Datasheet

LTC1646
13
1646fa
APPLICATIO S I FOR ATIO
WUUU
5V
IN
(Pin 12) and 5V
SENSE
(Pin 11) for the 5V supply. For
the 3.3V supply, a sense resistor is connected between
3V
IN
(Pin 8) and 3V
SENSE
(Pin 9). The current limit and the
current foldback current level are given by Equations 4
and 5:
I
mV
R
I
mV
R
LIMIT XVOUT
SENSE XVOUT
FOLDBACK XVOUT
SENSE XVOUT
()
()
()
()
=
=
55
20
(5)
where XV
OUT
= 5V
OUT
or 3V
OUT
.
As a design aid, the current limit and foldback level for
commonly used values for R
SENSE
is shown in Table 3.
Table 3. I
LIMIT(XVOUT)
and I
FOLDBACK(XVOUT)
vs R
SENSE
R
SENSE
(Ω)I
LIMIT(XVOUT)
I
FOLDBACK(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 XV
OUT
= 3V
OUT
or 5V
OUT
.
5V
IN
–5V
SENSE
100mV/DIV
GATE
10V/DIV
FAULT
5V/DIV
5µs/DIV
1646 F06
(4)
Figure 6. Short-Circuit Fault on 5V
4
3
21
+
–
5V
IN
5V
IN
R
SENSE
5V
SENSE
V
CB
V
CB(MAX)
= 65mV
V
CB(NOM)
= 56mV
V
CB(MIN)
= 50mV
12
11
I
LOAD(MAX)
LTC1646*
*ADDITIONAL DETAILS
OMITTED FOR CLARITY
1646 F07
–
+
Figure 7. Circuit Breaker Equivalent
Circuit for Calculating R
SENSE
Calculating R
SENSE
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.
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 V
CB(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 I
2
R power being dissipated by it.
The first step in calculating the value of R
SENSE
is based on
I
TRIP(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 MAX
CB MIN
TRIP MAX
()
()
()
=
(6)
The second step is to determine the nominal value of the
sense resistor which is dependent on its tolerance