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LMC7221: Device soft start with delay between applying V+ and V-

franziskus
Expert 7568 points
Part Number: LMC7221
Other Parts Discussed in Thread: TL331, TLV1701

Team,

my customer wants to implement some kind of soft start where there is a delay between applying V+ voltage and V- voltage.
The delay between applying V+ and V- is 150msec.

  • Do you see any issue with this delay?

They want to supply the device with +15V and -15V. As I see it, the device can handly a max range of 16V (V+ minus V-)

  • Is my understanding of the datasheet correct?

Thanks!

  • 0
    TI__Mastermind 24560 points

    Franziskus

    Having a delay between V+ and V- being applied is not a problem as long as none of the abs max conditions are violated.  One area of concern would be the voltages applied to the inputs.  The inputs always must not exceed the supply pins by more than 0.3V.

    Your second question relates to the abs max supply voltage which has a maximum of 16V.  That means that the difference between V+ and V- can never be greater than 16V.  If the customer wants to apply +15V to V+ and -15V to the V- pin, that will be a supply voltage of 30V and violates the abs max rating.  If those are the required supply voltages of the system, I suggest using devices such as the TL331 or TLV1701 which are also open-collector devices that can accept supply voltages as high as 36V.

    Chuck

  • 0 in reply to Chuck Sins
    TI__Expert 7568 points

    Thank you Chuck!

  • 0 in reply to Chuck Sins
    TI__Expert 7568 points

    Chuck,

    a quick follow-up question regarding the delay between applying V+ and V-:

    Is this statement in general valid?
    I have a couple other comparators and op amps that are part of the same discussion.

    Thanks!

  • 0 in reply to franziskus
    TI__Mastermind 24560 points

    I would make the same comment regardless.  You really just need to look at what is connected to the inputs to determine if the delay is a problem.

    Another good tip when there is a concern about turn on delay is to put a resistor in series with the inputs or use a device that has a fail safe input.  Many of our new comparators like TLV701x/2x/3x/4x have this feature integrated.  A good way to look for this is in the abs max table.  If the input pin voltage rating is related to a voltage value instead of VDD or VCC or V+, than a voltage can be applied to the inputs regardless of the supply. 

    Chuck

  • 0
    Guru 140200 points

    Hi Franziskus,

    this post might interest you:

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 24560 points

    Thanks Kai

    Franziskus

    If you need anything else, please let us know.  Otherwise, we will consider this closed.

    Thanks

    Chuck

  • 0 in reply to Chuck Sins
    TI__Expert 7568 points

    Thanks Chuck and thanks Kai!