JAJSLY1G July 2013 – June 2021 TPS54561
PRODUCTION DATA
The TPS54561 effectively uses the lower voltage of the internal voltage reference or the SS/TR pin voltage as the power-supply's reference voltage and regulates the output accordingly. A capacitor on the SS/TR pin to ground implements a soft start time. The TPS54561 has an internal pull-up current source of 1.7 μA that charges the external soft start capacitor. The calculations for the soft start time (10% to 90%) are shown in Equation 5. The voltage reference (VREF) is 0.8 V and the soft start current (ISS) is 1.7 μA. The soft start capacitor should remain lower than 0.47 μF and greater than 0.47 nF.
At power up, the TPS54561 will not start switching until the soft start pin is discharged to less than 54 mV to ensure a proper power up, see Figure 7-3.
Also, during normal operation, the TPS54561 will stop switching and the SS/TR must be discharged to 54 mV, when the VIN UVLO is exceeded, EN pin pulled below 1.2 V, or a thermal shutdown event occurs.
The FB voltage will follow the SS/TR pin voltage with a 42-mV offset up to 85% of the internal voltage reference. When the SS/TR voltage is greater than 85% on the internal reference voltage the offset increases as the effective system reference transitions from the SS/TR voltage to the internal voltage reference (see Figure 6-23). The SS/TR voltage will ramp linearly until clamped at 2.7 V typically as shown in Figure 7-3.