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LMZ31710: INH connected to VIN internally?

Franz Schellhase
Expert 3605 points
Part Number: LMZ31710
Other Parts Discussed in Thread: LMZ30606

Hi team,

One of my customers wants to use LMZ31710 in the next project. They are a bit worried about what the inner circuit looks like. Because they want to control the INH-pin with logic.

With the LMZ30606 for example the INH-pin comes dangerously close to the VIN, which they use already. INH is connected to VIN via an internal resistor in that IC. Is this the case for LMZ31710 as well?

I know that in both datasheets there is the open drain suggestion at the INH-pin.

Thank you,

Franz

  • 0
    TI__Mastermind 35165 points

    Hi Franz,

    The LMZ31710 does not have an internal connection between the VIN and INH pins. The buck converter used in the LMZ31710 has a current source which pulls up the INH pin voltage if the INH pin is floating. The current source is specified by the 1.15 µA and 3.3 µA specifications in the electrical characteristics. When floating the INH pin to enable the module, it is important to minimize leakage on the pin to ensure the 1.15 µA current source is strong enough to pull the INH pin above the 1.1V turn on threshold.

    Anthony

  • 0 in reply to Anthony F
    TI__Expert 3605 points

    Hi Anthony,

    Thank you for the elaborate reply!

    We will actually use the UVLO feature with the LMZ31710. Since the controller should switch off at about 10V, we have simply taken the values from table 8 of the data sheet. In addition we added a FET, because the whole board (Ultrascale+ board) should be switchable from outside by ENABLE signal.

    Where was my thinking error, when I noticed that the 10V combination 68K/9K3 somehow does not fit at all to the UVLO threshold of 3.85V?
    Now that you mention the INH threshold of 1.1V, the voltage divider becomes a bit more plausible (for the INH function). But I still don't understand how the UVLO at 3.85V is established. How can I understand this voltage divider for 10V?

    Thank you,

    Franz

  • 0 in reply to Franz Schellhase
    TI__Mastermind 35165 points

    The UVLO of 3.85V is an internal fixed threshold at the VIN pin. The resistor divider to the INH pin has no influence on it. The resistor divider to INH is not actually moving the VIN pin threshold, it is just gives the capability of creating another threshold higher than the internal VIN pin threshold. The voltage at both the VIN pin and the INH pin need to be above their respective thresholds for the output of the LMZ31710 to be enabled.

    Hope that helps clarify. Just let me know if anything is still unclear.

  • 0 in reply to Anthony F
    TI__Expert 3605 points

    Hi Anthony,

    I'm getting closer to understanding. I had assumed a shift by the resistor circuit, but you have described that this is not the case.

    I still haven't figured out the (UVLO) chain of effects at 10V. For me it looks more like a Vin-dependent disable below 10V.

    I don't understand the difference between disable and UVLO on the same pin. If UVLO is tightly coupled to Vin, then the UVLO designation on the INH pin has no place. It is simply a voltage dependent INH. Probably TI developed the combination INH/UVLO from this., right?

    Best regards,

    Franz

  • 0 in reply to Franz Schellhase
    TI__Mastermind 35165 points

    Hi Franz,

    In this context, enable/disable and UVLO have very similar meanings. I would say the distinction is we call it a UVLO when it is dependent on the input voltage or some power rail being in regulation.

    There is a fixed VIN pin UVLO threshold at 3.85V and a fixed enable/disable threshold at the INH pin. The feature provided by the INH pin is it can either be driven by some logic signal or you can use a resistor divider from VIN to set an enable/disable threshold dependent on the VIN voltage. When the resistor divider makes the enable/disable at the INH pin become dependent on VIN it also becomes a UVLO.

    If either VIN is below its UVLO and INH is below its threshold it has the same effect. The output of the LMZ31710 is disabled. Both pins need to be above there respective thresholds for the output of the LMZ31710 to be enabled.

    Again, just let me know if there is still any doubts.

    Anthony

  • 0 in reply to Franz Schellhase
    TI__Guru 56925 points

     

    Under Voltage Lockout (UVLO) is, by definition, a voltage dependent inhibit function that prevents operation and holds the device is a known state when the input voltage is low. 

    TI uses separate pin labeling designations based on the intended use / design of the pin for digital or analog interface.

    Pins that are intended to be interfaced with digital logic are typically labeled Enable, Disable, or Inhibit.

    Pins that are intended to be interfaced with a resistor divider provide an analog programmable low-voltage disable (lock-out) feature are labeled UVLO.

    Using a pin as a UVLO function requires the pin to have several features that are not critical for digitally interfaced pins.  Low/controlled leakage, a precision threshold, and relatively low hysteresis.  When device pins are designed to meet these requirements, we label the pin UVLO and provide customers the guidance they need to select an external resistor divider to realize an programmable Under voltage Lock-out (or voltage dependent inhibit) for their circuit.

    Some devices, such as the LMZ31710, are design to be able to be interfaced with either an analog resistor divider (ULVO) and digital logic (EN, DIS, INH) and the pins can be labeled with both functions, so the INH/UVLO pin can be driven with an external open-drain logic circuit  and/or a resistor divider from VIN, but both the INH/UVLO and VIN pins must be above their respective thresholds before the LMZ31710 will begin switching and regulate the output.