DLPS052 October 2015 DLPA3000
The PAD devices can be supplied from either a battery pack or an adapter. The PAD devices use several warning and detection levels, as indicated in the previous paragraphs, to prevent system damage in case the supply voltage becomes too low or even interrupted.
Interruption of the supply voltage occurs when, for instance, the adapter is switched to another mains outlet. In case a battery pack is installed, the system power control should switch at that moment to the battery pack. A change of supply voltage from, for instance, 20 V to 8 V can occur, and thus the OVP level (which is ratio-metric; see Ratio Metric Overvoltage Protection) could become lower than VLED. An OVP fault will be triggered and the system will switch off.
The Auto_LED_Turn_Off functionality can be used to prevent the system from turning off in these circumstances. This function disables the LEDs when the supply voltage drops below LED_AUTO_OFF_LEVEL (reg 0x18h). It is advisable to have this level the same as the BAT_LOW_WARN level. When the Auto_LED_Turn_Off functionality is enabled (reg 0x01h), once a supply voltage drop is detected to below LED_AUTO_OFF_LEVEL, the LEDs will be switched off and the system should start sending lower current levels to have a lower VLED. After start using lower currents, the LEDs can be switched on again by disabling AUTO_LED_TURN_OFF function. As a result, the system can continue working at the lower supply voltage using a lower intensity. The system has to monitor the BAT_LOW_WARN status, and once the mains adapter is plugged in again (seen by BAT_LOW_WARN being low), the Auto_LED_Turn_Off functionality can be enabled again. Now the LED currents can be restored to their original levels from before the supply voltage drop.