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RC5040
PRODUCT SPECIFICATION
14
Table 6. RC5040 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:
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 7 illustrates the inductor
current waveform for the RC5040 DC-DC converter at maxi-
mum load.
Figure 7. Typical DC-DC Converter
Inductor Current Waveform
Short Circuit Comparator
V
threshold
(mV)
120
100
140
Typical
Minimum
Maximum
R
SENSE
V
SC
I
SC
= Output short circuit current
=
I
SC
I
inductor
3
I
Load, max
I
------------2
I
–
(
)
+
=
t
I
T=1/f
s
T
ON
T
OFF
I
LOAD
(Ipk-imin)/2
Ipk
Imin
The calculation of this ripple current is as follows:
where: V
in
= input voltage to Converter
V
SW
=voltage across Switcher (MOSFET)
=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 3.3V, an
inductor value of 1.3
m
H and a switching frequency of
650KHz (using C
EXT
= 39pF), the inductor current can be
calculated as
follows:
Therefore, the peak current, I
PK
, through the inductor for a
14.5A load is found to be:
As a result, the short circuit detection threshold must be at
least 15.5A.
The next step is to determine the value of the sense resistor.
Including sense resistor tolerance, the sense resistor value
can be approximated as follows
Where TF = Tolerance Factor for the sense resistor.
There are several different type of sense resistors. Table 7
describes tolerance, size, power capability, temperature
coefficient and cost of various type of sense resistors.
I
---------------------------
I
–
2
(
)
V
-------------------------L
V
–
V
–
(
)
V
IN
V
D
+
+
(
)
V
SW
–
V
D
(
)
-V
′
=
I
-----------2
I
–
(
)
-5.0
14.5
1.3
0.037
10
′
–
3.3
–
(
)
6
–
′
′
=
0.5
0.037
+
′
)
5.0
14.5
–
0.5
+
(
)
-------------------3.3
650
10
3
′
---------1
′
1.048A
=
I
SC
I
inductor
3
I
Load, max
I
------------2
I
–
(
)
+
14.5
1
+
15.5A
=
=
=
R
SENSE
V
I
SC
----------------
1
TF
–
(
)
′
V
I
Load,max
+
1.0
----------------------------------
1
TF
–
(
)
′
=
=