23
LTC4230
4230f
4
3
2
1
+
SLOW
COMPn
V
CCn
V
OUTn
V
CBn
SENSEn
R
SENSE
*ADDITIONAL DETAILS
OMITTED FOR CLARITY
4230 F12
LTC4230*
V
CB(MAX)
= 60mV
V
CB(NOM)
= 50mV
V
CB(MIN)
= 40mV
I
LOAD(MAX)
V
CCn
5V
+
Figure 12. Circuit Breaker Equivalent Circuit
for Calculating R
SENSE
The power rating of the sense resistor should accommo-
date steady-state fault current levels so that the compo-
nent is not damaged before the circuit breaker trips.
Table 4 in the Appendix lists suggested sense resistors
that can be used with the LTC4230s circuit breaker.
For example:
If a sense resistor with 7m& ?% R
TOL
is used for current
limiting, the nominal trip current I
TRIP(NOM)
= 7.1A. From
Equations 11 and 12, I
TRIP(MIN)
= 5.4A and I
TRIP(MAX)
= 9A
respectively.
For proper operation and to avoid the circuit breaker
tripping unnecessarily, the minimum trip current
(I
TRIP(MIN)
) must exceed the circuits maximum operating
load current. For reliability purposes, the operation at the
maximum trip current (I
TRIP(MAX)
) must be evaluated
carefully. If necessary, two resistors with the same R
TOL
can be connected in parallel to yield an R
SENSE(NOM)
value
that fits the circuit requirements.
IRC-TT SENSE RESISTOR
LR251201R010F
OR EQUIVALENT
0.01&, 1%, 1W
CURRENT FLOW
TO LOAD
CURRENT FLOW
TO LOAD
TO
V
CCn
TO
SENSE
n
TRACK WIDTH W:
0.03" PER AMP
ON 1 OZ COPPER
W
4230 F11
Figure 11. Making PCB Connections to the Sense Resistor
CALCULATING CIRCUIT BREAKER TRIP CURRENT
For a selected R
SENSE
value, the nominal load current that
trips the circuit breaker is given by Equation 10:
I
V
R
mV
R
TRIP NOM
CB NOM
SENSE NOM
SENSE NOM
(
)
(
)
(
)
(
)
=
=
50
(10)
The minimum load current that trips the circuit breaker is
given by Equation 11.
I
V
R
mV
R
TRIP MIN
CB MIN
SENSE MAX
SENSE MAX
(    )
(    )
(    )
(    )
=
=
40
(11)
where
R
R
R
SENSE MAX
SENSE NOM
TOL
(    )
(
)
"
=
+
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
1
100
The maximum load current that trips the circuit breaker is
given in Equation 12.
I
V
R
mV
R
TRIP MAX
CB MAX
SENSE MIN
SENSE MIN
(    )
(    )
(    )
(    )
=
=
60
(12)
where
R
R
R
SENSE MIN
SENSE NOM
TOL
(    )
(
)
"   
=
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
1
100
POWER MOSFET SELECTION CRITERIA
To start the power MOSFET selection process, choose the
maximum drain-to-source voltage, V
DS(MAX)
, and the
maximum drain current, I
D(MAX)
of the MOSFET. The
V
DS(MAX)
rating must exceed the maximum input supply
voltage (including surges, spikes, ringing, etc.) and the
I
D(MAX)
rating must exceed the maximum short-circuit
current in the system during a fault condition. In addition,
consider three other key parameters: 1) the required gate-
source (V
GS
) voltage drive, 2) the voltage drop across the
drain-to-source ON resistance, R
DS(ON)
and 3) the maxi-
mum junction temperature rating of the MOSFET.
Power MOSFETs are classified into two categories: stan-
dard MOSFETs (R
DS(ON)
specified at V
GS
= 10V) and logic-
level MOSFETs (R
DS(ON)
specified at V
GS
= 5V). The absolute
APPLICATIO  S I FOR  ATIO
U
U
U
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