is the charging and discharging current of the Gate? Is there any circuit combination to quick charging and discharging of the gate?

    Or it is due to any other reason?

    Please help me to understand.

    Thanks & regards

  • 0 in reply to Venkatesh P17
    TI__Mastermind 39530 points
    Hi Venkatesh,
    I would need to know more about your circuit implementation to provide better guidance. Are you using an op-amp to drive the gate of the FET? Have you confirmed that the circuit is stable thru modeling or by measuring the loop response (i.e. bode plots)? This FET has relatively high gate charge and input capacitance. The drive circuit will not to be able to source and sink 10's to 100's of mA depending on how fast you need to slew the gate of the FET. For example, if you want to change the gate voltage by 1V in 1us:

    i = C x dV/dt, using Ciss = 8770pF from the datasheet, then the current needs to be i = 8770pF x 1V/1us = 8.8mA. If you want to slew the gate voltage 10x faster, then the current would be 88mA.

    If the drive circuit does not have adequate current capability to slew the gate of the FET, then you can look at adding a totem pole circuit using complimentary bipolar transistors or FETs. Please let me know if this helps or if you have additional questions.
  • 0 in reply to John Wallace1
    TI__Mastermind 39530 points
    Correction to a typo: The drive circuit will need to be able to source and sink 10's to 100's of mA depending on how fast you need to slew the gate of the FET.
  • 0 in reply to John Wallace1
    Intellectual 895 points
    Hi Mr John Wallace,

    Thanks for your suggestions, It seems very good point, I will my design by this aspect and let you know. Currently I am using LTC2057 as my error amplifier. Please suggest suitable TI part alternate to and compatible to CSD18510KCS. I will also search.


    Thanks and Regards,
  • 0 in reply to Venkatesh P17
    TI__Mastermind 39530 points
    Hi Venkatesh,
    The LTC2057 is a high precision, high voltage op amp. As you may know, TI has a lot of op amps and without knowing the detailed requirements, it is difficult to narrow down the choices. I did identify the OPA277 as a potential candidate. Your TI FAE should be able to help you find the right TI op amp for your application.
  • 0 in reply to John Wallace1
    Intellectual 895 points

    Hi John Wallace,

    The exact requirement of design is as follows. We need to produce 0V to 5V and 0A - 5A. (nominal is 3.3V, 5A)

    We are approaching the linear voltage regulator method as follows.

    We sure we can adjust the output voltage by adjusting the Vref of Error amplifier. It works properly with expected resolution.

    But for sudden transient load the output voltage is not being stable. currently we are using CSD18510KCS as pass transistor and LT2057 as Error amplifier. 

    By your previous answer and some theoretical study, we looking for a high output op-amp as Error amplifier preferred from TI product. By your suggestion we would use OPA277. We also think to use either LM224 or LM7171 from TI

    We also looking to use any compensation circuit at Voltage divider (Vfb) side and at gate of the MOSFET by referring the video lecture of TI (training.ti.com/linear-regulator-fundamentals-nmos)

    1. Please suggest a suitable op-amp for this MOSFET application.

    2. or please suggest other suitable MOSFET and OP-amp meets my requirement.

    3. Suggest the compensation method for better bandwidth

    in addition

    i)  Please mention any low gate capacitance MOSFETs in TI

    ii) Please mention any high output current op-amps in TI

    iii) Which amplifier is very much suitable for linear voltage regulation? whether voltage feedback amp or current feedback amp?

  • 0 in reply to Venkatesh P17
    TI__Mastermind 39530 points
    Hi Venkatesh,
    I have asked our amplifiers team to look into your questions as they are more knowledgeable on the op-amp products.
  • 0 in reply to John Wallace1
    TI__Guru**** 167546 points
    Venkatesh,

    Can you post the Ti-tina file so we can simulate the circuit?
    It will likely need some compensation for stability and your 12V node is programmed to 5V in the image.
    What is the minimum load current? I ask because the regulator correction might overshoot a little (and it needs to drain back to desired voltage)
  • 0 in reply to Ron Michallick
    Intellectual 895 points

    Hi Mr.Ron Michallick,

    Thanks for your reply.

    I was trying to import csd18510kcs file in TINA but it shows some error like "Invalid Device". I think it is encrypted file. Can you please guide me to create macro model for this part? I plan to use LM224  as error amplifier. 

    For LM224 pspice import, it shows multiple subcircuit error. Please guide me regarding this also.

    And help me to understand suitable compensation circuits for my design.

  • 0 in reply to Venkatesh P17
    TI__Mastermind 39530 points
    Hi Venkatesh,
    The CSD18501 model is encrypted. I'll contact you directly via email to provide an unencrypted model. Ron should be able to assist you with the LM224 model.