Datasheet

LTC1644
16
1644f
APPLICATIO S I FOR ATIO
WUU
U
Calculating R
SENSE
An equivalent circuit for one of the LTC1644’s circuit
breakers useful in calculating the value of the sense
resistor is shown in Figure 7. To determine the most
appropriate value for the sense resistor first requires the
maximum current required by the load under worst-case
conditions.
–
+
14
LTC1644*
1644 F07
*ADDITIONAL DETAILS OMITTED FOR CLARITY
V
CB(MAX)
= 70mV
V
CB(NOM)
= 55mV
V
CB(MIN)
= 40mV
13
5V
SENSE
5V
IN
R
SENSE
I
LOAD(MAX)
5V
IN
V
CB
+
–
Two other parameters affect the value of the sense resis-
tor. First is the tolerance of the LTC1644’s circuit breaker
threshold. The LTC1644’s nominal circuit breaker thresh-
old is V
CB(NOM)
= 55mV; however, it exhibits ±15mV
tolerance over process and temperature. Second is the
tolerance (RTOL) in the sense resistor. Sense resistors are
available in RTOLs of ±1%, ±2% and ±5% and exhibit
temperature coefficients of resistance (TCRs) between
±75ppm/°C and ±100ppm/°C. How the sense resistor
changes as a function of temperature depends on the I
2
R
power being dissipated by it. 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.
The first step in calculating the value of R
SENSE
is based on
I
LOAD(MAX)
and the lower limit for the circuit breaker
threshold, V
CB(MIN)
. The maximum value for R
SENSE
in this
case is expressed by Equation 6:
R
V
I
SENSE
CB MIN
LOAD MAX
=
()
()
(6)
Figure 7. Circuit Breaker Equivalent
Circuit for Calculating R
SENSE
The second step is to determine the nominal value of
the sense resistor which is dependent on its tolerance
(RTOL␣ = ±1%, ±2%, or ±5%) and standard sense resistor
values. Equation 7 can be used to calculate the nominal
value from the maximum value found by Equation 6:
R
R
RTOL
SENSE N M
SENSE MAX
()
()
0
1
100
=
+






(7)
Often, the result of Equation 7 may not yield a standard
sense resistor value. In this case, two sense resistors with
the same RTOL can be connected in parallel to yield
R
SENSE(NOM)
.
The last step requires calculating a new value for I
TRIP(MAX)
(I
TRIP(MAX,NEW)
) based on a minimum value for R
SENSE
(R
SENSE(MIN)
) and the upper limit for the circuit breaker
threshold, V
CB(MAX)
. The new value for I
TRIP(MAX,NEW)
is
given by Equation 8:
I
V
R
TRIP MAX NEW
CB MAX
SENSE MIN
(,)
()
()
=
(8)
where R R
RTOL
SENSE MIN SENSE NOM() ( )
=•−












1
100
Table 4 lists I
TRIP(MIN)
and I
TRIP(MAX)
versus some sug-
gested values of R
SENSE
. Table 8 lists manufacturers and
part numbers for these resistor values.
Table 4. I
TRIP
vs R
SENSE
Table
R
SENSE
(1% RTOL) I
TRIP(MIN)
I
TRIP(MAX)
0.005Ω 7.92A 14.14A
0.007Ω 5.66A 10.10A
0.011Ω 3.60A 6.43A
0.028Ω 1.41A 2.53A
0.055Ω 0.72A 1.29A