PRODUCT DATASHEET
AAT2687
TM
SystemPower
PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
stability. Equation 3, 4, 5 and 6 relate the compensation
network’s poles and zeros to the components R1, R3,
R5, C5, C6, and C10:
Channel 1 Feedback and
Compensation Networks
C6
1
Eq. 3:
FZ1
=
2 · π · R1 · C5
C5
C10
R5
VOUT1
R1
1
Eq. 4:
FZ1
FP1
=
2 · π · (R3 + R5) · C10
COMP1
1
R3
Eq. 5:
=
C5 · C6
2 · π · R1 ·
C5 + C6
1
FB1
Eq. 6:
FP2
=
R4
2 · π · R5 · C10
REF
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 1.
Figure 1: AAT2687 Feedback and Compensation
Networks for Type III Voltage-Mode Control Loop.
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:
Channel 1 Thermal Protection
The AAT2687 has an internal thermal protection circuit
which will turn on when the device die temperature
exceeds 135°C. The internal thermal protection circuit
will actively turn off the high side regulator output
device to prevent the possibility of over temperature
damage. The Buck regulator output will remain in a
shutdown state until the internal die temperature falls
back below the 135°C trip point. The combination and
interaction between the short circuit and thermal protec-
tion systems allows the Buck regulator to withstand
indefinite short-circuit conditions without sustaining per-
manent damage.
1
Eq. 1:
FLC
=
2 · π · L · COUT
1
Eq. 2:
FESR
=
2 · π · ESR · COUT
The feedback and compensation networks provide a
closed loop transfer function with the highest 0dB cross-
ing frequency and adequate phase margin for system
VOUT =3.3V
VIN = 6V-24V
VOUT = 5.0V
VIN = 6V-24V
Network
Components
R4
R3
C10
R5
C5
C6
R1
C4
R2
R6
R7
R8
1.96kΩ
8.87kΩ
2.2nF
453Ω
2.2nF
150pF
3.92kΩ
220nF
2kΩ
1.96kΩ
8.87kΩ
2.2nF
453Ω
2.2nF
150pF
3.92kΩ
220nF
2kΩ
Feedback
Feed-forward
Compensation
Current Limit
Open
2kΩ
165kΩ
Open
2kΩ
165kΩ
Table 1: AAT2687 Feedback, 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
2687.2008.06.1.0
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