TPS54618RTET Datasheet, PDF (25/39 Pages) Texas Instruments – 2.95-V to 6-V Input, 6-A Output, 2-MHz, Synchronous Step-Down Switcher With Integrated FETs ( SWIFT™)

TPS54618
www.ti.com
SLVSAE9A – NOVEMBER 2010 – REVISED OCTOBER 2012
To get started, the modulator pole, fpmod, and the esr zero, fz1 must be calculated using Equation 36 and
Equation 37. For Cout, the derated capacitance value is 82.5 µF. Use Equation 38 and Equation 39 to estimate a
starting point for the crossover frequency, fc. For the example design, fpmod is 6.43 kHz and fzmod is 643 kHz.
Equation 38 is the geometric mean of the modulator pole and the esr zero and Equation 39 is the mean of
modulator pole and the switching frequency. Equation 38 yields 64.3 kHz and Equation 39 gives 56.7 kHz. The
lower value of Equation 38 or Equation 39 is the maximum recommended crossover frequency. For this example,
a lower fc value of 40 kHz is specified. Next, the compensation components are calculated. A resistor in series
with a capacitor is used to create a compensating zero. A capacitor in parallel to these two components forms
the compensating pole (if needed).
¦p mod =
Iout max
2p ´ Vout ´ Cout
(36)
vertical spacer
1
¦z mod =
2p ´ Resr ´ Cout
(37)
vertical spacer
¦_C = ¦p mod ´ ¦z mod
(38)
vertical spacer
¦_C =
¦p mod ´ ^¦sw
2
(39)
vertical spacer
The compensation design takes the following steps:
1. Set up the anticipated cross-over frequency. Use Equation 40 to calculate the compensation network’s
resistor value. In this example, the anticipated cross-over frequency (fc) is 40 kHz. The power stage gain
(gm_ps) is 25 A/V and the error amplifier gain (gm_ea) is 245 μA/V.
R3 = ^{2p × ¦c ´ Vo ´ Co}
Gm ´ Vref ´ VI_gm
(40)
2. Place compensation zero at the pole formed by the load resistor and the output capacitor. The compensation
network’s capacitor can be calculated from Equation 41.
C4 = ^{Ro ´ Co}
R3
(41)
3. An additional pole can be added to attenuate high frequency noise. In this application, it is not necessary to
add it.
From the procedures above, the compensation network includes a 7.50 kΩ resistor and a 3300 pF capacitor.
APPLICATION CURVES
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