PRODUCT DATASHEET
AAT2688
TM
SystemPower
4.5A PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters
5. Connect unused signal pins to ground or input to
Layout Considerations
avoid unwanted noise coupling.
The suggested PCB layout for the AAT2688 is shown in
Figures 5 through 8. The following guidelines should be
used to help ensure a proper layout.
6. The critical small signal components include feed-
back components, and compensation components
should be placed close to the FB1 and COMP1 pins.
The feedback resistors should be located as close as
possible to the FB1 pin with its ground tied straight
to the signal ground plane which is separated from
power ground plane.
7. C4 should be connected close to the RS1 and OS1
pins, while R2 should be connected close to the
inductor.
1. The power input capacitors (C1 and C15) should be
connected as close as possible to high voltage input
pin (IN1) and power ground.
2. C1, L1, Q1, C7, C8, and C9 should be place as close
as possible to minimize any parasitic inductance in
the switched current path which generates a large
voltage spike during the switching interval. The con-
nection of inductor to switching node should be as
short as possible.
8. R7 should be connected directly to the output pin of
inductor L1 to sense precisely its DCR.
9. For good thermal coupling, a 4-layer PCB layout is
recommended and PCB vias are required from the
exposed pad (EP) for the TQFN45-24 paddle to the
middle plans and bottom plane. The EP is internally
connected to IN.
3. The feedback trace or FB1 pin should be separated
from any power trace and connected as close as
possible to the load point. Sensing along a high-
current load trace will degrade DC load regulation.
4. The resistance of the trace from the load returns to
PGND should be kept to a minimum. This will help to
minimize any error in DC regulation due to differ-
ences in the potential of the internal signal ground
and the power ground.
VOUT1
3.3V/4.5A
U1
L1
4.7μH
1
24
C3
0.1μF
LX1
LX1
LX1
LX1
DL
C10
23
20
2
5
2.2nF
R3
C4
220nF
R2
2k
D1
BAS16
9.09k
R5
150
Q1
BST1
VL1
R6
open
2
1
D1
18
19
16
13
14
15
J1
FB1
RS1
C7
22μF
C8
C9
22μF 22μF
C6
150pF
R1
VIN1
6.0V - 24.0V
VAL1AT2688
R7
0
3.92K
R4
1.96k
22
3
IN
OS1
N/C
EP
C5
2.2nF
COMP1
OUT2
N/C
C14
2.2μF
4
6
11
12
17
7
N/C
EN1
IN2
VOUT2
1.8V/0.6A
3
2
1
R8
open
10
VIN2
EN1
GND
C12
2.2μF
C11
open
C1
220μF
25V
C13
1μF
25V
9
8
+
N/C
EN2
GND2
N/C
3
2
C15
open
C2
2.2μF
21
1
TQFN45-24
EN2
U1
C1
AAT2688 Analogic Technologies, Hi-Voltage Buck/LDO, TQFN45-24
Cap, MLC, 220μF/25V, Electrolytic cap
Cap, MLC, 2.2μF, 6.3V, 0805
C2, C12, C14
C3
C4
Cap, MLC, 0.1μF/6.3V, 0603
Cap, MLC, 220nF/6.3V, 0402
C5, C6, C10, C11
Cap, MLC, misc, 0603
C7, C8, C9
Cap, MLC, 22μF/10V, 1206
C13
D1
Q1
L1
Cap, MLC, 1μF, 25V, 0805
BAS16, Generic, Rectifier, 0.2A/85V, Ultrafast, SOT23
SOP8 MOSFET, Si4686DY, Vishay or FDS8884, Fairchild
RCH108NP-4R7M, Sumida, 4.7μH, ISAT = 5.7A, DCR = 11.7mꢀ; Unshielded
or Wurth 744 771 004, 4.7μH, ISAT
Carbon film resistor, 0402
= 6.8A, DCR = 11mꢀ, Shielded
R1-R5
Figure 4: AAT2688 Evaluation Board Schematic for VIN = 6V-24V and VOUT = 3.3V.
w w w . a n a l o g i c t e c h . c o m
2688.2008.06.1.0
15
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