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Part Number: LMR14020-Q1
We are currently developping a LED driver for our customer ; final application is an automotive dashboard, planned for a mass production (about 500k / year).
We built an evaluation board made of :
- 5 RGB LEDs
- one LED driver : TLC5941-Q1
- one buck controller : LMR14020-Q1
- all automotive components
The board is supplied by :
- 5V power supply for LED driver (Vcc)
- 12V power supply for VIN buck controller (12V automotive battery)
The principle is that buck controller generates 5V at its output to supply the LEDs ("V_LED", on anode side), while the LED driver outputs connected at the LED cathodes allow to control LED current (dot correction + grey scale).
When "V_LED" is directly supplied by a 5V power supply, the LED driver controls precisely all the LED currents in all cases (whatever VBAT, dot correction or grey scale values are).
When "V_LED" is supplied by the buck controller output (set at 5V), in some cases LED currents are not correct because the buck output voltage falls below 3 or 4V ; this depends on VBAT/dot correction/grey scale values : sometimes its correct, sometimes its not. Currents in LEDs have a square wave shape, I suppose that buck controller has difficulty to control voltage during too high load transients.
Looking at other LED drivers datasheet, I read that "high-side sensing hysteretic buck current source" are preferred for LED supply because of their higher current control bandwith (example : TPS92661-Q1, §8.1 page 38), while LMR14020-Q1 is a "step-down converter".
Does that explain why voltage regulation is not correct in certain cases ?
Shall I better use for example the LM3489-Q1, which is an hysteretic buck controller ?
Or should I better :
- avoid using a buck controller which is not directly included in the LED driver as it is the case for TLC5941 ?
- ... and connect directly "V_LED" to VBAT or use a linear power supply ?
Final system requirements are :
- 15 led outputs used on TLC5941 driver (LED driver reference will not change)
- 20mA nominal current for each LED ; up to 40mA max
- system supplied from 12V battery
We are glad that we were able to resolve this issue, and will now proceed to close this thread.
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