LTC1646
W U U
APPLICATIO S I FOR ATIO
U
Calculating RSENSE
An equivalent circuit for one of the LTC1646’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.
5V –5V
IN
SENSE
100mV/DIV
I
R
LOAD(MAX)
SENSE
1
2
GATE
10V/DIV
5V
IN
3
4
12
5V
11
5V
SENSE
IN
+
–
FAULT
5V/DIV
V
LTC1646*
CB
–
+
V
V
V
= 65mV
= 56mV
= 50mV
CB(MAX)
CB(NOM)
CB(MIN)
5µs/DIV
1646 F06
*ADDITIONAL DETAILS
OMITTED FOR CLARITY
1646 F07
Figure 6. Short-Circuit Fault on 5V
5VIN (Pin 12) and 5VSENSE (Pin 11) for the 5V supply. For
the 3.3V supply, a sense resistor is connected between
3VIN (Pin 8) and 3VSENSE (Pin 9). The current limit and the
current foldback current level are given by Equations 4
and 5:
Figure 7. Circuit Breaker Equivalent
Circuit for Calculating R
SENSE
Two other parameters affect the value of the sense resis-
tor. First is the tolerance of the LTC1646’s circuit breaker
threshold. The LTC1646’s nominal circuit breaker
threshold is VCB(NOM) = 56mV; however, it exhibits a
–6mV/+9mV tolerance due to process variations. 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.
55mV
RSENSE(XVOUT)
(4)
(5)
ILIMIT(XVOUT)
=
20mV
RSENSE(XVOUT)
IFOLDBACK(XVOUT)
=
where XVOUT = 5VOUT or 3VOUT
.
As a design aid, the current limit and foldback level for
commonly used values for RSENSE is shown in Table 3.
Table 3. I
and I
I
vs R
I
The first step in calculating the value of RSENSE is based on
ITRIP(MAX) and the lower limit for the circuit breaker
threshold,VCB(MIN).ThemaximumvalueforRSENSE inthis
case is expressed by Equation 6:
LIMIT(XVOUT)
(Ω)
FOLDBACK(XVOUT)
SENSE
R
SENSE
LIMIT(XVOUT)
FOLDBACK(XVOUT)
0.005
0.006
0.007
0.008
0.009
0.01
11A
9.2A
7.9A
6.9A
6.1A
5.5A
4A
3.3A
2.9A
2.5A
2.2A
2A
VCB(MIN)
ITRIP(MAX)
RSENSE(MAX)
=
(6)
The second step is to determine the nominal value of the
where XVOUT = 3VOUT or 5VOUT
.
sense resistor which is dependent on its tolerance
1646fa
13
Linear [ Linear ]
