SDAA108 October 2025 LM5171 , LM5171-Q1

4 Design of Additional OVP Circuit

In Figure 4-1, both the hiccup mode protection and latched protection are shown. Pull SS/DEMx pin low to achieve hiccup mode protection. Pull DT/SD pin low to shut down both channels and latch.

For the latched OVP, pull DT/SD below 0.5V to shut down both channels and latch. An open-drain comparator is a good fit here.

For the hiccup OVP, a small-signal MOSFET and a comparator are required. The SS/DEMx pins are multifunction pins. The SS/DEMx pins serve as ISETx soft start, and program each channel to operate in the DEM or FPWM. To create hysteresis, positive feedback is required. However, connecting the feedback resistor to SS/DEMx pin affects the soft start current and the DEM/FPWM control. A small-signal MOSFET is used so that the feedback resistor is not connected to SS/DEM2.

Figure 4-1 LM5171 with Additional OVP circuit

As shown in Figure 4-1, the hiccup OVP trigger voltage is,

Equation 6.

V_{O V P H} = \frac{R_{O V P T} + (R_{O V P B} ‖R_{H Y S})}{R_{O V P B} ‖R_{H Y S}} \times V_{C}

The OVP voltage hysteresis is,

Equation 7.

V_{H Y S} = \frac{R_{O V P B} ‖R_{O V P T}}{R_{H Y S} + (R_{O V P B} ‖R_{O V P T})} \times \frac{R_{O V P T} + (R_{O V P B} ‖R_{H Y S})}{R_{O V P B} ‖R_{H Y S}} \times V D D

Similar to selecting VSETH and VSETL, pay attention to the comparator common mode input voltage range when selecting V_C. With the supply of VDD, widely used comparators such as the LM393 series have a common mode input voltage range of 0V to VDD-2V over temperature. In this case, 1V to 2.5V is a reasonable range for V_C. Use a resistor divider from VREF pin to set V_C.