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LM27222: Switching into a 2A Load

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Replies: 16

Views: 4339

Part Number: LM27222

Hi,

We am trying to use LM27222 as a standard push pull driver, driving two N-Ch FETs into a load of 2A. The input to PWM pin is either DC or pulsed - when pulsed, its a frequency of ~ 100Hz with high period of ~100usec. The output switching needs to be fast, changing within 100nsec of the PWM input rise/fall edges. The VCC as well as PWM input levels are 5V. The high side mosfet Vin is also 5V.

My queries are:

1) Can the device handle this kind of a switching, as the datasheet mainly intends to show this only in a buck regulator  configuration.

2) We will chose a low Rdson mosfet (~40mOhm) to minimize drop over the high mosfet and have almost 5V appear at the output. Is that ok?

3) There shall be no L/C configuration at the switching node SW. The output shall directly be fed to the load. Is that ok?

Regards

Gaurav

  • In reply to Richard Herring:

    Hi Richard,
    Thanks for reply.
    1) As of now there is no gate resistor, the HG/LG pins directly drive the fet.
    2) Yes, PCB layout may be a reason as we have wired up the circuit on a prototyping board for a quick test.
    3) I didnt get your last point regarding HB capacitance. We have used the circuit as shown in your earlier block diagram, there is no separate capacitance used except for the load capacitors that are in range of 0.1uF-1uF.
    Regards
    Gaurav
  • In reply to Gaurav Agrawal1:

    Hello Gaurav,
    I should have been more clear on the HB capacitance. The HB capacitor should be placed close to the HB and HS pins to minimize the layout trace inductance, as this is part of the total inductance seen in the gate drive current loop from the driver to the FET gate and FET source back to the driver.
    Also a consideration for the HB capacitor is making sure the capacitor value is at least 10x the gate to source capacitance. Another consideration is to make sure there is a low ESR ceramic capacitance on the HB to HS to minimize the high frequency impedance, X7R is a good choice. All of the high frequency impedances from the layout, to the quality of the HB capacitor can affect the gate drive rise and fall time performance.
    If the driver is wired into the circuit, which I understand for experimentation, the prototype wiring will add significant inductance and limit the driver performance.

    Regards,
    Richard Herring

    Richard Herring

  • In reply to Richard Herring:

    Hi Richard,

    OK I assume you mean the CB (HB) and SW (HS) pins?

    We have a 1uF/X7R cap here. As you wrote in your last post, we will try adding a high frequency ceramic capacitor (22-100nF) in parallel with the HB capacitance to reduce the high frequency impedance.

    Regards
    Gaurav
  • In reply to Richard Herring:

    Hello Richard,

    The solution worked fine for us, and the noise issue was resolved with the change in the HB cap.

    We are now implementing this in our PCB where multiple such circuits will be needed to drive an array of switching loads. My query is if a single SN6505B isolated source acting as the bootstrap can be used across multiple LM27222 circuits, all of which have the same high side FETs.

    Regards

    Gaurav

  • In reply to Gaurav Agrawal1:

    Hello Gaurav,

    It is good to hear the LM27222 with the floating bias is working in your application.

    Regarding the multiple LM27222 circuits and the shared bias. The floating high side bias needs to be referenced to the CB and SW pins of each respective driver, so if there are multiple MOSFET's and drivers with the same connection to the high side MOSFET source the bias can be shared. But in that case the same driver may possibly be used to drive MOSFET's in parallel if the driving capability is adequate.

    If there are multiple LM27222 drivers and MOSFETs connected to difference SW nodes in the power circuit, a common floating bias cannot be shared since it needs to be referenced to the SW node.

    Regards,

    Richard Herring

    Richard Herring

  • In reply to Richard Herring:

    Thanks Richard! We can close this thread now.

    Regards

    Gaurav

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