TPS54618RTET Datasheet, PDF (24/39 Pages) Texas Instruments – 2.95-V to 6-V Input, 6-A Output, 2-MHz, Synchronous Step-Down Switcher With Integrated FETs ( SWIFT™)

TPS54618
SLVSAE9A – NOVEMBER 2010 – REVISED OCTOBER 2012
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The slow start capacitor value can be calculated using Equation 32. For the example circuit, the slow start time is
not too critical since the output capacitor value is 110 μF which does not require much current to charge to 1.8 V.
The example circuit has the slow start time set to an arbitrary value of 4ms which requires a 10 nF capacitor. In
TPS54618, Iss is 2.2 μA and Vref is 0.799 V.
Css(nF) = ^{Tss(ms) ´ Iss(mA)}
Vref(V)
(32)
BOOTSTRAP CAPACITOR SELECTION
A 0.1 μF ceramic capacitor must be connected between the BOOT to PH pin for proper operation. It is
recommended to use a ceramic capacitor with X5R or better grade dielectric. The capacitor should have 10 V or
higher voltage rating.
OUTPUT VOLTAGE AND FEEDBACK RESISTORS SELECTION
For the example design, 100 kΩ was selected for R6. Using Equation 33, R7 is calculated as 80 kΩ. The nearest
standard 1% resistor is 80.6 kΩ.
Vref
R7 =
R6
Vo - Vref
(33)
Due to the internal design of the TPS54618, there is a minimum output voltage limit for any given input voltage.
The output voltage can never be lower than the internal voltage reference of 0.799 V. Above 0.799 V, the output
voltage may be limited by the minimum controllable on time. The minimum output voltage in this case is given by
Equation 34
Voutmin = Ontimemin ´ Fsmax ´ (Vinmax - loutmin ´ RDSmin)- Ioutmin ´ (RL + RDSmin)
Where:
Voutmin = minimum achievable output voltage
Ontimemin = minimum controllable on-time (75 ns typical. 120 ns no load)
Fsmax = maximum switching frequency including tolerance
Vinmax = maximum input voltage
Ioutmin = minimum load current
RDSmin = minimum high-side MOSFET on resistance (See Electrical Characteristics)
RL = series resistance of output inductor
(34)
There is also a maximum achievable output voltage which is limited by the minimum off time. The maximum
output voltage is given by Equation 35
Voutmax
=
Vin
´
çèæ¹
-
Offtimemax
ts
ö
÷ø
-
Ioutmax
´
(RDSmax
+
RI)-
(0.7
-
Ioutmax
´
RDSmax
)´
æ
çè
tdead
ts
ö
÷ø
Where:
Voutmax = maximum achievable output voltage
Vin = minimum input voltage
Offtimemax = maximum off time (90 ns typical for adequate margin)
ts = 1/Fs
Ioutmax = maximum current
RDSmax = maximum high-side MOSFET on resistance (See Electrical Characteristics)
RI = DCR of the inductor
tdead = dead time (60 ns)
(35)
COMPENSATION
There are several industry techniques used to compensate DC/DC regulators. The method presented here is
easy to calculate and yields high phase margins. For most conditions, the regulator has a phase margin between
60 and 90 degrees. The method presented here ignores the effects of the slope compensation that is internal to
the TPS54618. Since the slope compensation is ignored, the actual cross over frequency is usually lower than
the cross over frequency used in the calculations. Use SwitcherPro software for a more accurate design.
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