TPS54388-Q1 Datasheet, PDF (14/38 Pages) Texas Instruments – 2.95-V to 6-V Input, 3-A Output, 2-MHz, Synchronous Step-Down Switcher With Integrated FETs

TPS54388-Q1
SLVSAF1B – OCTOBER 2010 – REVISED JULY 2012
www.ti.com
SLOW START/TRACKING PIN
The TPS54388-Q1 regulates to the lower of the SS/TR pin and the internal reference voltage. A capacitor on the
SS/TR pin to ground implements a slow start time. The TPS54388-Q1 has an internal pull-up current source of 2
μA 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, Iss is the internal slow start charging current of 2 μA, and Vref is
the internal voltage reference of 0.800 V.
vertical spacer
Css(nF) = ^{Tss(mS) ´ Iss(mA)}
Vref(V)
(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 TPS54388-Q1 stops switching. When the VIN goes above UVLO, EN is released or pulled
high, or a thermal shutdown is exited, then SS/TR is discharged to below 60 mV before reinitiating a powering up
sequence. The VSENSE voltage will follow the SS/TR pin voltage with a 50 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.
SEQUENCING
Many of the common power supply sequencing methods can be implemented using the SS/TR, 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 23 shows the sequential method. The power good is coupled to the EN pin on the
TPS54388-Q1 which enables the second power supply once the primary supply reaches regulation.
Ratio-metric start up can be accomplished by connecting the SS/TR 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 ratio metric method is illustrated in Figure 25.
PWRGD
EN
SS
TPS54388-Q1
EN
EN1
SS
EN2
PWRGD
VO1
VO2
Figure 23. Sequential Start-Up Sequence
Figure 24. Sequential Startup using EN and
PWRGD
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