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
= 60 pF
1
C
C1
=
Sf
LC
R
C1
= 1.66 nF
R
C1
=
f
CROSSOVER
f
LC
'
V
RAMP
V
IN
R
FB1
=
80 kHz
17.4 kHz
10 k:
1.2 V
5.0 V
= 11 k:
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Table 3. Compensation Value Parameters
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
1.2V
f
SW
500 kHz
where ΔV
RAMP
is the oscillator peak-to-peak ramp voltage (nominally 0.8 V, however, 1.2V was used here
to cover a broad frequency and input voltage combination range), 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
SNVA480A–May 2011–Revised May 2013 AN-2140 LM21212-2 Evaluation Board
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