ADP3209 ON Semiconductor, ADP3209 Datasheet - Page 28

• Manufacturer Part Number: ADP3209
• Description: 5-bit, Programmable, Single-phase, Synchronous Buck Controller
• Manufacturer: ON Semiconductor

ADP3209
With the multimode feedback structure of the ADP3209, it is
necessary to set the feedback compensation so that the
converter’s output impedance works in parallel with the output
decoupling. In addition, it is necessary to compensate for the
several poles and zeros created by the output inductor and
decoupling capacitors (output filter).
A Type III compensator on the voltage feedback is adequate
for proper compensation of the output filter. Figure 36 shows the
Type III amplifier used in the ADP3209. Figure 37 shows the
locations of the two poles and two zeros created by this amplifier.
The following equations give the locations of the poles and
zeros shown in Figure 37:
f
f
f
f
Z
Z
P
P
GAIN
1
2
1
2
=
=
=
=
Figure 37. Poles and Zeros of Voltage Error Amplifier
π 2
2
π 2
π 2
( π
0dB
×
×
×
–20dB/DEC
C
3
C
C
C
R
A
VOLTAGE ERROR
1
COMP
A
FB
A
1
f
A
Figure 36. Voltage Error Amplifier
+
P1
R
×
×
+
AMPLIFIER
A
×
1
C
C
R
C
R
C
B
A
B
B
FB
)
B
×
×
C
R
C
A
f
FB
A
Z2
2
f
FB
Z1
R
C
–20dB/DEC
FB
REFERENCE
FB
VOLTAGE
f
P2
ADP3209
FREQUENCY
VOLTAGE
OUTPUT
Rev. 2 | Page 28 of 32 | www.onsemi.com
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The expressions that follow compute the time constants for the
poles and zeros in the system and are intended to yield an
optimal starting point for the design; some adjustments may be
necessary to account for PCB and component parasitic effects
(see the Tuning Procedure for ADP3209 section):
where:
R' is the PCB resistance from the bulk capacitors to the ceramics
and is approximately 0.4 mΩ (assuming an 8-layer motherboard).
R
A
V
L
Panasonic SP capacitors).
The compensation values can be calculated as follows:
The standard values for these components are subject to the
tuning procedure described in the Tuning Procedure for
ADP3209 section.
X
DS
D
RT
is the ESL of the bulk capacitors (450 pH for the two
is 5.
is the total low-side MOSFET for on resistance.
is 1.25 V.
T
T
T
T
C
C =
C
R
R =
C
A
B
D
E
A
FB
A
B
=
=
=
=
=
=
=
2
(
C
V
R
C
C
R
T
T
R
R
R
×
T
R
RT
O
X
C
B
X
X
O
A
B
E
D
A
L
×
+
×
×
+
×
×
C
×
(
A
(
T
R
R
R
⎛
⎜
⎜
⎝
C
V
X
1 (
R
L
A
D
O
'
VID
B
X
−
×
O
−
−
×
−
R
×
R
−
(
O
×
C
R
R
A
n
O
R
2
)
R
DS
'
D
×
Z
)
×
×
×
'
)
+
E
×
×
D
V
C
+
+
f
L
R
R
SW
R
))
VID
C
X
R
DS
X
O
O
2
Z
×
L
V
×
×
V
⎞
⎟
⎟
⎠
×
VID
R
R
RT
V
O
O
R
RT
−
X
R
+
'
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