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PRODUCT SPECIFICATION
RC5050
13
Table 6. RC5050 Short Circuit Comparator
Threshold Voltage
When designing the external current sense circuitry, the
designer must pay careful attention to the output limitations
during normal operation and during a fault condition. If the
short circuit protection threshold current is set too low, the
DC-DC converter may not be able to continuously deliver
the maximum CPU load current. If the threshold level is too
high, the output driver may not be disabled at a safe limit and
the resulting power dissipation within the MOSFET(s) may
rise to destructive levels.
The design equation used to set the short circuit threshold
limit is as follows:
V
I
SC
where I
pk
and I
min
are peak ripple current
and
I
load, max
= maximum output load current
The designer must also take into account the current
(I
pk
–I
min
), or the ripple current flowing through the induc-
tor under normal operation. Figure 6 illustrates the inductor
current waveform for the RC5050 DC-DC converter at
maximum load.
Figure 6. Typical DC-DC Converter Inductor
Current Waveform
The calculation of this ripple current is as follows:
where:
V
in
= input voltage to converter
V
SW
= I
LOAD
x R
DS,ON
V
D
= Forward Voltage of the Schottky diode
T = the switching period of the converter = 1/f
S
,
where f
S
= switching frequency.
For an input voltage of 5V, an output voltage of 2.8V, an
inductor value of 1.3
m
H and a switching frequency of
285KHz (using C
EXT
= 100pF), the inductor current can be
calculated as follows:
I
I
–
(
)
2
1.3
10
′
Therefore, for a continuous load current of 14.5A, the peak
current through the inductor, I
pk
, is found to be:
For continuous operation at 14.5A, the short circuit detection
threshold must be at least 17.5A.
The next step is to determine the value of the sense resistor.
Including tolerance, the sense resistor value can be approxi-
mated as follows:
where TF = Tolerance Factor for the sense resistor.
There are several different types of sense resistors. Table 7
describes tolerance, size, power capability, temperature
coefficient and cost of various sense resistors.
Short Circuit Comparator
V
threshold
(mV)
120
100
140
Typical
Minimum
Maximum
R
SENSE
--------, where: I
SC
= Output short circuit current
=
I
SC
I
inductor
3
I
Load, max
I
----------–
I
2
(
)
+
=
t
I
T=1/f
s
T
ON
T
OFF
I
LOAD, MAX
(I
pk
-I
min
)/2
Ipk
Imin
I
---------------------------
I
–
2
(
)
V
----------------------------------------------------
V
–
V
–
(
)
L
V
V
IN
D
+
)
V
SW
–
V
D
(
)
-------------------------+
T
′
′
=
----------------------------
-5.0
14.5
0.037
–
′
–
2.8
–
(
)
6
′
=
0.5
0.037
+
′
)
5.0
14.5
–
0.5
+
(
)
-------------------2.8
285
10
3
′
---------1
′
3A
I
SC
I
inductor
3
I
Load, max
I
-----------------------------
I
–
2
(
)
+
14.5
3
+
17.5A
=
=
=
R
SENSE
V
SC
----I
1
TF
–
(
)
′
V
Load,max
3.0
1
TF
–
(
)
′
=
=