SLUAAF9 September   2021 UCC28782

 

  1.   Trademarks
  2. 1 Initial Board Visual Inspection and Start-up Check
  3. 2 Typical System Operating Waveforms
    1. 2.1 SBP2 Mode
    2. 2.2 SBP1 Mode
    3. 2.3 LPM mode
    4. 2.4 LPM to ABM Mode Transition
    5. 2.5 ABM Mode
    6. 2.6 ABM to AAM Mode Transition
    7. 2.7 AAM Mode
  4. 3Typical System Protection Waveforms
    1. 3.1 Over-Power Protection (OPP)
    2. 3.2 Output Overvoltage Protection (OVP)
    3. 3.3 Output Short-Circuit Protection (SCP)
  5. 4Common Issues and Solutions
    1. 4.1 VDD Boost Converter Survival Mode
      1. 4.1.1 Survival Mode Due To Boost Inductor DCR Too High.
      2. 4.1.2 System Stays In Survival Mode When the Output Voltage is Low Such as 3.3V/5V/9V
    2. 4.2 10% Load Efficiency Might not Meet spec. in USB-PD Application, Especially at 5 V/9 V Output Condition
    3. 4.3 Transient
      1. 4.3.1 BIN/BSW Pin Damage During LPM to ABM Mode Transition
      2. 4.3.2 SR MOSFET VDS Overstress at Survival Mode
      3. 4.3.3 SR MOSFET Vds Voltage Overstress Due to PWMH Partial Turn On At Load Transition
  6. 5References

4.1.2 System Stays In Survival Mode When the Output Voltage is Low Such as 3.3V/5V/9V

If this occurs, it might be due to BIN and VDD capacitance are too small. At no load, energy in each survival mode pulse is intended to go to BIN and VDD networks on the primary side of transformer. If the auxiliary winding resistance RAUX is high, it can be difficult to pass this energy to BIN capacitors, and most of the energy goes to output instead. This results in more survival mode pulses than expected to get energy into BIN and VDD which overcharges Vout.

Solution: Use higher BIN and VDD capacitance to attract energy to AUX side. ~47 uF on BIN and ~22 uF on VDD is a good start.

Higher capacitors at BIN pin CBIN keeps BIN voltage lower than reflected Vout so pulse current steers to BIN instead of Vout. Higher CVDD keeps VDD higher so fewer survival mode pulses are required. Low RAUX allows survival mode pulse energy to steer easily in to lower-value BIN cap.