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Hi Team,
We are having an issue adjusting the LED current when trying to vary below 200mA.
We use a TPS92512 LED Driver(I will email schematic offline). In this case (R308 =0.1 Ohm), at the lowest output of DAC (U301), the voltage of the pin 7 of the U302 is measured 0.020V, i.e. the current of LED is 200mA.
As we found the LED is too bright, we want to adjust the current much lower.
When I changed the R308 to 1 Ohm, the Voltage at pin 7 of U302 is always 0.179, i.e. the current is 179 mA, whatever the VIADJ is. Not adjustable at all.
When the R308 is 0.5 Ohm, the measurement values are as following:
VIADJ (V) Vpin7 (V)
0.836 0.138
0.669 0.110
0.500 0.099
0.335 0.099
0.167 0.099
Below is in reference to the waveforms attached.
VLED - Voltage at L300 LED side
PH – Voltage at L300 PH side
RSense = 0.5 Ohm
VLED1, PH1 - VIADJ = 4 mV, VIsense = 104.7mV
VLED2, PH3 - VIADJ = 501.1 mV, VIsense = 106.3mV
VLED3, PH3 - VIADJ = 837 mV, VIsense = 138.7.7mV
The waveforms look similar, the current cannot go below 200mA.
We would like to have the current between 40mA to 200mA.
The ripple current is pretty high, datasheet recommends >75mA which is too much for 40mA working current.
As there is no room to increase the inductor size, I think increasing the switching frequency (change the R306 to 100K) to 2MHz should be able to improve the ripple. Do you think it works? Any suggestions?
Attached waveforms below:
