A capacitor from the SS pin to GND defines the SS time, t_SS. The TPS40170 enters into soft-start immediately after completion of the overcurrent calibration. The SS pin goes through the device internal level-shifter circuit before reaching one of the positive inputs of the error amplifier. The SS pin must reach approximately 0.65 V before the input to the error amplifier begins to rise above 0 V. To charge the SS pin from 0 V to 0.65 V faster, at the beginning of the soft-start in addition to the normal charging current, (11.6 μA, typ.), an extra charging current (40.4 μA, typ.) is switched-in to the SS pin. As the SS capacitor reaches 0.5 V, the extra charging current is turned off and only the normal charging current remains. Figure 6-5 shows the soft-start function block.

Figure 6-5 Soft-Start Schematic Block

As the SS pin voltage approaches 0.65 V, the positive input to the error amplifier begins to rise (see Figure 6-6). The output of the error amplifier (the COMP pin) starts rising. The rate of rise of the COMP voltage is mainly limited by the feedback loop compensation network. After V_COMP reaches the valley of the PWM ramp, the switching begins. The output is regulated to the error amplifier input through the FB pin in the feedback loop. After the FB pin reaches the 600 mV reference voltage, the feedback node is regulated to the reference voltage, V_REF. The SS pin continues to rise and is clamped to VDD.

The SS pin is discharged through an internal switch during the following conditions:

• Input (VIN) undervoltage lock out UVLO pin less than V_UVLO
• Overcurrent protection calibration time (t_CAL)
• VBP less than threshold voltage (VBP_(off))

Because it is discharged through an internal switch, the discharging time is relatively fast compared with the discharging time during the fault restart which is discussed in the Soft-Start During Overcurrent Fault section.

Figure 6-6 Soft-Start Waveforms

For 0 < V_{SS_EAMP} < V_REF

Equation 8.

For V_{SS_EAMP} > V_REF

Equation 9.