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RC5037
PRODUCT SPECIFICATION
8
P
Switching Regulator
where Rx: R8, R9, R10, and R11.
For example, for 2.8V:
Short Circuit Considerations
The RC5037 uses a current sensing scheme to limit the load
current if an output fault condition occurs. The current sense
resistor carries the peak current of the inductor, which is
greater than the maximum load current due to ripple current
flowing in the inductor. The RC5037 will begin to limit the
output current to the load by reducing the duty cycle of the
top-side FET driver when the voltage across the current-
sense resistor exceeds the short circuit comparator threshold
voltage (V
th
). When this happens the output voltage will
temporarily go out of regulation. As the voltage across the
sense resistor becomes larger, the duty cycle of the top-side
MOSFET will continue to be reduced until the current limit
value is reached. At this point, the RC5037 will continuously
deliver the limit current at a reduced output voltage level.
The short circuit comparator threshold voltage is typically
90mV, with a tolerance of
±
10mV. The ripple current flowing
through the inductor in Figure 10 is 0.6A. There needs to be
a 29% margin for the sense resistor when using a mother-
board PC trace resistor. Refer to Application Note 48 for
detailed discussions. The sense resistor value can be approx-
imated as follows:
Where TF = Tolerance Factor for the sense resistor and 0.6A
accounts for the inductor ripple current.
There are several different types of sense resistors. Table 3
describes the tolerance, size, power capability, temperature
coefficient and cost of various types of sense resistors.
Based on the Tolerance in Table 3:
For an embedded PC trace resistor:
For a discrete Copel resistor:
Table 4 lists recommended values for sense resistors for var-
ious load currents using an embedded PC trace resistor or a
discrete resistor. If the calculated value is not available,
round down.
Table 4. R
SENSE
for Various Load Currents,
Switching Regulator
V
OUT
1.5
Rx
R7
+
R7
è
′
=
V
OUT
1.5
R11
R7
+
R7
è
′
1.5
1.74k
2.0k
+
2.0k
è
′
2.8V
=
=
=
I
LOAD, MAX
(A)
3
R
SENSE
PC Trace Resistor
(m
W
)
15.8
R
SENSE
Discrete Resistor
(m
W
)
20.0
R
SENSE
V
I
PK
----------------
1
TF
–
(
)
′
V
I
LOAD,MAX
+
0.6A
---------------------------------------------
1
TF
–
(
)
′
=
=
R
SENSE
LOAD,MAX
+
0.6
.71
′
=
R
SENSE
I
LOAD,MAX
0.6
+
--------------0.08
.90
′
=
Table 3. Comparison of Sense Resistors
Motherboard
Trace Resistor
±
29%
Discrete
Iron Alloy
resistor
(IRC)
±
5%
(
±
1% available)
0.45" x 0.065" x
0.2"
1 watt (3 and 5
watts available)
+30 ppm
Discrete
Metal Strip
surface mount
resistor
(Dale)
±
1%
Discrete
MnCu Alloy
wire
resistor
±
10%
Discrete
CuNi Alloy
wire resistor
(Copel)
±
10%
Tolerance Factor
(TF)
Size
(L x W x H)
Power capability
2" x 0.2" x 0.001”
(1 oz Cu trace)
>50A/in
0.25" x 0.125" x
0.025"
1 watt (3 and 5
watts available)
±
75 ppm
0.2" x 0.04" x
0.16"
1 watt
0.2" x 0.04" x
0.1"
1 watt
Temperature
Coefficient
Cost@10,000
piece quantity
+4,000 ppm
±
30 ppm
±
20ppm
Low; included in
motherboard
$0.31
$0.47
$0.09
$0.09