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LM3404EVAL: 25000 to 1 contrast ratio for dimming LEDs

Gregory Dyczko
Prodigy 30 points
Part Number: LM3404EVAL
Other Parts Discussed in Thread: LM3404, LM3404HV

TI customer service has referred me to this site after responding to several case studies.  Basically we ne to control LED lighting with contrast ratio of around 25000 to 1.  We are using the LM3402-04 application board.  In the AN-1839 paper it says we can achieve higher than a 50000 to 1 contrast ratio.  I have several questions that are in the following paragraph that I typed for the case study.  All help you can give us is greatly appreciated.

Original details: Please see CS 0082366. My colleagues and I still need some spoon feeding. The LM3402-04App board does not match the schematic in your users guide SNVA342E AN-1839 … . I think we have that figured. TP2 VCC on the board is VDIM in the paper and TP4 Dim goes to pin 3 of the LM3404. We used TP4 Dim to control the LED intensity but only got about a 10000 to 1 contrast ratio. Reviewing your paper, I believe we should be feeding our PWM signal into J15 (Conn-1). Is that true and what do we do with TP4 Dim? What is TP2 Vcc (Vdim) for? We are shooting for a minimum of 25000 to 1 contrast ratio. What is J1 (JMP-1) used for. Are there any other papers that can help us with this eval board. The last part of your eval paper in the example 4 portion discusses both the Dim pin operation and the External Mosfet dimming but we could use more information on an circuit diagram for the high ratio dimming. Thank you for your time.

  • 0
    TI__Mastermind 28655 points

    Hello Gregory,

    It looks like J1 connects the VCC of the LM3404 to the push-pull to drive the SHUNT MOSFET.  The VCC for this can be applied separately as well via TP2 (leaving J1 open).  The command for shunt dimming goes to J15 which is CONN-1 on the schematic.  TP4 is not SHUNT dimming so it will not be as fast, leave TP4 open as there is an internal current source that pulls up on that pin.  Also, when SHUNT FET dimming the output capacitance should be very small or not exist.  Sorry the documentation is not real clear on this.

    Summary for SHUNT FET dimming:

    Leave TP4 open

    Connect PWM to J15/CONN-1

    Either install J1 to provide VCC to the push-pull or leave J1 open and apply a VCC to TP2.

    Also make sure C4 is removed or a very small value, it becomes switching loss when SHUNT FET dimming.

    I'm not sure what "CS 0082366" is.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 30 points

    Thank you for the information; very helpful.  Do you have any suggested circuit's for 25000 to 1 dimming to drive J1?  When/why would you use TP 2 VCC, with J1 open, instead of the LM3404 internal source with J1 closed?  Is the limit of 5 LEDs with the eval board because the external MOSFET is 20 volts?  If yes, can you suggest a MODFET for 18 LEDs in a series string.  to answer your question, CS 0082366 is a case study with TI that has my original questions.   Thank you for your time. 

  • 0 in reply to Gregory Dyczko
    TI__Mastermind 28655 points

    Hello Gregory,

    I believe J1 it is setup to run from a function generator or other PWM source.

    If the push pull runs from the VCC of the IC it will dissipate more power, especially at high input voltages.  It also allows the use of different voltages to drive the Shunt FET when not using the LM3404 VCC.

    The limit of this is how fast you want to dim and the SHUNT FET gate drive to make it switch fast enough (that is the push-pull stage).  The 20V MOSFET has a very low Qgate making it capable of turning on and off quickly with limited drive.  If a higher voltage MOSFET is used Qgate will (most likely) go up with Vds rating if RDSon stays fairly constant.  It will also be more difficult to switch that large of a string that fast.  Also, the MOSFET drain has to slew 18 X 3V = 54V (typical LED Vf) during SHUNT FET PWM dimming instead of approximately.  This also means you would need to use the LM3404HV part since the LM3404 is rated to 42V maximum.  I would look for a MOSFET with similar RDSon (if your current is similar) and low gate charge plus it should have a VDS rating greater than the maximum LED string voltage.  Also be aware of the Vgs threshold with this gate drive, it cannot be too high.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points

    Hello Gregory,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue.
    If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

    Best Regards,