SBVS318B July 2017 – January 2019 TPS7A92
PRODUCTION DATA.
Soft-start refers to the gradual ramp-up characteristic of the output voltage after the EN and UVLO thresholds are exceeded. Reducing how quickly the output voltage increases during startup also reduces the amount of current needed to charge the output capacitor, referred to as inrush current. Inrush current is defined as the current going into the LDO during start-up. Inrush current consists of the load current, the current used to charge the output capacitor, and the ground pin current (that contributes very little to inrush current). This current is difficult to measure because the input capacitor must be removed, which is not recommended. However, the inrush current can be estimated by Equation 4:
where
The TPS7A92 features a monotonic, voltage-controlled soft-start that is set by the user with an external capacitor (CNR/SS). This soft-start helps reduce inrush current, minimizing load transients to the input power bus that can cause potential start-up initialization problems when powering FPGAs, digital signal processors (DSPs), or other high current loads.
To achieve a monotonic start-up, the TPS7A92 error amplifier tracks the voltage ramp of the external soft-start capacitor until the voltage exceeds approximately 97% of the internal reference. The final 3% of VNR/SS is charged through the noise-reduction resistor (RNR), creating an RC delay. RNR is approximately 280 kΩ and applications that require the highest accuracy when using a large value CNR/SS must take this RC delay into account.
The soft-start ramp time depends on the soft-start charging current (INR/SS), the soft-start capacitance (CNR/SS), and the internal reference (VREF). The approximate soft-start ramp time (tSS) can be calculated with Equation 5:
The value for INR/SS is determined by the state of the SS_CTRL pin. When the SS_CTRL pin is connected to GND, the typical value for the INR/SS current is 6.2 µA. Connecting the SS_CTRL pin to IN increases the typical soft-start charging current to 100 µA. The larger charging current for INR/SS is useful if shorter start-up times are needed (such as when using a large noise-reduction capacitor).