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TPS61150 Datasheet Clarification

Gary Sinde
Prodigy 100 points
Other Parts Discussed in Thread: TPS61150, TPS61150A

I have been using the TPS61150 for an LCD with 2 Strands of 4 White LED's each. The voltage ends up around 14V with the current in each strand set by Vset/Rset * 920 or 1.229 / 54.9K * 920 = 20 mA each. I use the On/Off pins in I/O mode and not PWM mode. This has been working fine but I am confused by the following two statements in the datasheet which appear to state that I am exceeding the parts design limits:

Page 5

I(IFB_MAX) - Current sink max output current (IFB current = 330mV) = Minimum of 34 mA 

and

Page 12

The maximum output current can also be limited by the current capability of the current sink circuitry. It is designed to provide maximum 35mA current regardless of the current capability of the boost converter.

and

Page 8

Graph Shows Io curves efficiency out above 50 mA

In my current design which has been working fine for many months, I am not sure if I am exceeding the Iout capacity or not (but could not find a limit on Iout in the Electrical Specifications Table.)

Thanks

  • 0
    TI__Guru*** 147165 points

    The electrical specifications table on page 5 gives the minimum tested value for the current.  The graph is for total LED current, which is the sum of the current in both strings, with each set for 25mA. 

  • 0 in reply to Jeff F
    Prodigy 100 points

    So, I have been looking at both the TPS61150 and TPS61150A Datasheets and I am still unclear as to what is the maximum current for each strand and total maximum current.

    I have 2 Strands with each using 20mA. Although this has been working, Am I exceeding the Iout specification or is there an internal current limiting taking place or is this fine. I will try and get a "real" measurement but getting in series with my LCD backlight strands will take a little bit of cuttting. Thanks

  • 0 in reply to Gary Sinde
    TI__Guru*** 147165 points

    If each string is 20mA, then you are under the 34 or 35 mA maximum per string and therefore should have no issues.  The total current is 68ma or 70mA.  The datasheet graphs shows data for a typical configuation of 25mA per string. I am inquiring with our product test folks as to why one part has 34mA and one part as 35mA but that should not affect you. 

  • 0 in reply to Jeff F
    Prodigy 100 points

    OK, Thanks. I am using it on 3 potential LCD's for a product and they all have different current requirements but all under 40mA total. Good News.