PRODUCT DATASHEET
AAT2689
TM
SystemPower PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters
The feedback and compensation networks provide a
closed loop transfer function with the highest 0dB cross-
ing frequency and adequate phase margin for system
Channel 1 Feedback and
Compensation Networks
C3
stability. Equation 3, 4, 5 and 6 relate the compensation
network’s poles and zeros to the components R1, R3,
R5, C2, C3, and C8:
C2
C8
VOUT1
R1
R5
COMP1
1
R3
Eq. 3:
FZ1
=
=
2 · π · R1 · C2
FB1
R4
1
Eq. 4:
Eq. 5:
FZ2
FP1
2 · π · (R3 + R5) · C8
REF
Figure 1: AAT2689 Feedback and Compensation
Networks for Type III Voltage-Mode Control Loop.
1
=
C2 · C3
C2 + C3
2 · π · R1 ·
The transfer function of the error amplifier is dominated
by the DC gain and the L COUT output filter of the regula-
tor. This output filter and its equivalent series resistor
(ESR) create a double pole at FLC and a zero at FESR in the
following equations:
1
Eq. 6:
FP2
=
2 · π · R5 · C8
Components of the feedback, feed-forward, compensa-
tion, and current limit networks need to be adjusted to
maintain the systems stability for different input and
output voltages applications as shown in Table 2.
1
Eq. 1:
FLC
=
2 · π · L · COUT
1
Eq. 2:
FESR
=
2 · π · ESR · COUT
VOUT = 3.3V
VIN = 12V fixed
VOUT = 3.3V
VIN = 6V to 24V
VOUT = 5.0V
VIN = 6V to 24V
Network
Components
R3
R4
C8
R5
C2
C3
R1
C7
R2
R6
R7
R8
6.04kΩ
27.4kΩ
470pF
1.37kΩ
1nF
56pF
11.8kΩ
22nF
10kΩ
Open
11kΩ
600kΩ
6.04kΩ
27.4kΩ
330pF
499Ω
470pF
56pF
24.3kΩ
220nF
2kΩ
6.04kΩ
44.2kΩ
330pF
499Ω
220pF
56pF
24.3kΩ
220nF
2kΩ
Feedback
Feed-forward
Compensation
Current Limit
Open
2kΩ
133kΩ
Open
2kΩ
133kΩ
Table 2: AAT2689 Feedback, Feed-Forward, Compensation, and Current Limit Components
for VOUT = 3.3V and VOUT = 5.0V.
w w w . a n a l o g i c t e c h . c o m
12
2689.2008.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
