Datasheet
EN1=2V/div
PWRGD1=2V/div
Vout1=1/div
Vout2=1V/div
Time=5msec/div
SS
TPS54218
EN
PWRGD
SS
EN
PWRGD
SS SS
SS
ref
T (mS) I ( A)
C (nF) =
V (V)
´ m
-
×
- + × ´
6
STOP
1.18 R1
R2 =
V 1.18 R1 3.2 10
-
× -
´
START STOP
6
0.944 V V
R1 =
2.59 10
TPS54218
www.ti.com
SLVS974B –SEPTEMBER 2009–REVISED JULY 2013
(2)
(3)
SLOW START PIN
The TPS54218 regulates to the lower of the SS pin and the internal reference voltage. A capacitor on the SS pin
to ground implements a slow start-time. The TPS54218 has an internal pull-up current source of 1.8μA which
charges the external slow start capacitor. Equation 4 calculates the required slow start capacitor value where Tss
is the desired slow start time in ms, I
SS
is the internal slow start charging current of 1.8 μA, and Vref is the
internal voltage reference of 0.8 V. TI recommends keeping the soft-start time in the range of 1 to 10 ms.
(4)
If during normal operation, the VIN goes below the UVLO, EN pin pulled below 1.2 V, or a thermal shutdown
event occurs, the TPS54218 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
TPS54218 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
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