SC480 SEMTECH [Semtech Corporation], SC480 Datasheet - Page 19

• Manufacturer Part Number: SC480
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: SEMTECH [Semtech Corporation]

accurate signal. The layout can be generally considered
in three parts; the control section referenced to VSSA, the
VTT output, and the switcher power section.
Looking at the control section fi rst, locate all components
referenced to VSSA on the schematic and place these
components at the chip. Connect VSSA using a wide
(>0.020”) trace. Very little current fl ows in the chip ground
therefore large areas of copper are not needed. Connect
the VSSA pin directly to the thermal pad under the device
as the only connection from PGND1 and PGND2 from
VSSA.
Decoupling capacitors for VCCA/VSSA and VDDP/PGND1
should be placed is as close as possible to the chip. The
feedback components connected to FB, along with the
VDDQ sense components, should also be located at the
chip. The feedback trace from the VDDQ output should
route from the top of the output capacitors, in a quiet
layer back to the FB components.
Next, looking at the switcher power section, there are a
few key guidelines to follow:
1.
2.
3.
POWER MANAGEMENT
Application Information (Cont.)
© 2006 Semtech Corp.
There should be a very small input loop, well
decoupled.
The phase node should be a large copper pour, but
still compact since this is the noisiest node.
Input power ground and output power ground should
not connect directly, but through the ground planes
instead.
19
Finally, connecting the control and switcher power sections
should be accomplished as follows:
1.
2.
3.
4.
Route VDDQ feedback trace in a “quiet” layer, away
from noise sources.
Route DL, DH and LX (low side FET gate drive, high
side FET gate drive and phase node) to the chip using
wide traces with multiple vias if using more than one
layer. These connections are 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 VBAT and ground at high frequencies, thus
should be kept as short as practical. DH has LX as its
return path.
BST is also a noisy node and should be kept as short
as possible.
Connect PGND1 pins on the chip directly to the VDDP
decoupling capacitor and then drop vias directly to
theground plane. Locate the current limit resistor at
the chip with a kelvin connection to the phase node.
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SC480