LT8697 datasheet(19/40 Pages) AD

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the same current as the output. Since the local ground

at the LT8698S is separated by a current carrying cable

from the remote ground at the point of load, the ground

reference points for these two locations are different.

Use a differential probe across the remote output at the

end of the cable to measure output voltage at that point,

as shown in Figure 4b. Do not simultaneously tie an oscil-

loscope’s probe ground leads to both the local LT8698S

ground and the remote point of load ground, as shown

in Figure 4a. Doing so will result in high current flow

in the probe ground lines and a strange and incorrect

measurement. Figure 4c shows this strange behavior. A

1.3A load step is applied to the LT8698S output through

3 meters of AWG 24 twisted-pair cable. On one curve, the

resultant output voltage is measured correctly using a dif-

ferential probe tied across the point of load. On the other

curve, the oscilloscope ground lead is tied to the remote

ground. This poor probing causes both a DC error due to

the lower ground return resistance and an AC error show-

ing increased overshoot. Do not add your oscilloscope,

lab bench, and input power supply ground lines into your

measurement of the LT8698S remote output.

Reducing Output Overshoot

A consequence of the use of cable drop compensation is

that the local output voltage at the LT8698S BUS/ISN pin

is regulated to a voltage that is higher than the remote

output voltage at the point of load. Several hundred mΩ

of line resistance can separate these two outputs, so at 2A

of load current, VBUS/ISN may be up to 1.05V higher than

the nominal 5V output at the point of load. Ensure that any

components tied to the LT8698S output can withstand

this increased voltage.

The LT8698S has several features designed to mitigate

any effects of higher output voltage due to cable drop

compensation. First, the LT8698S error amplifier, in addi-

tion to regulating the voltage on the USB5V pin to 5V

for the primary output, also regulates the BUS/ISN pin

voltage to less than 6.05V. This 6.05V upper limit on the

maximum BUS/ISB voltage protects components tied to

the LT8698S output such as a portable device from an

overvoltage condition, but reduces the possible amount

of cable drop compensation to 1.05V.

LT8698S/LT8698S-1

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

(a)

20µs/DIV

VLOAD through

0.5Ω Resistor

200mV/DIV

VLOAD through

0.5Ω Cable

200mV/DIV

IBUS

0.5A/DIV

8698S F03a

(b)

LOAD AT END OF 3M AWG 24 CABLE

40µs/DIV

VLOAD without

CLOAD

200mV/DIV

VLOAD with

10µF CLOAD

200mV/DIV

ILOAD

500mA/DIV

8698S F3b

Figure 3. Effect of Cable Inductance on Load Step

Transient Response

Figure 3b shows the effect of remote capacitance at the

load side of the cable on the LT8698S transient response.

The load step is a 60mA/µs, 0.5A load step through a 0.5Ω,

3 meter long cable. Without remote capacitance CLOAD at

the load step location, there is no LC tank and therefore

no observed ringing. In this case, given 100% of the dI/dt

of the load step occurs across the cable resistance R and

inductance L, the peak to peak VLOAD deviation is ±300mV.

When 10µF of remote CLOAD is added at the load step loca-

tion, the cable L and load capacitance form a tank circuit

leading to modest ringing. In this case, given the effective

dI/dt across the cable R and L is reduced, the peak to peak

VLOAD deviation is reduced to less than ±250mV.

Probing a Remote Output Correctly

Take care when probing the LT8698S’s remote output

to obtain correct results. With cable drop compensation

the local regulator output has a different voltage than

the remote output at the end of a cable due to the cable

resistance and high load current. The same is true for the

ground return line which also has resistance and carries

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