sc1486aitstr Semtech Corporation, sc1486aitstr Datasheet - Page 22

• Manufacturer Part Number: sc1486aitstr
• Description: Complete Ddr1/2/3 Power Supply Controller
• Manufacturer: Semtech Corporation

Layout Guidelines (Cont.)
Key points for the power section:
1) there should be a very small input loop, well decoupled.
2) the phase node should be a large copper pour, but compact since this is the noisiest node.
3) input power ground and output power ground should not connect directly, but through the ground planes instead.
4) The two outputs should not share their input capacitors, and these should have separate PWR_SRC and PGND
(component-side) copper pours.
5) The two output inductors should not be placed adjacent to each other to avoid crosstalk.
6) Notice in Figures 10 and 11 on the previous page placement of 0 resistor at the bottom of the output capacitor
to connect to VSSA1/2 for each output.
Connecting the control and power sections should be accomplished as follows (see Figure 16 below):
1) Route VSSA1/2 and their related feedback traces as differential pairs routed in a “quiet” layer away from noise
sources.
2) Route DL, DH and LX (low side FET gate drive, high side FET gate drive and phase node) to chip using wide traces
with multiple vias if using more than one layer. These connections to be as short as possible for loop minimization,
with a length to width ratio less than 20:1 to minimize impedance. DL is the most critical gate drive, with power
ground as its return path. LX is the noisiest node in the circuit, switching between PWR_SRC and ground at high
frequencies, thus should be kept as short as practical. DH has LX as its return path.
3) BST is also a noisy node and should be kept as short as possible.
4) Connect PGND pins on the chip directly to the VDDP decoupling capacitor and then drop vias directly to the
ground plane.
POWER MANAGEMENT
Application Information (Cont.)
2006 Semtech Corp.
PHASE NODES (BLACK) TO BE COPPER ISLANDS (PREFERRED) OR WIDE COPPER TR
GATE DRIVE TRACES (RED) AND PHASE NODE TRACES (BLUE) TO BE WIDE COPPER
TRACES (L:W < 20:1) AND AS SHORT AS POSSIBLE, WITH DL THE MOST CRITICAL
L1
2u4
8
7
6
5
8
7
6
5
IRF7811AV
FDS6676S
D
D
D
D
D
D
D
D
Figure 16: Connecting Control and Power Sections
G
G
S
S
S
S
S
S
Q2
Q3
1
2
3
4
1
2
3
4
27
22
24
26
23
25
28
3
1
6
7
5
4
2
U1
VDDP1
PGND1
DH1
BST1
LX1
ILIM1
DL1
PGD1
EN/PSV1
VOUT1
FB1
TON1
VCCA1
VSSA1
22
SC1486A
REFOUT
PGND2
VSSA2
VDDP2
PGD2
VCCA2
REFIN
TON2
ILIM2
FB2
BST2
DH2
DL2
LX2
17
15
20
21
19
18
16
13
12
8
10
9
11
14
FDS6982S
Q1
4
3
8
2
5
6
1
7
L2
3u9
SC1486A
www.semtech.com