The device has a soft-start feature that starts the device typically with 500-mA peak current limit until V_O = 1.8 V and 500 µs elapsed when the input current limit is set to unlimited (see Section 7.3.4). Afterwards, the output voltage ramps in a series of discrete steps (see Figure 7-6).

• When V_O ≤ 1.8 V, peak current is limited to 500 mA typical for 500 µs.
• When V_O > 1.8 V, each step is 100 mV high and has a duration of 125 µs.

The total soft-start ramp-up time can be calculated with Equation 2.

Equation 2.

where

• t_r(SS) is the rise time of the output voltage in milliseconds
• V_O is the output voltage in volts

Figure 7-6 shows a typical start-up case.

Figure 7-6 Start-Up Behavior

Figure 7-7 illustrates the start-up step size behavior.

Figure 7-7 Typical Soft-Start Ramp Step Size

Table 7-1 shows the typical start-up time for a number of standard output voltages.

If the output is prebiased – that is, the initial output voltage is not zero – the start-up behavior is as follows:

• If the prebias voltage is lower than the target voltage, the device does not start switching until the ramping output voltage is greater than the prebias voltage (see Figure 7-8).
• If the prebias voltage is higher than the target voltage, the device does not start to switch until the output voltage has decreased to the target voltage (see Figure 7-9). The device cannot actively discharge the output to the target voltage and relies on the load current to discharge the output capacitor and decrease the output voltage to the target value.

Figure 7-8 Start-Up Behavior into Prebiased (Low) Output

Figure 7-9 Start-Up Behavior into Prebiased (High) Output