JAJSD10B March 2017 – July 2018 LP8863-Q1
PRODUCTION DATA.
The LP8863-Q1 boost DC/DC converter generates the bias voltage for the LEDs. During normal operation, boost output voltage is adjusted automatically based on the LED current sink headroom voltages. This is called adaptive boost control. The number of used LED outputs is auto detected and only the active LED outputs are monitored to control the adaptive boost voltage. Any LED strings with open or short faults are also removed from the adaptive voltage control loop. The LED driver pin voltages are periodically monitored by the control loop and the boost voltage is raised if any of the LED outputs falls below the low headroom threshold. The boost voltage is lowered if all the LED outputs are above the high headroom threshold. See Figure 10 for how the low and high headroom thresholds automatically scale based on the LED string current and VSAT level being used.
The external resistive divider (RFB1, RFB2) defines both the minimum and maximum adaptive boost voltage levels. The FB circuit operates the same in boost and SEPIC topologies. Choose maximum boost voltage based on the maximum LED string voltage specification. Before the LED drivers are active the boost starts up to the initial boost level. The initial boost voltage is approximately 90% of maximum boost voltage. Once the LED driver channels are active, the boost output voltage is adjusted automatically based on LED current sink headroom voltage. The FB pin resistor divider also scales the boost OVP and undervoltage levels.