sc483itstrt Semtech Corporation, sc483itstrt Datasheet - Page 21

• Manufacturer Part Number: sc483itstrt
• Description: Sc483 Dual Synchronous Buck Pseudo-fixed-frequency Power-supply Controller
• Manufacturer: Semtech Corporation

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 Figure 13 placement of 0
each output.
Connecting the control and power sections should be accomplished as follows (see Figure 14 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.
5) Locate the current limit sense resistors between the LX and ILIM pins at the device.
Phase nodes (black) to be copper islands (preferred) or wide copper traces. 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.
POWER MANAGEMENT
2005 Semtech Corp.
Figure 14: Connecting Control and Power Sections
22
23
24
25
26
27
28
10
11
12
13
14
8
9
U1
EN/PSV1
TON1
VOUT1
VCCA1
FB1
PGD1
VSSA1
EN/PSV2
TON2
VOUT2
VCCA2
FB2
PGD2
VSSA2
resistor at the bottom of the output capacitor to connect to VSSA1/2 for
SC483
PGND1
PGND2
VDDP1
VDDP2
ILIM1
ILIM2
BST1
BST2
DH1
DH2
DL1
DL2
LX1
LX2
7
6
5
4
3
2
1
21
20
19
18
17
16
15
21
R5
0402
R10 7k87
0402
IRF7811AV
FDS6676S
Q2
Q1
Q3
Q4
L1
L2
2u2
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SC483