PRODUCT DATASHEET
AAT2688
TM
SystemPower
4.5A PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters
The critical parameter recommendations for the external
synchronous minimum 25V MOSFET are as follows:
1
Eq. 3:
FZ1
=
=
2 · π · R1 · C5
QG (Total Gate Charge) = 5nC to 15nC (max) (VGS: 4.5V
to 5V)
RDS(ON) = 10mΩ to 30mΩ (max) (VGS: 4.5V to 5V)
1
Eq. 4:
FZ2
FP1
2 · π · (R3 + R5) · C10
1
Channel 1 Input Capacitor Selection
Eq. 5:
=
C5 · C6
C5 + C6
2 · π · R1 ·
For low cost applications, a 220μF/25V electrolytic
capacitor is selected to control the voltage overshoot
across the high side MOSFET. A small ceramic capacitor
with voltage rating at least 1.05 times greater than the
maximum input voltage is connected as close as possible
to the input pin (Pin 14) for high frequency decoupling.
1
Eq. 6:
FP2
=
2 · π · R5 · C10
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 voltage applications as shown in Table 2.
Channel 1 Feedback and
Compensation Networks
C6
VOUT =3.3V
Network
Components
VIN=6V-24V
R4
R3
C10
R5
C5
C6
R1
C4
R2
R6
R7
R8
1.96kΩ
9.09kΩ
2.2nF
150Ω
2.2nF
150pF
3.92kΩ
220nF
2kΩ
C5
C10
R5
VOUT1
R1
Feedback
Feed Forward
Compensation
COMP1
R3
FB1
R4
Current Limit
Open
0
Open
REF
Figure 1: AAT2688 Feedback and Compensation
Networks for Type III Voltage-Mode Control Loop.
Table 2: AAT2688 Feedback, Compensation, and
Current Limit Components for VOUT =3.3V.
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 AAT2688 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
FESR
=
2 · π · L · COUT
1
Eq. 2:
=
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
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:
w w w . a n a l o g i c t e c h . c o m
12
2688.2008.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
