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LM25119: Pin ratings and Functions; Suitability for Cuk Topology

Tim Williams1
Prodigy 210 points
Part Number: LM25119

I have three questions:

1. What are the absolute maximum ratings of these pins:

COMP1, COMP2

2. What are the functions of these pins:

SW1, SW2: the block diagram shows these only serving as ground reference for the high side driver. Is there no other functionality associated with these?

RAMP1, RAMP2, RES: what are the V/I characteristics of these pins in the on and off states?  How fast do the switches operate?

(I see the voltage rating has been answered already, ref: https://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/t/262250 )

COMP1, COMP2: V_OH and V_OL are provided, but one must still guess at the output type.  The ratings and compensation connection suggest an op-amp voltage output, which therefore cannot be connected to other loads.  But this is contradicted by the recommended two-phase connection, where COMP pins are strapped together.  Is it actually a transconductance amplifier?  If so, why not use an R+C to GND for compensation?  If an external error amplifier is used, what voltage range can the COMP pins be driven to?

3. I'm looking at an application for an inverting type output.  I would like to use a Cuk converter, for reduced ripple, and LM25119 is presently selected for other channels in the same circuit, so it would be convenient to reuse.  (Also, that there are very few offerings from any major manufacturers, including TI, for multi-phase, synchronous Cuk converter applications, but if you can recommend something more suitable, I would be interested.)  From what I can see in the datasheet, this should be acceptable, but in lieu of answers to the above, I cannot be certain that the "diode emulation" function will work correctly, or that an external error amplifier can be used.

Thank you for your time,

Tim

  • 0
    TI__Mastermind 24075 points
    Hi Tim,

    1. The ABS rating of COMP1 and COMP2 should be -0.3V to 15V.
    COMP1 and COMP2 are op-amp voltage output. In two-phase mode, the COMP2 is disabled and has a high output impedance. This informaiton is availble on datasheet page 14.

    2. SW1 and SW2 only serves as the grouding reference of high side driver. No other functionality.

    3. The "diode emulation" function can be disabled by pulling DEMB pin high if you have concern.

    Regarding RAMP1, RAMP2 and RES pins, I don't know what does "V/I characteristics" of the pins mean. Can you explain the question with more detials?


    Thanks
    Qian
  • 0 in reply to Qian Chen
    Prodigy 210 points
    Is the diode emulation driven by the current sense inputs?

    For RAMP and RES, an NMOS is shown on the diagram, what is the on resistance? Is that all it is, i.e., a CCS, NMOS switch, and whatever ESD protection corresponds to the absolute maximum rating?

    Tim
  • 0 in reply to Tim Williams1
    TI__Mastermind 24075 points
    Yes, the diode emulaton is driven by the current sense input.

    RAMP pin is an input pin, it is in high impedance.
    RES pin is an output pin, it is a current source during hiccup time.
  • 0 in reply to Qian Chen
    Prodigy 210 points
    Please confirm, RAMP does not ever sink current (have a switch to ground)?

    What voltage does the RES current source pull up to?

    Could you elucidate on the COMP disable function -- what is maximum +/- COMP output current when FB is tied to VCC? Does this apply only to COMP2, or can COMP1 be disabled in the same way as well?

    Tim
  • 0 in reply to Tim Williams1
    TI__Mastermind 24075 points

    Hello,

    We need to know which customer we are supporting.
    Can you use your business email to send an email ?

    Thanks
    Qian

  • 0 in reply to Tim Williams1
    TI__Mastermind 24075 points
    Got your email. Thanks.

    There is an internal switch to discharge RAMP voltage. Please refer to the funciton block on datasheet page 12.

    When FB is tied to VCC, COMP is disabled and it does not have source and sink capability.
    COMP1 can be disabled in the same way.

    I don't know the maximum source and sink capbalitty of COMP pin when the COMP pin is active, probably around 10mA.
  • 0 in reply to Qian Chen
    Prodigy 210 points

    Thanks.  Can you tell what is the resistance of the internal switch, for RAMP and RES pins?

    Approximately what voltage supplies the RES pin current source, or in other words, what voltage does it pull up to in steady state?

    Regarding my Cuk converter application, it sounds like it will be suitable.

  • 0 in reply to Tim Williams1
    Prodigy 210 points
    Can you provide an answer?

    Thanks,
    Tim
  • 0 in reply to Tim Williams1
    TI__Mastermind 24075 points
    Sorry, I don't have the resistance value of internal switch for RAMP and RES poin.

    During hiccup, the RES cap will be charged to 1.25V and then be discharged by internal switch.

    Thanks