SLUAA69 July   2020  – MONTH  TPS548D22

 

  1.   Trademarks
  2. 1Introduction
    1. 1.1 LED Driver Methods
    2. 1.2 Power Supply Solutions for Common-Cathode LED Display
  3. 2Principle of Synchronous Buck with Sinking Current Application
  4. 3 Design Considerations and Analysis
    1. 3.1 Choose an IC with Sufficient Current Sinking
    2. 3.2 Choose IC Supporting Negative OCP
    3. 3.3 Choose an IC Supporting Pre-Bias Startup
    4. 3.4 Analysis of System Startup
  5. 4 TI Devices and Functionalities
    1. 4.1 Negative OCP Functionality
    2. 4.2 Hiccup Mode and Latch-off Mode
    3. 4.3 UVP and OVP Functionality
  6. 5 TI Solution
  7. 6 Bench Test and Result
    1. 6.1 Bench Test Configuration
    2. 6.2 Startup Waveforms and Behaviors Analysis Overview
    3. 6.3 Startup Waveforms and Behaviors Analysis at the First OVP
    4. 6.4 Startup Waveforms and Behaviors Analysis after the First OVP
    5. 6.5 Waveforms and Behaviors Analysis of Startup Solution with Lazy Loading
  8. 7 Conclusion
  9. 8References

4.1 Negative OCP Functionality

TPS548D22 has cycle-by-cycle overcurrent limit control. The inductor current is monitored by the voltage across low-side MOSFET RDS (on) during the OFF state. The controller maintains the OFF state during the period when the inductor current is larger than the overcurrent trip level. The device uses the GND pin as the positive current sensing node. As the comparison occurs during the OFF state, VILIM sets the valley level of the inductor current. Also, this device performs both positive and negative OCP with the same magnitudes. Positive current limit is normally used to protect the inductor from saturation which causes damage to MOSFETs. Negative current limit is normally used to protect the low-side MOSFET during OVP discharge. While in a synchronous buck converter with sinking current application, a negative current limit is also used to protect the inductor from saturation, just as the positive current limit does.

GUID-20200604-SS0I-6GHK-FSPQ-KXPMHJPQXF0V-low.gifFigure 4-1 OCP Functionality Block Diagram of TPS548D22
Figure 4-1 shows the OCP functionality block diagram of TPS548D22. The resistor RILIM is connected between ILIM pin and GND to set OCP level. The current coming out from the internal current source flows through RILIM and determines the voltage VILIM on the ILIM pin. VILIM is attenuated to 1/16 of the original value and then connected to the inverting input of the zero-crossing detector. VILIM is also attenuated to -1/16 of the original value and then connected to the non-inverting input of the OCP detector. The switching node is connected to the non-inverting input of zero-crossing detector and the inverting input of OCP detector.

For positive OCP, once the magnitude of sensing voltage VSW (VSW is negative, since current follows from source to drain) exceeds -1/16 VILIM, the OCP comparator outputs high and a positive OCP event is recorded. For negative OCP, the detect threshold is set at the same absolute value as positive OCP but with negative polarity. Once the magnitude of sensing voltage VSW (VSW is positive, since current follows from drain to source) exceeds 1/16 VILIM, the ZC comparator outputs high and a negative OCP event is recorded. Note that the negative OCP threshold still represents the valley value of the inductor current.