LT8697
applicaTions inForMaTion
5.8
5.8
5.6
5.4
5.2
V
OUT
V
OUT
5.6
5.4
5.2
I
= 2.1A
I
= 2.1A
LOAD
LOAD
CABLE = 4 METERS AWG
20 TWISTED-PAIR COPPER
CABLE = 4 METERS AWG
20 TWISTED-PAIR COPPER
V
LOAD
V
LOAD
5.0
4.8
5.0
4.8
–20
20
40
60
80
100
–20
20
40
60
80
100
0
0
TEMPERATURE (°C)
TEMPERATURE (°C)
8697 F02c
8697 F02d
ꢁigure 2c. Cable Drop Compensation ꢂhrough 4m of AWG 20
ꢂwisted-Pair Cable (260mΩ) with ꢂemperature Compensation
Using NꢂC RCBL
ꢁigure 2d. Cable Drop Compensation ꢂhrough 4m of AWG 20
ꢂwisted-Pair Cable (260mΩ) with ꢂemperature Compensation
Using Copper RSENSE
The NTC resistor does not give a perfectly linear transfer
function versus temperature. Here, for typical component
values, theworsecaseerroris<10%ofthecablecompen-
sation output, or <1% of the total output voltage accuracy.
Better output voltage accuracy versus temperature can be
ꢂable 2. Copper Wire Inductors for Use as Sense Resistors
VENDOR
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
Coilcraft
PARꢂ NUMBER
NA5931-AL
NA5932-AL
NA5933-AL
NA5934-AL
NA5935-AL
NA5936-AL
DC RESISꢂANCE (mΩ)
15.7 5%
21.8 5%
32.4 5%
achieved if R
resistor values are optimized for a nar-
CBL
34.3 5%
rower temperature range. Contact LTC for help designing
an R resistor network.
44.1 5%
CBL
47.2 5%
ChoosinganR
resistorwithatemperaturecoefficient
SENSE
Effect of Cable Inductance on Load Step ꢂransient
Response
that matches the cable resistance temperature coefficient
canreducethisoutputvoltageerrorovertemperatureifthe
senseresistorisatroughlythesameambienttemperature
The inductance of long cabling limits the peak-to-peak
transient performance of a 2-wire sense regulator to fast
load steps. Since a 2-wire sense regulator like the LT8697
detects the output voltage at its local output and not at
the point of load, the load step response degradation due
to cable inductance is present even with cable resistance
compensation. The local regulator output capacitor and
the input capacitor of the remote load form a LC tank
circuit through the inductive cabling between them. Fast
loadstepsthroughlongcablingshowalargepeak-to-peak
transientresponseandringingattheresonantfrequencyof
the circuit. This ringing is a property of the LC tank circuit
and does not indicate regulator instability.
as R
. Small value copper wire inductors can be used
SENSE
in this way if the inductor resistance is well specified.
Figure 2d shows the resultant cable drop compensation
output at several temperatures using a copper R
.
SENSE
Use of an R
that varies over temperature will make
SENSE
the LT8697 output current limit vary over temperature. To
achievetheratedoutputcurrentoverthefulloperatingtem-
perature range, a higher room temperature output current
limit may be necessary. Table 2 shows the manufacturer
specified DCR of several copper wire inductors that may
be used for R
.
SENSE
8697p
14
For more information www.linear.com/LT8697
LINEAR_DIMENSIONS [ Linear Dimensions ]
