LT8697 datasheet(17/40 Pages) AD

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LT8698S/LT8698S-1

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

Short USB5V to the VBUS output if no cable drop com-

pensation is desired.

The LT8698S limits the maximum voltage of VBUS by limiting

the voltage on the BUS/ISN pin to 6.05V. If the cable drop

compensation is programmed to compensate for more than

1.05V of cable drop at the maximum IBUS, this limit will pre-

vent VBUS/ISN from rising higher and the voltage at the point

of load VBUS will drop below 5V. Equation 2 shows how to

derive the LT8698S output voltage.

VOUT =5V+

46 •IBUS •RSEN •RCDC

RCBL

(2)

Refer to Figure 1 for load lines of VBUS/ISN and VBUS to see

how cable drop compensation works.

Figure 1. Example Cable Drop Compensation Load Line

VIN = 12V

RSEN = 10mΩ

RCBL = 1.82k

VBUS

VLOAD

LOAD CURRENT (A)

0.5

1.5

2.5

4.50

4.75

5.00

5.25

5.50

5.75

6.00

6.25

6.50

8698S F01

Cable Drop Compensation Over a Wide

Temperature Range

Cable drop compensation with zero temperature variation

may be used in many applications (see Figure 2a,b,c).

However, matching the cable drop compensation temper-

ature variation to the cable resistance temperature varia-

tion may result in better overall output voltage accuracy

over a wide operating temperature range. For example, in

an application with 0.49Ω of wire resistance and a maxi-

mum output current of 1.5A, cable drop compensation

adds 0.735V at 25°C to the output at max load for a fully

compensated wire resistance. If the wire in this example

is copper, the copper resistance temperature coefficient

(a) Cable Drop Compensation Through 3m of AWG 24 Twisted-

Pair Cable (490mΩ) without Temperature Compensation

IBUS = 1.5A

RCBL = 1.87k

VBUS

VLOAD

TEMPERATURE (°C)

–50 –25

75 100 125 150

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

6.2

8698S F02a

1.45k

RCBL

8698S F02b

11k

MURATA

NCP15XC680E03RC

68 THERMISTOR

(b) RCBL Resistor Network for Matching Copper

Wire Temperature Coefficient

24 Twisted-Pair Cable (490Ω) with Temperature

Compensation Using NTC RCBL

IBUS = 1.5A

RCBL = Figure 2b

VBUS

VLOAD

TEMPERATURE (°C)

–50 –25

75 100 125 150

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

6.2

8698S F2c

Figure 2. Cable Drop Compensation Applications

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