The device has a dual-level UVLO circuit. During power-on, if the BIAS pin voltage is greater than 2.7 V, and the UVLO pin voltage is in between the enable threshold (V_EN) and the UVLO threshold (V_UVLO) for more than 1.5 μs (see Section 8.3.5 for more details), the device starts up and an internal configuration starts. The device typically requires a 65-μs internal start-up delay before entering standby mode. In standby mode, VCC regulator and RT regulator are operational, SS pin is grounded, and no switching at the GATE output.

Figure 8-1 Line UVLO and Enable

When the UVLO pin voltage is above the UVLO threshold, the device enters run mode. In run mode, a soft-start sequence starts if the VCC voltage is greater than 4.5 V, or 50 μs after the VCC voltage exceeds the 2.85-V VCC UV threshold (V_VCC-UVLO), whichever comes first. UVLO hysteresis is accomplished with an internal 50-mV voltage hysteresis and an additional 5-μA current source that is switched on or off. When the UVLO pin voltage exceeds the UVLO threshold, the current source is enabled to quickly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the UVLO threshold, the current source is disabled causing the voltage at the UVLO pin to fall quickly. When the UVLO pin voltage is less than the enable threshold (V_EN), the device enters shutdown mode after a 35-μs (typical) delay with all functions disabled.

Figure 8-2 Boost Start-Up Waveforms Case 1: Start-Up by 2.85-V VCC UVLO, UVLO Toggle After Start-Up

Figure 8-3 Boost Start-Up Waveforms Case2: Start-Up When VCC > 4.5 V, EN Toggle After Start-Up

The external UVLO resistor divider must be designed so that the voltage at the UVLO pin is greater than 1.5 V (typical) when the input voltage is in the desired operating range. The values of R_UVLOT and R_UVLOB can be calculated as shown in Equation 1 and Equation 2.

Equation 1.

where

• V_SUPPLY(ON) is the desired start-up voltage of the converter.
• V_SUPPLY(OFF) is the desired turnoff voltage of the converter.

Equation 2.

UVLO capacitor (C_UVLO) is required in case the input voltage drops below the V_SUPPLY(OFF) momentarily during the start-up or during a severe load transient at the low input voltage. If the required UVLO capacitor is large, an additional series UVLO resistor (R_UVLOS) can be used to quickly raise the voltage at the UVLO pin when the 5-μA hysteresis current turns on.

Figure 8-4 Line UVLO using Three UVLO Resistors

Do not leave the UVLO pin floating. Connect to the BIAS pin if not used.