LTC1646CGN View Datasheet(PDF) - Linear Technology
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Description
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LTC1646CGN Datasheet PDF : 20 Pages
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LTC1646
APPLICATIO S I FOR ATIO
5VIN –5VSENSE
100mV/DIV
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.
GATE
10V/DIV
ILOAD(MAX)
5VIN
1 RSENSE 2
3
4
FAULT
5V/DIV
5µs/DIV
1646 F06
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:
ILIMIT(XVOUT)
=
55mV
RSENSE(XVOUT)
(4)
IFOLDBACK(XVOUT)
=
20mV
RSENSE(XVOUT)
(5)
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. ILIMIT(XVOUT) and IFOLDBACK(XVOUT) vs RSENSE
RSENSE (Ω)
ILIMIT(XVOUT)
IFOLDBACK(XVOUT)
0.005
11A
4A
0.006
9.2A
3.3A
0.007
7.9A
2.9A
0.008
6.9A
2.5A
0.009
6.1A
2.2A
0.01
5.5A
2A
where XVOUT = 3VOUT or 5VOUT.
12
5VIN
–+ VCB
11
5VSENSE
LTC1646*
–+
VCB(MAX) = 65mV
VCB(NOM) = 56mV
VCB(MIN) = 50mV
*ADDITIONAL DETAILS
OMITTED FOR CLARITY
1646 F07
Figure 7. Circuit Breaker Equivalent
Circuit for Calculating RSENSE
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.
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). The maximum value for RSENSE in this
case is expressed by Equation 6:
RSENSE(MAX)
=
VCB(MIN)
ITRIP(MAX)
(6)
The second step is to determine the nominal value of the
sense resistor which is dependent on its tolerance
13
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