Hi there,
I'm trying to power my TFT LCD display, which is rated at 500cd/m2 luminance rating. The only issue being that the screen clearly does not look as bright as it should be, which makes me think something is up with the regulator and the feedback loop.
Screen datasheet: https://www.farnell.com/datasheets/3164295.pdf
Backlight datasheet: https://www.ti.com/lit/ds/symlink/tps61161.pdf?ts=1631153302041&ref_url=https%253A%252F%252Fwww.google.com%252F
Currently, I'm using a 5 ohm feedback resistor to achieve 40mA through the LEDs, as the constant current equation for IOUT is FB voltage/sense resistor = 0.2/5 = 40mA. Currently, across the feedback resistor, I measure 17mV with my multimeter. The LED configuration for my screen seems to be 6p2s. Currently I’m measuring 17mV across the rsense which seems far too low. The device is powered from 5V on VIN and CTRL is @ logic 3v3.
The control pin (CTRL) on the TPS61160A for analog dimming is pulled high to 3v3 (so it should be @ 100% duty cycle) via a GPIO pull-up on our Raspberry Pi. I did read here (https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/89365/tps61160-tps61161-tps61165-pwm-dimming-frequency-range) that the PWM pin should only have a frequency of 5kHz to 100kHz, otherwise the backlight may not start up properly. However, the datasheet mentions specifically that: "When the CTRL pin is constantly high, the FB voltage is regulated to 200 mV typically. However, the CTRL pin allows a PWM signal to reduce this regulation voltage". These two statements seem to be conflicting one another. I would assume the frequency of a constantly high signal would have been 0. The TPS61160A cannot be controlled via one-wire interface unlike the non-A variant as you would be aware, so it cannot be entering EasyScale mode.
I've attached pictures of the schematics, as well as a comparison picture to another screen I've used. layout below. I know that the switching node path should be kept as short as possible so I've tried my best to follow the layout guidelines, but maybe someone might notice some issue. Perhaps the screen just doesn't get that bright? The main parts of the circuit include the following:
22uH inductor: VLCF4020T-220MR56
Diode: MBR0540T1G
Schematic: https://imgur.com/vBM9lJp
Layout: https://imgur.com/jy9TwvC
Screen problems: https://imgur.com/VcTQCgT
Thanks,
Ben
