LT3697 datasheet(18/40 Pages) AD

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LT8698S/LT8698S-1

Rev. A

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of about 4000ppm/°C results in an output voltage error of

294mV at 125°C and 191mV at –40°C. Figure 2a shows

this behavior.

See Table 1 for a list of copper wire resistances vs gauge.

Table 1. Copper Wire Resistance vs Wire Gauge

AWG

RESISTANCE OF CU WIRE AT 20°C (mΩ/m)

10.4

13.2

16.6

21.0

26.4

33.3

42.0

53.0

66.8

84.2

106

134

169

213

268

339

427

538

679

856

1080

1360

1720

2160

2730

3440

Cable drop compensation can be made to vary positively

versus temperature with the addition of a negative tem-

perature coefficient (NTC) resistor as a part of the RCBL

resistance. This circuit idea assumes the NTC resistor is

at the same temperature as the cable. Figure 2b shows

an example resistor network for RCBL that matches cop-

per resistance variation over a wide –40°C to 150°C tem-

perature range. Figure 2c shows the resultant cable drop

compensation output at several temperatures using RCBL

with negative temperature variation.

APPLICATIONS INFORMATION

The NTC resistor does not give a perfectly linear transfer

function versus temperature. Here, for typical component

values, the worst case error is <20% of the cable compen-

sation output, or <4% of the total output voltage accuracy.

Better output voltage accuracy versus temperature can

be achieved if RCBL resistor values are optimized for a

narrower temperature range.

Effect of Cable Inductance on Load Step Transient

Response

The inductance of long cabling limits the peak-to-peak

transient performance of a 2-wire sense regulator to fast

load steps (see Figure 3). Since a 2-wire sense regulator

like the LT8698S 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 load steps through long cabling show a large

peak-to-peak transient response and ringing at the reso-

nant frequency of the circuit. This ringing is a property

of the LC tank circuit and does not indicate regulator

instability.

Figure 3a shows the LT8698S load step transient response

to a 125mA/µs, 0.5A load step. Two cable impedances are

compared: resistive only and then resistive plus inductive.

First, a surface mount 0.5Ω resistor is tied between the

LT8698S output and the load step generator. This resis-

tor stands in for a purely resistive “cable”. Second, AWG

24 twisted-pair cabling 3 meters long with 0.5Ω of total

resistance and about 0.8µH of inductance is connected

between the LT8698S output and the load step genera-

tor. Even though the resistance in these two circuits is

the same, the transient load step response in the cable

is worse due to the inductance. The degree that cable

inductance degrades LT8698S load transient response

performance depends on the inductance of the cable and

on the load step rate. Long cables have higher inductance

than short cables. Cables with less separation between

supply and return conductor pairs show lower inductance

per unit length than those with separated conductors. A

faster load step rate exacerbates the effect of inductance

on load step response.

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