sc488mltrt Semtech Corporation, sc488mltrt Datasheet - Page 17

• Manufacturer Part Number: sc488mltrt
• Description: Sc488 Complete Ddr1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

Finally, we calculate the current limit resistor value. As de-
scribed in the current limit section, the current limit looks
at the “valley current”, which is the average output cur-
rent minus half the ripple current.
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under nor-
mal operating conditions.
For our example:
Thermal Considerations
The junction temperature of the device may be calculated
as follows:
where T
temperature, PD is the total SC488 device dissipation. The
SC488 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
POWER MANAGEMENT
Application Information (Cont.)
© 2006 Semtech Corp.
I
VALLEY
PD = VCCA • ICCA + VDDP • IDDP + VTT • |ITT|
R
ILIM
= 8.31A, R
I
T
J
VALLEY
J
is the junction temperature, T
= T
AMB
I
VALLEY
+ θ
I
DS(ON)
OUT
JA
= 4mΩ, giving R
1.2
I
RIPPLE_VBA
R
DS
10
2
(ON)
10
( T
MIN
ILIM
AMB
6
1.4
)
= 5.62kΩ
is the ambient
A
Ohms
17
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
PD = 0.808W
T
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
J
D
= T
= 5V • 0.0015 A + 5V • 0.025A + 0.9V • |0.75|A
AMB
+ P
D
• T
JA
ICCA = 1.5mA
VCCA = VDDP = 5V
VTT = 1.25V
ITT = 0.75A (average)
Ambient = 45 degrees C
Thermal resistance = 29
= 45 + 0.808W • 29°C/W = 68.4°C
IDDP = 25mA
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SC488