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CSD18510KCS: Need operational amplifier part number apt for this MOSFET

Part Number: CSD18510KCS
Other Parts Discussed in Thread: TLV9002, TLV9001, TLV9061, TLV9062, , OPA2990, TLV9302, OPA552

Hi TI team,

I am using this MOSFET as pass transistor for simple linear voltage regulator. Can you please suggest me an op-amp in TI which is good apt with this MOSFET and work as an error amplifier?

Please suggest the error amplifier op-amp with concern of good stability. 

Thanks and Regards,

Venkatesh P

  • Hi Venkatesh,

    Take a look at using the TLV9002 (or TLV9001 if you would like use a single). This is a good performance, low cost 1 MHz part which will be fairly easy to stabilize due to its resistive output impedance.

    If you need more bandwidth, then the TLV9062 (or TLV9061) are good options at 10 MHz GBW.

    I recommend that you download the spice models for the op-amp you choose and the MOSFET and simulate your circuit in TINA. The op-amp model for these devices are current and accurate and will give you a way to get a good understanding of your circuit's stability.

  • Hi Paul,

    Thanks for the reply.

    Yes, I will check these op-amps in simulation. I hope it would resolve my issue.

    My search is to find an apt op-amp for the device CSD18510KCS. Because I am trying to avoid compensation circuit when I for a linear voltage regulator using this device.

    Thanks and regards,

    Venkatesh P

  • Hi Paul,

    For these IC's the absolute maximum supply voltage is 6V only.
    But the device CSD18510KCS is NMOSFET so if we employ any error amplifier, it should be capable to drive the device by giving more voltage to gate than its Vds, ritht?
    here my Voltage at drain will be more than 6V so I think this op-amp will not help in this case.
    Please suggest any other op-amps.
    Please correct me if I misunderstood.

    Thanks and Regards,
  • Hi Venkatesh,

    My apologies, you are correct. For a higher voltage application, I would recommend the OPA2990 or the TLV9302. Both are op-amps which can support up to 40V supply. For a higher bandwidth part, look at the OPA552.

  • Hi Paul,

    Thanks for the identification of op-amps. I will check the features and TINA simulation and will finalize.

    My primary objective is to choose the correct op-amp apt to the device CSD18510KCS for good stability. I don't want to use any compensation circuit for my linear voltage application. 

    I referred the application report Technical Review of Low Dropout Voltage Regulator Operation and Performance but in the transient response part, this document explains for PMOS device voltage regulator, but in my case I need to derive it for NMOSFET. 

    Can you help me to draw the small signal diagram for this CSD18510kcs and op-amp? I have few confusion in op-amp small signal model, like whether the output resistor will be in series or parallel.

    Please see the figure 17 and 19 of this document please suggest me to draw and derive

    Thanks and Regards,

  • Hi Venkatesh,
    An LDO has three poles when you are trying to design it discretely. 1st pole comes from Op Amp's pole itself (internal), the second pole comes from the gate capacitance of the power FET and impedance at gate (this is output impedance of an Op Amp), and the third pole comes from the output capacitor (at the regulated node of LDO) and the impedance at that of that node ( due to parallel of Rload and Rout of FET).

    P1= OP Amp Pole.
    P2= 1/(2*pi*Ramp*Cgate)

    The system is unstable with three poles if you do not do something (compensate). You cannot do much about the Op Amp pole, but you can play with the other two poles to build a stable system around the application you need.

    From the NexFET (NMOS) datasheet, the total gate charge at 4.5V is 58nC. That is 12-13 nF of capacitance. There are very few Op Amps that are stable driving that kind of capacitance. You can put a zero to compensate this pole (resistor in series).

    The output pole is dependent on parallel combination of the impedance of 1/Gm of NMOS and Rload with output capacitance. You can have a continuous load at the output to have higher Gm (NMOS has source side to load so inherently it is low impedance) and if the output capacitor low enough capacitance, it will push that pole to high frequency.

    This will make your LDO look like it has two poles and one zero and make it stable.

    Start with 1M Hz GBW Op Amp with the supply voltage that fits the requirement. Like Paul suggested TLV9001, TLV9002 for supply within 6V and OPA2990, TLV9302 for high voltage supply. It will very hard (if not impossible) to make a high bandwidth discrete LDO without some internal compensation which is used in a single IC solution.

    I have a few questions.

    Is the stability of LDO primary concern?
    What is the voltage you are trying to regulate?
    What are the available supply rails for your LDO system? 6V? 20V?
    What is the current load at the output of the LDO?
    What is the nature of Load? What kind of transients? What frequency?
    What is your reference voltage for you LDO?

    Sanjeev Manandhar