Datasheet

SS
TPS54418
EN
PWRGD
SS
TPS54418
EN
PWRGD
EN1=2V/div
Vout1=1V/div
Vout2=1V/div
Time=5msec/div
EN1=2V/div
PWRGD1=2V/div
Vout1=1/div
Vout2=1V/div
Time=5msec/div
SS
TPS54418
EN
PWRGD
SS
EN
PWRGD
TPS54418
www.ti.com SLVS946C –MAY 2009–REVISED JULY 2013
If during normal operation, the VIN goes below the UVLO, EN pin pulled below 1.2 V, or a thermal shutdown
event occurs, the TPS54418 stops switching and the SS is discharged to 0 volts before reinitiating a powering up
sequence.
SEQUENCING
Many of the common power supply sequencing methods can be implemented using the SS, EN and PWRGD
pins. The sequential method can be implemented using an open drain or collector output of a power on reset pin
of another device. Figure 25 shows the sequential method. The power good is coupled to the EN pin on the
TPS54418 which enables the second power supply once the primary supply reaches regulation.
Ratiometric start up can be accomplished by connecting the SS pins together. The regulator outputs ramp up
and reach regulation at the same time. When calculating the slow start time the pull up current source must be
doubled in Equation 4. The ratiometric method is illustrated in Figure 27.
Figure 25. Sequential Start-Up Sequence Figure 26. Sequential Startup using EN and
PWRGD
Figure 27. Schematic for Ratiometric Start-Up Figure 28. Ratiometric Start-Up using Coupled SS
Sequence Pins
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