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I have a display with the backlight LEDs arranged 6S4P (so 4 parallel strings of 6 LEDs joined internally, so must be driven as single string) and have been trying to find a suitable driver for it - I thought the TPS61169 looked promising but now I've run the numbers I think I'm busting the 1.2A switch current limit.

basic schematic

Using TI's reference circuit & calculations from the datasheet (plus measured voltage on an example screen):

  • Vin = 5v
  • Total LED voltage (converter Vout) = ~26V
  • Total LED current (Iout) = 240mA
  • Assuming ~80% efficiency
  • Inductor = 10uH
  • Switch frequency = 1.2MHz (fixed)
  • VFB = 204mV
  • Diode Vf = 0.5v

I calculate:

  • Sense resistor (RLED) = 0R85
  • Input current (ILDC) = 1560mA
  • Inductor ripple current (ILP-P) = 337mA
  • Peak inductor current (ILP) = 1728mA

So assuming ILP will flow through the switch that's going to let the magic smoke out of the IC.

So, are my calculations correct? If so, without wanting to violate the "no shopping questions" rule, pointers to a more suitable device (ideally in the same footprint) would be most appreciated.

John U
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  • Schematic, schematic, schematic. – Andy aka Nov 16 '20 at 17:39
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    Also, parallel strings are going to be a no-no with this design without some clever current sharing circuit that seems to go unmentioned in your words. – Andy aka Nov 16 '20 at 17:47
  • @Andyaka - schematic is the standard one in the datasheet (basic version is on page 1), and unfortunately cheap Chinese display = commoned strings, I don't like it either but I have to work with it. – John U Nov 17 '20 at 08:48

1 Answers1

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Expanding on AndyAka's comment:
The standard schematic you are referring to can only drive correctly one series of LEDs without other means of current sharing (it also says in the top right corner of your image 10s1p).

You either need one TP61169 per parallel string, or even better a four channel backlight driver (for instance, TI's LP8862 has two channels).
Since you are going for SOT23 package, i suppose you are trying to save space, therefore i would suggest the single driver solution, this way you'll need to place components only for a single boost stage instead of four, albeit said components will be slightly bigger.

This quick search shows that multi channel LED driver are inherently bigger, but perhaps you can find one that suits your needs, also depending on what SMD mounting process you have available.

Jack
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  • I've just edited my question to clarify that all the strings are internally joined in the display, so I cannot drive them separately - the display needs ~26v@240mA into the LED pins. Yes it's horrible, no I can't change it. – John U Nov 17 '20 at 09:45
  • You mean both the anodes and the cathodes are joined? – Jack Nov 17 '20 at 11:58
  • Yep, one wire in for LED+, one wire in for LED-, because China. – John U Nov 17 '20 at 12:16