When detecting low battery voltage using LPCOMP and ADC, the detection voltage changes (occurs approximately 5%)

[pkr2258]

HI

We are mass producing products using nRF52832.

However, during production, a PCB was found to not detect low battery voltage for about 5%, so we are inquiring about it during review.

1-1) The circuit configuration is as follows.

1-2) SW configuration is as follows.

- Basic LPCOMP settings -> LPCOMP event occurs -> Switch to ADC to read battery voltage -> When below the reference voltage, LED display

- LPCOMP

static void lpcomp_event_handler(nrf_lpcomp_event_t event)
{
    uint8_t bsp = 0;
    if (event == NRF_LPCOMP_EVENT_DOWN)
    {
       
        //voltage_falls_detected++;
        //voltage_falls_total++;
        
        if( getlpcomp_chceck() == 0 )
        {
           bsp =  getBsp_led_indication_stage(); 
           if( ( bsp == BSP_INDICATE_IDLE ) || ( bsp == BSP_INDICATE_CONNECTED_OFF ) || ( bsp == BSP_INDICATE_USER_STATE_OFF ) )
           {
                bsp_indication_set( BSP_INDICATE_USER_STATE_BAT_ON );


           }
        }
    }
}


static void lpcomp_init(void)
{
    uint32_t                err_code;

    nrf_drv_lpcomp_config_t config = NRF_DRV_LPCOMP_DEFAULT_CONFIG;
    config.input = NRF_LPCOMP_INPUT_2;
    // initialize LPCOMP driver, from this point LPCOMP will be active and provided
    // event handler will be executed when defined action is detected
    err_code = nrf_drv_lpcomp_init(&config, lpcomp_event_handler);
    APP_ERROR_CHECK(err_code);
    nrf_drv_lpcomp_enable();
}

- ADC

         nrf_drv_lpcomp_disable();
         temp_bat = getBatVoltage();
         nrf_drv_lpcomp_enable();



int32_t getBatVoltage( void )
{
    uint8_t i;
    int32_t temp_adc = 0;

    for( i = 0; i < BAT_VOLTAGE_CNT; i++ )
    {
       temp_adc += BatVoltage();
    }

    temp_adc /= ( BAT_VOLTAGE_CNT  );
    gBatVoltage = (int32_t)( ( (float)temp_adc / 0.166666667L * 0.6L / 1024L ) * 1000 );



    return gBatVoltage;
}

2-1) Review Symptoms

- LPCOMP: When it is lower than the reference voltage, check that it wakes up normally

- ADC: After LPCOMP wakes up, if the ADC is read and it is lower than the reference voltage, the LED should turn on, but the LED remains OFF

-> The strange thing is that after lowering the voltage below the low battery voltage and checking the wake-up by LPCOMP,

If the low voltage LED does not turn on by ADC, and the voltage is slightly increased by 0.1V, the low voltage is detected by ADC.

[hmolesworth]

What colour is the LED?

I ask since below 2.06 volts Vbatt must be below 3V (2M/1M voltage divider), and some LEDs would not light up until the voltage is raised, say by 0.1 volts .. most red LEDs would light up at a lower voltage, worth a test. 5% voltage variation in LED visible voltage would be common, some would work some may not.

[pkr2258]

 HI

The VF of the LED we are using is 2.0V. Could this be a problem?

[hmolesworth]

Depends on the circuit driving the LED; can you share that part of the circuit schematic along with the supply voltage or regulator (if any)? If it were a faulty hardware issue such as a damaged pin raising the voltage by 0.1V, as  described above, would be unlikely to fix it.