The LM5176 has a dual function enable and undervoltage lockout (UVLO) circuit. The EN/UVLO pin has three distinct voltage ranges: shutdown, standby, and operating (see Section 7.4.1). When the EN/UVLO pin is below the standby threshold, V_EN(STBY), the converter is held in a low power shutdown mode. When EN/UVLO voltage is greater than the standby threshold, V_EN(STBY), but less than the operating threshold, V_EN(OP), the internal bias rails and the VCC regulator are enabled but the soft-start (SS) pin is held low and the PWM controller is disabled. A pullup current I_EN(STBY) is sourced out of the EN/UVLO pin in standby mode to provide hysteresis between the shutdown mode and the standby mode. When EN/UVLO is greater than the operating threshold, V_EN(OP), and VCC is above the undervoltage threshold, V_UV(VCC), the controller starts operation. A hysteresis current ΔI_HYS(OP) is sourced out of the EN/UVLO pin when the EN/UVLO input exceeds the operating threshold to provide hysteresis that prevents on/off chattering in the presence of noise with a slowly changing input voltage.

The V_IN UVLO threshold is typically set by a resistor divider from V_IN to AGND (Figure 7-2). The turnon threshold V_IN(UV) is calculated using Equation 1 where R_UV2 is the upper resistor and R_UV1 is the lower resistor in the EN/UVLO resistor divider:

Equation 1.

The hysteresis between the UVLO turnon threshold and turnoff threshold is set by the upper resistor in the EN/UVLO resistor divider and is given by:

Equation 2.

Figure 7-2 UVLO Threshold Programming