I'm experiencing a problem using the TPS61181 WLED driver, driving an LCD LED backlight (specced as 6 strings of 18V 18mA).
When the driver is enabled it illuminates the backlight very briefly, then cuts out. EN needs to be toggled before it comes on again, at which point it repeats the same effect.
Here's an oscilloscope capture. CH1 is the "VO" node driving the backlight common anode and CH2 is an "IFBx" node connected to a cathode. Other IFB nodes looked similar.
The output voltage appears to ramp smoothly and the backlight forward voltage is around 18V as expected, but the voltage seems to keep rising until the overvoltage cutout kicks in.
Here's the relevant schematic. Very similar to the sample in the datasheet, apart from omitting the optional "fault" FET:
VBAT is 15.5V. EN & DCTRL are both driven low when VBAT is first powered, and several seconds later are driven to 3.3V simultaneously.
For the momentary period that the screen is on, before overvoltage protection kicks in, the backlight is even and constant and appears to have the correct brightness.
Applying PWM input to DCTRL results in the backlight dimming as expected, and it stays on for proportionally longer before cutting out. Oscilloscope traces show that the ramp of VO is less rapid.
Grounds are connected to a single internal ground plane (this is a four layer board). I can supply a picture of the layout if required.
Do you have any suggestions for further diagnosing or solving this problem?
Thank you,
Angus