Datasheet
1
2Sf
ESR
R
C2
C
C3
=
= 898 pF
f
ESR
- f
LC
R
C2
=
R
FB1
f
LC
= 166:
C
C2
=
C
C1
Sf
SW
R
C1
C
C1
-1
= 90 pF
1
C
C1
=
Sf
LC
R
C1
= 2.49 nF
R
C1
=
f
CROSSOVER
f
LC
'
V
RAMP
V
IN
R
FB1
=
80 kHz
17.4 kHz
10 k:
0.8 V
5.0 V
= 7.4 k:
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Table 3. Parameters Needed for Compensation Values
Parameter Value
V
IN
5.0V
V
OUT
1.2V
I
OUT
12A
f
CROSSOVER
80 kHz
L 0.56 µH
R
DCR
1.8 mΩ
C
O
150 µF
R
ESR
1.0 mΩ
ΔV
RAMP
0.8V
f
SW
500 kHz
where ΔV
RAMP
is the oscillator peak-to-peak ramp voltage (nominally 0.8 V), f
CROSSOVER
is the frequency at
which the open-loop gain is a magnitude of 1, R
DCR
is the effective DC resistance of the inductor, R
ESR
is
the effective resistance of the output capacitor, and C
O
is the effective output capacitance at the
programmed output voltage. It is recommended that f
CROSSOVER
not exceed one-fifth of the switching
frequency. The output capacitance, C
O
, depends on capacitor chemistry and bias voltage. For Multi-Layer
Ceramic Capacitors (MLCC), the total capacitance will degrade as the DC bias voltage is increased.
Measuring the actual capacitance value for the output capacitors at the output voltage is recommended to
accurately calculate the compensation network. Note that it is more conservative, from a stability
standpoint, to err on the side of a smaller output capacitance value in the compensation calculations
rather than a larger, as this will result in a lower bandwidth but increased phase margin.
First, the value of R
FB1
should be chosen. A typical value is 10kΩ. From this, the value of R
C1
can be
calculated to set the mid-band gain so that the desired crossover frequency is achieved.
(7)
Next, the value of C
C1
can be calculated by placing a zero at half of the LC double pole frequency.
(8)
Now the value of C
C2
can be calculated to place a pole at half of the switching frequency.
(9)
R
C2
can then be calculated to set the second zero at the LC double pole frequency.
(10)
Last, C
C3
can be calculated to place a pole at the same frequency as the zero created by the output
capacitor ESR.
(11)
The standard values used for the above calculations are given in the Bill of Materials.
9
SNVA467B–February 2011–Revised May 2013 AN-2107 LM21212-1 Evaluation Board
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