SLVS974F September 2009 – May 2020 TPS54218
PRODUCTION DATA.
The TPS54218 device regulates to the lower of the SS pin and the internal reference voltage. A capacitor on the SS pin to ground implements a soft-start time. The TPS54218 device has an internal pullup current source of 2.07 μA that charges the external soft-start capacitor. Equation 4 calculates the required soft-start capacitor value where tSS is the desired soft-start time in ms, ISS is the internal soft-start charging current of 2.07 μA, and 0.9 V is the SS pin voltage at the SS to reference crossover point. Equation 4 represents a first order linear approximation of the SS time using the well-known relationship I = C × dV / dt. A constant current charging a capacitor results in a fixed value for dV / dt and a linear charge from 0 V to the 0.9 V SS to reference crossover voltage. In use, there are two factors that cause the actual output voltage to deviate from this linear ideal ramp. At the initial start, the COMP pin voltage can be below the minimum skip voltage. The TPS54218 does not begin to switch and the output voltage does not start to rise until the RC compensation network from COMP to GND has charged above the skip threshold. At the end of the SS time, when the output has reached 90% of its final regulated value, a “soft handoff” occurs to transition from the SS pin voltage to the internal reference at the non-inverting terminal of the error amplifier. When the SS pin voltage is at 0.9 V, the output voltage is defined to be within 98% typical of the final regulated voltage.
where
It is recommended to maintain the soft-start time in the range between 1 msec and 10 msec.
There are two conditions that can cause the SS pin to be intentionally discharged. If a fault condition occurs that results in the COMP pin voltage reaching its maximum voltage as during an overcurrent condition, the comp pin is discharged to below 20 mV. Only the discharge voltage is specified, not the discharge current. This allows the TPS54218 to gracefully recover from an overcurrent condition that can reduce the output voltage. If during normal operation, the input voltage goes below the UVLO, the EN pin is pulled below 1.2 V, or a thermal shutdown event occurs, the TPS54218 stops switching and the SS capacitor is discharged before reinitiating a powering up sequence. See Figure 46 for an example of this discharge behavior.