Datasheet

LTC4211
22
4211fb
OPERATION
SENSE RESISTOR CONSIDERATIONS
The fault current level at which the LTC4211’s internal
electronic circuit breaker trips is determined by a sense
resistor connected between the LTC4211’s V
CC
and SENSE
pins and two separate trip points. The first trip point is
set by the SLOW COMP’s threshold, V
CB(SLOW)
= 50mV,
and occurs should a load current fault condition exist for
more than 20μs. The current level at which the electronic
circuit breaker trips is given by Equation 8:
I
TRIP(SLOW)
=
V
CB(SLOW)
R
SENSE
=
50mV
R
SENSE
(8)
The second trip point is set by the FAST COMP’s threshold,
V
CB(FAST)
= 150mV, and occurs during fast load current
transients that exist for 300ns or longer. The current level
at which the circuit breaker trips in this case is given by
Equation 9:
I
TRIP(FAST)
=
V
CB(FAST)
R
SENSE
=
150mV
R
SENSE
(9)
As a design aid, the currents at which the electronic circuit
breaker trips for common values for R
SENSE
are shown
in Table 3.
Figure 11. Making PCB Connections to the Sense Resistor
Table 2. t
SLOWCOMP
vs C
FILTER
C
FILTER
t
SLOWCOMP
100pF 82μs
220pF 156μs
330pF 224μs
470pF 310μs
680pF 440μs
820pF 527μs
1000pF 638μs
Table 3. I
TRIP(SLOW)
and I
TRIP(FAST)
vs R
SENSE
R
SENSE
I
TRIP(SLOW)
I
TRIP(FAST)
0.005Ω 10A 30A
0.006Ω 8.3A 25A
0.007Ω 7.1A 21A
0.008Ω 6.3A 19A
0.009Ω 5.6A 17A
0.01Ω 5A 15A
For proper circuit breaker operation, Kelvin-sense PCB
connections between the sense resistor and the LTC4211’s
V
CC
and SENSE pins are strongly recommended. The
drawing in Figure 11 illustrates the correct way of making
connections between the LTC4211 and the sense resis-
tor. PCB layout should be balanced and symmetrical to
minimize wiring errors. In addition, the PCB layout for the
sense resistor should include good thermal management
techniques for optimal sense resistor power dissipation.
The power rating of the sense resistor should accommodate
steady-state fault current levels so that the component is
not damaged before the circuit breaker trips. Table4 in
the Appendix lists sense resistors that can be used with
the LTC4211’s circuit breaker.
IRC-TT SENSE RESISTOR
LR251201R010F
OR EQUIVALENT
0.01, 1%, 1W
CURRENT FLOW
TO LOAD
CURRENT FLOW
TO LOAD
TO
V
CC
TO
SENSE
TRACK WIDTH W:
0.03" PER AMP
ON 1 OZ COPPER
W
4211 F11
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
TRIP(NOM)
=
V
CB(NOM)
R
SENSE(NOM)
=
50mV
R
SENSE(NOM)
(10)
The minimum load current that trips the circuit breaker is
given by Equation 11.
I
TRIP(MIN)
=
V
CB(MIN)
R
SENSE(MAX)
=
40mV
R
SENSE(MAX)
(11)
where
R
SENSE(MAX)
= R
SENSE(NOM)
s 1+
R
TOL
100
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