LP8866S-Q1: Boost output is not stable

Hi, Vidhya, 

I'd like to know more details

1. If the boost circuit is ok when there is load

2. What is the meaning of no load? (Is LEDs floating or ILED=0? ).

3. Will Vboost keep 0v until next powering up cycle? 

Input voltage or VDD UVLO can cause Boost circuit shut down. 

Pls check register status below:  

a). 0Eh SUPPLY_STATUS 

b). 10h BOOST_STATUS  

c). 12h LED_STATUS 

Regards, 

X.G 

Hi Xiaoguang, 

Please find my response below:

1. If the boost circuit is ok when there is load:

No, even with load response is same as mentioned before

2. What is the meaning of no load? (Is LEDs floating or ILED=0? ).: No load means boost output is not connected to any peripherals

3. Will Vboost keep 0v until next powering up cycle? :

Yes

Input voltage or VDD UVLO can cause Boost circuit shut down. 

Pls check register status below:  

a). 0Eh SUPPLY_STATUS 

b). 10h BOOST_STATUS  

c). 12h LED_STATUS 

The device itself is not getting detected in i2c bus. So we cannot access the above register.

Hi xiaoguang, please help to follow this case.

Hi, Vidhya, 

Is it right that the Boost works if using LP8863? 

Could you help capture the waveform of VIN/SW/SD/GD when Vout 14V drops? thanks. 

Regards, 

X.G  

Hi Xiaoguang, 

Yes boost works if using LP8863.

Below is attached waveforms:CH1 : VDD, CH2 : SD, CH3 : GD, CH4 : OUT

When output drops, VDD is stable (3.3V), SD = High and GD = Low 

Regards,
Vidhya

Thank Vidhya, 

I have some comments as below and hope it helps. 

1. Is there any input filter? I didn't find any input capacitor across P3V3_IN and GND. If not, pls add enough capacitance. Pls also monitor P3V3_IN.

2. Can you set the oscilloscope in auto mode to see if there is any recovery event(there is still pulse from SD/GD)  after boost shut down or this is latch fault(If all LED strings are disabled due to faults)

3. If the circuit works when increasing input voltage up to 5V?  

4. test the condition with removing Q1,   R15=0, SD float, shorting SenseN and SenseP

5. It would be better if the customer can make I2C work and read fault register directly to help analysis. 

Regards, 

X.G 

Regards, 

Xiaoguang 

Hi Xiaoguang,

The issue has been resolved.

In the previous case, the backlight ic is connected to the external load of 160mA (constant current load). The boost output was falling to  0 volts in this scenario. 

With the LCD display connected, the output is stable and the backlight driver ic is working as expected.

Also, the driver IC is tested in both PWM mode and i2c mode. In both modes, ic works fine.

Thank you for the support.

Regards,

Vidhya