Datasheet
V = 100 mV / div (dc coupled, -3.13 V offset)
OUT
I = 2 A / div
OUT
Load step = 1.5 A to 4.5 A Slew rate = 100 mA / µsec
Time = 200 µsec / div
V = 5 V / div
IN
V = 1 V / div
OUT
Time = 2 msec / div
×
=
RESR Cout
C5
R4
×
=
×
Vout Cout
C4
Iout R4
2 fp × × ×
=
× ×
ea ps
c Vout Cout
R4
gm Vref gm
2
f
f f= ×
sw
co pmod
f f f= ×co pmod zmod
1
2
f
p
=
× × ×
zm od
RESR C out
2
f
p
=
× × ×
Iout
pmod
Vout Cout
TPS54622
SLVSA70B –MARCH 2011–REVISED JANUARY 2014
www.ti.com
First, the modulator pole, fpmod, and the esr zero, fzmod must be calculated using Equation 31 and Equation 32.
For Cout, use a derated value of 75 µF. use Equation 33 and Equation 34 to estimate a starting point for the
closed loop crossover frequency fco. Then the required compensation components may be derived. For this
design example, fpmod is 3.86 kHz and fzmod is 707.4 kHz. Equation 33 is the geometric mean of the modulator
pole and the esr zero and Equation 34 is the geometric mean of the modulator pole and one half the switching
frequency. Use a frequency near the lower of these two values as the intended crossover frequency fco. In this
case Equation 33 yields 52.2 kHz and Equation 34 yields 30.4 kHz. The lower value is 30.4 kHz. A slightly higher
frequency of 30 kHz is chosen as the intended crossover frequency.
(31)
(32)
(33)
(34)
Now the compensation components can be calculated. First calculate the value for R2 which sets the gain of the
compensated network at the crossover frequency. Use Equation 35 to determine the value of R2.
(35)
Next calculate the value of C3. Together with R2, C3 places a compensation zero at the modulator pole
frequency. Equation 36 to determine the value of C3.
(36)
Using Equation 35 and Equation 36 the standard values for R4 and C4 are 3.74 kΩ and 0.01 µF.
An additional high frequency pole can be used if necessary by adding a capacitor in parallel with the series
combination of R4 and C4. The pole frequency can be placed at the ESR zero frequency of the output cpacitor
as given by Equation 13. Use Equation 37 to calculate the required capacitor value for C5.
(37)
Application Curves
LOAD TRANSIENT STARTUP with VIN
Figure 30. Figure 31.
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