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RC5031
PRODUCT SPECIFICATION
10
P
Table 4 lists recommended values for sense resistors for vari-
ous load currents using an embedded PC trace resistor or a
discrete resistor.
Table 4. R
SENSE
for Various Load Currents,
Switching Regulator
Since the value of the sense resistor is often less than 10m
W
,
care should be taken in the layout of the PCB. Trace resis-
tance can contribute significant errors. The traces to the
IFBH and IFBL pins of the RC5031 should be Kelvin con-
nected to the pads of the current-sense resistor. To minimize
the influence of noise, the two traces should be run next to
each other.
Thermal Design Considerations
Good thermal management is critical in the design of high
current regulators. System reliability will be degraded if the
component temperatures become excessive. The following
guide should serve as a reference for proper thermal manage-
ment.
MOSFET Temperature
The maximum power dissipation of the MOSFET can be cal-
culated by using the following formula:
For IR 3103,
Q
JA
is 42
°
C/W. For reliability the junction tem-
perature of the MOSFET should not exceed 120
°
C. Assum-
ing that the ambient temperature is 40
°
C, then the maximum
power dissipation is calculated as:
The power that the MOSFET dissipates at the rated 6A load
is calculated as follows:
where V
D
is the forward voltage of the Schottky diode used.
I
LOAD, MAX
(A)
5
6
7
8
9
10
R
SENSE
PC Trace Resistor
(m
W
)
8.7
7.6
6.7
6.0
5.4
4.9
R
SENSE
Discrete Resistor
(m
W
)
11.7
10.1
8.9
8.0
7.0
6.6
P
D
T
----------------------------------
T
A
–
Q
JA
=
P
D
120
40
–
42
1.905W
=
=
P
MOSFET
V
I
LOAD
2
R
DS ON
I
)
Duty Cycle
(
)
-------------6
t
r
t
f
+
(
)
f
′
′
+
′
′
=
Duty Cycle
V
(
–
D
V
IN
V
D
I
LOAD
R
DS ON
)
′
)
+
----------------------------------------+
=
Using the above formula, for V
out
= 2.8V, I
LOAD
= 6A
Since the power at 6A is within the thermal guideline, a heat
sink is not required other than the PCB.
Schottky Diode
In Figure 11, MOSFET M1 and flyback diode DS1 are used
as complementary switches in order to maintain a constant
current through the output inductor L1. As a result, DS1 will
have to carry the full current of the output load when the
power MOSFET is turned off. The power in the diode is a
direct function of the forward voltage at the rated load cur-
rent during the off time of the FET. The following equation
can be used to estimate the diode power:
where I
D
is the forward current of the diode, V
D
is the forward
voltage of the diode, and DutyCycle is defined the same as
above.
For the Motorola MBR2030CTL Rectifier in Figure 11,
It is recommended that the diode T0-220 package be placed
down on the motherboard to utilize the power plane as a
heatsink and achieve a thermal resistance of 40
°
C/W.
Board Design Considerations
RC5031 Placement
The RC5031 should be placed as close to the core voltage
supply pins of the P55C as possible, preferably to have the
PC layer directly underneath the RC5031 for ground layer.
This serves as extra isolation from noisy power planes.
MOSFET Placement
Placement of the power MOSFET is critical in the design of
the switch-mode regulator. The FET should be placed in
such a way as to minimize the length of the gate drive path
from the RC5031 SDRV pin. This trace should be kept under
0.5" for optimal performance. Excessive lead length on this
trace will cause high frequency noise resulting from the par-
asitic inductance and capacitance of the trace. Since this
voltage can transition nearly 12V in around 100nsec, the
resultant ringing and noise will be very difficult to suppress.
This trace should be routed on one layer only and kept well
away from the “quiet” analog pins of the device; VREF,
CEXT, FBSW, IFBH, IFBL, and VFBL. A10
W
resistor in
series with the MOSFET gate can decrease this layout
critically. Refer to Figure 12.
Duty Cycle
0.57
6
(
+
–
0.019
′
)
+
2.8
61.8%
=
=
P
MOSFET
6A
2
0.019
W
61.8%
5V
6A
′
6
210ns
54ns
+
)
300KHz
′
′
+
′
′
=
P
MOSFET
0.82W
P
DIODE
I
D
V
D
1
DutyCycle
–
(
)
′
′
=
P
DIODE
10A
0.57
1
64.8%
–
(
)
′
′
2.0W
=
=