
RC5050
PRODUCT SPECIFICATION
14
Based on the Tolerance in the above table:
For an embedded PC trace resistor
and
I
load,max
= 14.5A:
V
3.0A
+
For a discrete resistor and I
load, max
= 14.5A:
V
3.0A
I
Load, max
+
For user convenience, Table 8 lists the recommended values
for sense resistor values at various load currents using an
embedded PC trace resistor or discrete resistor.
Table 8. R
sense
for Various Load Currents
RC5050 Short Circuit Current Characteristics
The RC5050 has a short circuit current characteristic that
includes a foldback function with hysteresis that prevents the
DC-DC converter from oscillating in the event of a short cir-
cuit. A typical V-I characteristic of the DC-DC converter
output using a sense resistor value of 6m
W
is presented in the
Typical Operating Characteristics section, page 5. The con-
verter performs with a typical voltage regulation characteris-
tic until the voltage across the resistor exceeds the internal
short circuit comparator threshold of 120mV. At this point,
the internal comparator trips and sends a signal to the con-
troller to turn off the gate drive to the power MOSFET.
This causes a drastic reduction in the output voltage as the
load regulation collapses into the short circuit control
mode. The output voltage will not return to the normal load
characteristic until the output short circuit current is reduced
to within the safe range for the DC-DC converter.
Schottky Diode Selection
The application circuit of Figure 1 shows a Schottky diode,
DS1. DS1 is used as a flyback diode to provide a constant
current path for the inductor when M1 is turned off. A vital
selection criteria for DS1 is that it exhibits a very low for-
ward voltage drop, as this parameter will directly impact the
regulator efficiency as the output voltage is reduced. Table 9
presents several suitable Schottky diodes for this application.
Note that the diode MBR2015CTL has a very low forward
voltage drop. This diode is most ideal for applications where
output voltages below 2.8V are required.
I
Load,max
(A)
10.0
11.2
12.4
13.9
14.0
14.5
R
SENSE
PC Trace
Resistor (m
W
)
5.5
5.0
4.6
4.2
4.2
4.1
R
SENSE
Discrete
Resistor (m
W
)
7.3
6.7
6.2
5.6
5.6
5.4
R
SENSE
I
Load, max
----------------------------------------
1
TF
–
(
)
′
=
=
3.0A
14.5A
+
------100mV
1
29%
–
(
)
′
4.1m
W
=
R
SENSE
----------------------------------------
1
TF
–
(
)
′
=
=
3.0A
14.5A
+
------100mV
1
5%
–
(
)
′
5.4m
W
=
Table 7. Comparison of Sense Resistors
Description
Tolerance
Factor (TF)
Size
(L x W x H)
Power capability >50A/in
Motherboard
Trace Resistor
±
29%
Discrete Iron
Alloy
resistor (IRC)
±
5%
(
±
1% available)
0.45" x 0.065" x
0.200"
1 watt (3W and
5W available)
+30 ppm
Discrete Metal
Strip surface
mount resistor
(Dale)
±
1%
Discrete MnCu
Alloy wire
resistor
±
10%
Discrete
CuNi Alloy
wire resistor
(Copel)
±
10%
2" x 0.2" x 0.001"
(1 oz Cu trace)
0.25" x 0.125" x
0.025"
1 watt
0.200" x 0.04" x
0.160"
1 watt
0.200" x 0.04" x
0.100"
1 watt
Temperature
Coefficient
Cost
@10,000 piece
+4,000 ppm
±
75 ppm
±
30 ppm
±
20 ppm
Low included in
motherboard
$0.31
$0.47
$0.09
$0.09