7.3.9 Slow-Start / Tracking Pin (SS/TR)

The TPS54260 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 slow-start time. The TPS54260 has an internal pullup current source of 2 μA that charges the external slow-start capacitor. The calculations for the slow-start time (10% to 90%) are shown in Equation 6. The voltage reference (V_REF) is 0.8 V and the slow-start current (I_SS) is 2 μA. The slow-start capacitor should remain lower than 0.47 μF and greater than 0.47 nF.

Equation 6.

At power-up, the TPS54260 will not start switching until the slow-start pin is discharged to less than 40 mV to ensure a proper power-up, see Figure 30.

Also, during normal operation, the TPS54260 will stop switching and the SS/TR must be discharged to 40 mV, when the VIN UVLO is exceeded, EN pin pulled below 1.25 V, or a thermal shutdown event occurs.

The VSENSE voltage will follow the SS/TR pin voltage with a 45-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 23). The SS/TR voltage will ramp linearly until clamped at 1.7 V.

Figure 30. Operation of SS/TR Pin when Starting