TPS54678_17 Datasheet, PDF (15/42 Pages) Texas Instruments – 2.95-V to 6-V Input, 6-A Output, 2-MHz, Synchronous Step-Down SWIFT Switcher

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TPS54678
SLVSBF3A – JUNE 2012 – REVISED NOVEMBER 2015
Feature Description (continued)
7.3.9 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. The sequential method is shown in Figure 23. The power good is coupled to the EN pin on the
TPS54678, which will enable the second power supply once the primary supply reaches regulation.
TPS54678
PWRGD
EN
SS
C_SS
TPS54678
EN
PWRGD
SS/TR
C_SS
Figure 23. Sequential Start-Up Sequence
7.3.10 Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
The switching frequency of the TPS54678 is adjustable over a wide range from approximately 200 kHz to
2000 kHz by placing a maximum of 210 kΩ and minimum of 18 kΩ, respectively, on the RT/CLK pin. The
RT/CLK is typically 0.5 V. To determine the timing resistance for a given switching frequency, use the curve in
Figure 24. To reduce the solution size one would typically set the switching frequency as high as possible, but
tradeoffs of the supply efficiency, maximum input voltage and minimum controllable on time should be
considered.
The minimum controllable on time is typically 85 ns at 3-A current load and 100 ns at no load, and will limit the
maximum operating input voltage or output voltage.
RT (kΩ) =
56183
éë_{FSW (KHz)}ùû^1.052
(5)
1800
1600
1400
1200
1000
800
600
400
200
0
0
20 40 60 80 100 120 140 160 180 200
RT Resistance (KΩ)
G009
Figure 24. Switching Frequency vs RT Set Resistor
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