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LTC1644

1644f

APPLICATIO S I FOR ATIO

Calculating RSENSE

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.

LTC1644*

1644 F07

*ADDITIONAL DETAILS OMITTED FOR CLARITY

VCB(MAX) = 70mV

VCB(NOM) = 55mV

VCB(MIN) = 40mV

5VSENSE

5VIN

RSENSE

ILOAD(MAX)

5VIN

VCB +

–

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 VCB(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 I2R

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 RSENSE is based on

ILOAD(MAX) and the lower limit for the circuit breaker

threshold, VCB(MIN). The maximum value for RSENSE in this

case is expressed by Equation 6:

SENSE

CB MIN

LOAD MAX

()

(6)

Figure 7. Circuit Breaker Equivalent

Circuit for Calculating RSENSE

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:

RTOL

SENSE N M

SENSE MAX

()

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

RSENSE(NOM).

The last step requires calculating a new value for ITRIP(MAX)

(ITRIP(MAX,NEW)) based on a minimum value for RSENSE

(RSENSE(MIN)) and the upper limit for the circuit breaker

threshold, VCB(MAX). The new value for ITRIP(MAX,NEW) is

given by Equation 8:

TRIP MAX NEW

CB MAX

SENSE MIN

)

()

(8)

where R

RTOL

SENSE MIN

SENSE NOM

()

(

)

=• − 



















100

Table 4 lists ITRIP(MIN) and ITRIP(MAX) versus some sug-

gested values of RSENSE. Table 8 lists manufacturers and

part numbers for these resistor values.

Table 4. ITRIP vs RSENSE Table

RSENSE (1% RTOL)

ITRIP(MIN)

ITRIP(MAX)

0.005

Ω

7.92A

14.14A

0.007

Ω

5.66A

10.10A

0.011

Ω

3.60A

6.43A

0.028

Ω

1.41A

2.53A

0.055

Ω

0.72A

1.29A

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