SNVAA19 May   2021 LP8863-Q1 , LP8864-Q1 , LP8864S-Q1 , LP8866-Q1 , LP8866S-Q1

 

  1.   Trademarks
  2. 1Introduction
  3. 2Boost Output Voltage Range Design Consideration
  4. 3Boost UVP Design Consideration
  5. 4Boost OVP Design Consideration
  6. 5Calculation of Feedback Resistor Values
  7. 6Feedback Resistor Calculation Example
  8. 7References

3 Boost UVP Design Consideration

When VBOOST_UVP is reached, in other words, when VFB is below VUVP of 0.886 V, it is a sign that the boost is no longer in regulation. During normal operation, VFB is regulated at VBG of 1.21 V. If undervoltage condition lasts longer than 110 ms, LP8863-Q1 (using LP8863-Q1 as an example and it is similar for LP886x-Q1) will report boost overcurrent, by generating fault interrupt and asserting BSTOCP_STATUS bit in INTERRUPT_STATUS_1 register, as shown in Table 3-1. The device will go to standby for 200 ms before restart attempt. If VBOOST_UVP,MAX (when ISEL is at maximum value) level is lower than (VSTRING + VHEADROOM,MAX), open LED fault may be reported unintentionally instead of overcurrent fault during an overcurrent event. Therefore, we recommend to design the maximum boost UVP threshold VBOOST_UVP,MAX when the current DAC ISEL is at maximum value, higher than VSTRING,MAX + VHEADROOM,MAX, as shown in Equation 4. The aim is to prevent false triggering of LED open fault during an overcurrent event when boost under voltage is detected.

Equation 3. GUID-20210423-CA0I-BPK2-XL8K-M4TNCDFDMFPL-low.gif
Table 3-1 Boost OCP, OVP and Open LED Fault Table
FAULT NAMECONDITIONACTION
Boost overcurrentFB pin voltages falls below VUVP level for > 110 ms.Device goes to standby and then attempts to restart 200 ms after fault occurs.
Open LED stringHeadroom voltage on one or more channels is below minimum level and boost has adapted to maximum level.Faulted LED string is disabled and removed from adaptive boost control loop. String is re-enabled next power cycle.
Boost OVP lowFB pin voltage rises above VFB_OVPL level.Boost stops switching until boost voltage level falls. The device remains in normal mode with LED drivers operational.
Boost OVP highFB pin voltage rises above VFB_OVPH level or DISCHARGE pin voltage rises above VBST_OVPH.Device goes to Fault Recovery and waits until output voltage falls below threshold before restarting.