Datasheet

EN1=2V/div

PWRGD1=2V/div

Vout1=1/div

Vout2=1V/div

Time=5msec/div

TPS54318

PWRGD

SS SS

ref

T (mS) I ( A)

C (nF) =

V (V)

´ m

- + × ´

STOP

1.18 R1

R2 =

V 1.18 R1 3.2 10

TPS54318

www.ti.com

SLVS975A –SEPTEMBER 2009–REVISED SEPTEMBER 2013

(3)

SLOW-START PIN

The TPS54318 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 TPS54318 has an internal pullup current source of 1.8 μA which

charges the external slow-start capacitor. Equation 4 calculates the required slow-start capacitor value where T

is the desired slow-start time in ms, I

is the internal slow-start charging current of 1.8 μA, and V

ref

is the internal

voltage reference of 0.8 V. TI recommends to keep the soft-start time in the range of 1 ms 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 TPS54318 stops switching and the SS is discharged to 0 V before reinitiating a powering-up

sequence.

SEQUENCING

Many of the common power-supply sequencing methods are implemented using the SS, EN, and PWRGD pins.

The sequential method is 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 TPS54318

which enables the second power supply once the primary supply reaches regulation.

Ratiometric startup is 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 pullup current source must be

doubled in Equation 4. The ratiometric method is illustrated in Figure 27.

Figure 25. Sequential Startup Sequence Figure 26. Sequential Start-Up Using EN and

PWRGD

Product Folder Links :TPS54318