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

4.2 10% Load Efficiency Might not Meet spec. in USB-PD Application, Especially at 5 V/9 V Output Condition

The result could be caused by improper current sense resistor Rcs setting and not using the IPC pin resistor. This process results in a higher burst switching frequency. At 10% load the switching losses become dominant, reduce switching frequency helps reduce switching losses and improve efficiency.

GUID-20210804-CA0I-1BTC-RRBZ-1NCP2L9KHQMN-low.png Figure 4-3 Operating at SBP1 Mode
GUID-20210804-CA0I-K5H9-CTMW-4V1LRXXSMTGT-low.png Figure 4-4 Operating at SBP2 Mode

Solution:

  1. Reduce current resistor Rcs and increase CS pin filter capacitor Ccs to push operation mode from SBP1 into SBP2.

    Figure 4-3 shows the waveform of SBP1 operation, Figure 4-4 shows the waveform after reducing Rcs or increase Ccs to push Fsw<8 kHz. this helps the system operates at SBP2 mode and improve efficiency significantly.

  2. Add a resistor between IPC and AGND

    Disconnecting a resistor from IPC to AGND is not a mandatory requirement, but connecting a resistor in here helps for improving the light load efficiency especially when 5 V/9 V can’t meet the average efficiency spec.

GUID-20210804-CA0I-F6PS-JPZF-V5940ZSPLTV9-low.gif Figure 4-5 IPC Pin Resistor Setting

There is a 50 uA current source out of the IPC pin, the voltage on IPC pin resistor (VIPC)can program the peak current threshold (VCST_IPC) at SBP2 mode. Higher peak current can further reduce fsw then improve 10% load efficiency, but higher peak current may cause audible noise, trade off between efficiency and noise level maybe need to be made.