Datasheet

( )
1
C11
2 R8 fc
=
× × ×p
( )
( )
Vout Co
C4
Iout R4
×
=
×
( )
( )
ESR Co
R4
2 C6
×
=
×
2
ea ps
gm Vref gm ESR
C6
c Vout
× × ×
=
× ×fp
1
2
zmod
RESR Co
=
× × ×
f
p
2
Iout
pmod
Vout Co
=
× × ×
f
p
TPS54521
SLVS981C –JUNE 2010–REVISED AUGUST 2013 www.ti.com
Voutmin = minimum achievable output voltage
Ontimemin = minimum controllable on-time (135 nsec maximum)
Fsmax = maximum switching frequency including tolerance
Vinmax = maximum input voltage
Ioutmin = minimum load current
RDS1min = minimum high side MOSFET on resistance (57 mΩ typical)
RDS2min = minimum low side MOSFET on resistance (50 mΩ typical)
RL = series resistance of output inductor (31)
Compensation Component Selection
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 TPS54521. Since the slope compensation is ignored, the actual crossover frequency is usually lower than the
crossover frequency used in the calculations. Use SwitcherPro™ software for a more accurate design.
With the low frequency zero from the Poscap output capacitor adding phase and by using type III compensation
to give an additional phase boost, a high bandwidth, high phase margin design can be realized. This design
targets a crossover frequency (bandwidth) of 1/10
th
of the switching frequency (70 kHz).
First, the modulator pole, fpmod, and the ESR zero, fzmod, must be calculated using Equation 32 and
Equation 33. They are at 723 Hz and 18.1 kHz, respectively.
(32)
(33)
Now the compensation components can be calculated. First, calculate the value for C6 for a crossover frequency
of 70 kHz. Using Equation 34, the nearest standard value for C6 is 220 pF.
(34)
Along with C6, R4 creates a pole to cancel the gain caused by the ESR zero of the power stage, fzmod. To keep
some of the phase from the zero, this pole is placed at roughly twice the frequency of the zero. The value of R4
needed to set the pole at the desired frequency is given by Equation 35.
(35)
Next calculate the value of C4. Together with R4, C4 places a compensation zero at the modulator pole
frequency, fpmod. Use Equation 36 to determine the value of C4.
(36)
Using Equation 35 and Equation 36, the standard values for R4 and C4 are 20 kΩ and 0.01 µF.
In order to provide a zero around the crossover frequency to boost the phase at crossover, a feedforward
capacitor (C11) is added in parallel to R8. The value of this capacitor is given by Equation 37.
(37)
The closest standard value is 47 pF.
Use of the feedforward capacitor, C11, creates a low AC impedance path from the output voltage to the VSENSE
input of the IC that can couple noise at the switching frequency into the control loop. Use of a feedforward
capacitor is not recommended for high output voltage ripple designs (greater than 15 mV peak to peak at the
VSENSE input) operating at duty cycles of less than 30%. When using the feedforward capacitor, C11, always
limit the closed loop bandwidth to no more than 1/10
th
of the switching frequency, fsw.
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