SLUSDB2A August   2018  – December 2021

PRODUCTION DATA

1. Features
2. Applications
3. Description
4. Revision History
5. Pin Configuration and Functions
6. Specifications
7. Detailed Description
8. Application and Implementation
1. 8.1 Application Information
2. 8.2 Typical Application
1. 8.2.1 Design Requirements
2. 8.2.2 Detailed Design Procedure
3. 8.2.3 Application Curves
9. Power Supply Recommendations
10. 10Layout
11. 11Device and Documentation Support
1. 11.1 Device Support
2. 11.2 Documentation Support
3. 11.3 Receiving Notification of Documentation Updates
4. 11.4 Community Resources

• PW|24

#### 8.2.2.10 Current Sense Network (CT, RCS, R7, DA)

The CT chosen for this design has a turns ratio (CTRAT) of 100:1 in Equation 97:

Equation 97. Calculate nominal peak current (IP1) at VINMIN:

The peak primary current is calculated using Equation 98:

Equation 98. The CS pin voltage where peak current limit will trip is:

Equation 99. Calculate current sense resistor (RCS) and leave 300 mV for slope compensation using Equation 100. Include a 1.1 factor for margin:

Equation 100. Select a standard resistor for RCS:

Equation 101. Estimate the power loss for RCS using Equation 102:

Equation 102. Calculate maximum reverse voltage (VDA) on DA using Equation 103:

Equation 103. Estimate the DA power loss (PDA) using Equation 104:

Equation 104. Calculate reset resistor R7:

Resistor R7 is used to reset the current sense transformer CT:

Equation 105. Resistor RLF1 and capacitor CLF form a low-pass filter for the current sense signal (Pin 15). For this design, chose the following values. This filter has a low frequency pole (fLFP) at 482 kHz, (which is appropriate for most applications) but may be adjusted to suit individual layouts and EMI present in the design.

Equation 106. Equation 107. Equation 108. The UCC28951 VREF output (Pin 1) needs a high frequency bypass capacitor to filter out high frequency noise. This pin needs at least 1 µF of high-frequency bypass capacitance (CREF).

Equation 109. The voltage amplifier reference voltage (Pin 2, EA +) can be set with a voltage divider (R1, R2), for this design example, the error amplifier reference voltage (V1) will be set to 2.5 V. Select a standard resistor value for R1 and then calculate resistor value R2.

UCC28951 reference voltage:

Equation 110. Set voltage amplifier reference voltage:

Equation 111. Equation 112. Equation 113. The voltage divider formed by resistor R3 and R4 are chosen to set the DC output voltage (VOUT) at Pin 3 (EA-).

Select a standard resistor for R3:

Equation 114. Calculate R4 using Equation 115:

Equation 115. Then choose a standard resistor for R4 using Equation 116:

Equation 116. Note:

TI recommends using an RCD clamp to protect the output synchronous FETs from overvoltage due to switch node ringing. Figure 8-5 Daughter Board Schematic