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I have a display assembly with 8 WLEDs in 4s2p configuration for backlight. However, the split between 2 strings is made within the display assembly so I have only a single anode/cathode pair in the display connector. Each WLED has a Vf of 3.2V and If of 20mA, so I need to source 12.8V @ 40mA to drive the LEDs. In my current design I have been using a TPS61165, which has been working great.

Now for the next iteration of the device in question, I want to make use of the Qualcomm PMI8994 PMIC which comes with the SoC module we have selected. It has an integrated WLED SMPS that is capable to supply the required voltage, but has 3 WLED sink drivers, which, judging by the datasheet, can sink max 30mA each.

Assuming I synchronously control each sink driver to set brightness, can I safely split the backlight cathode connection on my PCB and connect them to 2 of the LED sink drivers to get under the 30mA limit?

schematic

Note that the part within the dashed region is something I can't change.

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  • \$\begingroup\$ Show schematic. \$\endgroup\$
    – winny
    Aug 16 '18 at 11:12
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    \$\begingroup\$ It can be done, I have seen it done successfully, but there may be some reason not to do it. I would expect a comment in the datasheet if it was a known bad idea. \$\endgroup\$
    – Puffafish
    Aug 16 '18 at 11:18
  • \$\begingroup\$ added schematic. of course this isn't ideal, but it's just for a prototype anyway. the firmware doesn't forbid you to set the duty cycle of sink1 to 100% and the other to 0%, in which case the result would be somehwat unexpected. \$\endgroup\$
    – Genoil
    Aug 16 '18 at 11:38
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    \$\begingroup\$ If you can control the current sinks independently then yes, you can. Don't worry, most of the LCD panel backlights have the same configuration that you've shown. And also the current sink modules inside the chip should be most likely identical if they can be controlled independently. \$\endgroup\$ Aug 16 '18 at 12:41
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Combining controlled or Constant Current (CC) sinks (or sources) in parallel may be added to a single load (s) as long as the resulting voltage is within the specified range, which is true in this case.

Other info

The same is true, for PTC polyfuses which are resettable current limiters.

However, not part of this question, constant voltage sources ( power supplies ) with current sharing links to each other for redundancy can have stability issues under 10% of the rated load.

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