sc480imltrt Semtech Corporation, sc480imltrt Datasheet - Page 18

• Manufacturer Part Number: sc480imltrt
• Description: Sc480 Complete Ddr1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

Thermal Considerations
The junction temperature of the device may be calculated
as follows:
where T
temperature, PD is the total SC480 device dissipation,
The SC480 device dissipation can be determined using:
The fi rst two terms are losses for the analog and gate drive
circuits and generally do not present a thermal problem.
Typical ICCA (VCCA operating current) is roughly 1.5mA,
which creates 7.5mW loss from the 5V VCCA supply. The
VDDP supply current is used to drive the MOSFETs and
can be much higher, on the order of 30mA, which can
create up to 150mW of dissipation.
The last term, VTT * |ITT|, is the most signifi cant term
from a thermal standpoint. The VTT regulator is a linear
device and will dissipate power proportional to the VTT
current and the voltage drop across the regulator. If VTT
= VDDQ/2, then the voltage drop across the regulator
is always VDDQ2, regardless of whether the regulator is
sinking or sourcing current. In either case the power lost
in the VTT regulator is VTT * |ITT|. The average or long-
term value for ITT should be used.
The thermal resistance of the MLPQ package is affected
by PCB layout and the available ground planes and vias
which conduct heat away. A typical value is 29°C/watt.
Example:
P
P
T
© 2006 Semtech Corp.
POWER MANAGEMENT
POWER MANAGEMENT
Application Information (Cont.)
J
D
D
= T
= 0.808W
= 5V • 0.0015 A + 5V • 0.025A + 0.9V • |0.75|A
PD = VCCA • ICCA + VDDP • IDDP + VTT • |ITT|
AMB
J
is the junction temperature, T
+ P
D
• T
T
ICCA = 1.5mA
VCCA = VDDP = 5V
VTT = 1.25V
ITT = 0.75A (average)
Ambient = 45 degrees C
Thermal resistance = 29
J
= T
JA
= 45 + 0.808W • 29°C/W = 68.4°C
AMB
+ θ
JA
AMB
IDDP = 25mA
is the ambient
18
Layout Guidelines
One (or more) ground planes are recommended to
minimize the effect of switching noise and copper losses,
and maximize heat dissipation. The IC ground reference,
VSSA, should be connected to PGND1 and PGND2 as a
star connection at the thermal pad, which in connects
using 4 vias to the ground plane. All components that are
:
referenced to VSSA should connect to it directly on the
chip side, and not through the ground plane.
VDDQ: The feedback trace must be kept far away from
noise sources such as switching nodes, inductors and
gate drives. Route the feedback trace in a quiet layer if
possible, from the output capacitor back to the chip. Chip
supply decoupling capacitors (VCCA, VDDP) should be
located next to the pins (VCCA/VSSA, VDDP/PGND1) and
connected directly to them on the same side.
VTT: Because of the high bandwidth of the VTT regulator,
proper component placement and routing is essential to
prevent unwanted high-frequency oscillations which can
be caused by parasitic inductance and noise. The input
capacitors should be located at the VTT input pins (VTTIN
and PGND2), as close as possible to the chip to minimize
parasitics. Output capacitors should be directly located at
the VTT output pins (VTT and PGND2). The routing of the
feedback signal VTTS is critical. The trace from VTTS (pin
2) should be connected directly to the output capacitor
that is farthest from VTT (pin24); route this signal away
from noise sources such as the VDDQ power train or high-
speed digital signals.
The switcher power section should connect directly to the
ground plane(s) using multiple vias as required for current
handling (including the chip power ground connections).
Power components should be placed to minimize loops
and reduce losses. Make all the connections on one side
of the PCB using wide copper fi lled areas if possible. Do
not use “minimum” land patterns for power components.
Minimize trace lengths between the gate drivers and the
gates of the MOSFETs to reduce parasitic impedances
(and MOSFET switching losses); the low-side MOSFET is
most critical. Maintain a length to width ratio of <20:1 for
gate drive signals. Use multiple vias as required by current
handling requirement (and to reduce parasitics) if routed
on more than one layer. Current sense connections must
always be made using Kelvin connections to ensure an
www.semtech.com
SC480