DLPS052 October 2015 DLPA3000
The DLPA3000 illumination driver LED outputs are protected against open circuit use. In case no LED is connected and the PAD device is instructed to set the LED current to a specific level, the LED voltage (ILLUM_A_FB) will quickly rise and potentially rail to VIN. This should be prevented. The OVP protection circuit triggers once VLED crosses a predefined level. As a result the DLPA3000 will be switched off.
The same protection circuit is triggered in case the supply voltage (VINA) will become too low to have the DLPA3000 work properly given the VLED level. This protection circuit is constructed around a comparator that will sense both the LED voltage and the VINA supply voltage. The fraction of the VINA is connected to the minus input of the comparator while the fraction of the VLED voltage is connected to the plus input. Triggering occurs when the plus input rises above the minus input and an OVP fault is set. The fraction of the VINA must be set between 1 V and 4 V to ensure proper operation of the comparator.
The fraction of the ILLUM_A_FB voltage is set by the register 0x19h bits [4:0], while the setting of the fraction of the VINA voltage is done by register 0x0Bh bits [4:0]. In general an OVP fault is set when
|VLED/VLED_RATIO ≥ VINA/VINA_RATIO|
|VLED ≥ VINA × VLED_RATIO/VINA_RATIO.|
Clearly, the OVP level is ratio-metric, i.e. can be set to a fixed fraction of VINA.
For example: VLED should stay below 85% of VINA. The settings for the respective registers are:
The result is as follows: OVP triggers if VLED ≥ VINA × 4.98/5.85 = 0.85 VINA.
Additionally, for VIN_RATIO = 5.85, the VIN- input voltage for the comparator is between 1 V and 3.4 V for a supply voltage between 6 V and 20 V.