LT8697 Datasheet, PDF (12/28 Pages) Linear Dimensions Semiconductor – USB 5V 2.5A Output, 42V Input Synchronous Buck with Cable Drop Compensation

LT8697
Applications Information
Cable Drop Compensation
The LT8697 includes the necessary circuitry to implement
cable drop compensation. Cable drop compensation allows
the regulator to maintain 5V regulation on the USB V_LOAD
despite high cable resistance. The LT8697 increases its
local output voltage V_OUT above 5V as the load increases
to keep V_LOAD regulated to 5V. This compensation does
not require running an additional pair of Kelvin sense
wires from the regulator to the load, but does require the
system designer to know the cable resistance R_CABLE as
the LT8697 does not sense this value.
Program the cable drop compensation using the follow-
ing ratio:
R_CBL
=
20.55 •
R_SENSE • R_CDC
R_CABLE
where R_CDC is a resistor tied between the regulator output
and the USB5V pin, R_CBL is a resistor tied between the
RCBL pin and GND, R_SENSE is the sense resistor tied be-
tween the ISP and ISN pins in series between the regulator
output and the load, and R_CABLE is the cable resistance.
R_SENSE is typically chosen based on the desired current
limit and is typically 20mΩ for 2.1A systems and 50mΩ
for 0.9A. Please see the Setting the Current Limit section
for more information.
The current flowing into the USB5V pin through R_CDC is
identical to the current flowing out of the R_CBL resistor.
While the ratio of these two resistors should be chosen
per the equation above, choose the absolute values of
these resistors to keep this current between about 30µA
and 200µA at full load current. This restriction results in
R_CBL and R_CDC values between 5k and 33k. If I_USB5V is
too low, capacitive loading on the USB5V and RCBL pins
will degrade the load step transient performance of the
regulator. If I_USB5V is too high, the RCBL pin will go into
current limit and the cable drop compensation feature
will not work.
of the LT8697’s output. If R_CDC is changed, C_CDC should
also be changed to maintain roughly the same 10µs RC
time constant. If the capacitance across the remote load
is large compared to the LT8697 output capacitor tied to
the SYS pin, a longer R_CDC • C_CDC time constant may be
necessary for stability depending on the amount of cable
drop compensation used. Output stability should always
be verified in the end application circuit.
The LT8697 limits the maximum voltage of V_OUT by
limiting the voltage on the SYS pin V_SYS to 5.8V. If the
cable drop compensation is programmed to compensate
for more than 0.8V of cable drop at the maximum I_LOAD,
this V_SYS maximum will prevent V_OUT from rising higher
and the voltage at the point of load will drop below 5V.
The following equation shows how to derive the LT8697
output voltage V_OUT:
V_OUT
=
5V +
20.55
• I_LOAD• R_SENSE
R_CBL
• R_CDC
As stated earlier, the LT8697’s cable drop compensation
feature does not allow V_OUT to exceed the SYS regula-
tion point of 5.8V. If additional impedance is placed in
between the SYS pin and the OUT node such as R_SENSE
or a USB Switch, the voltage drop through these imped-
ances at the maximum I_LOAD must also be factored in to
this maximum allowable V_OUT value. Refer to Figure 1
for load lines of V_OUT and V_LOAD to see how cable drop
compensation works.
6.0
R_CABLE = 0.3Ω
5.8
R_SENSE = 20mΩ
R_CDC = 10kΩ
R_CBL = 13.7kΩ
5.6
V_OUT
5.4
5.2
V_LOAD
5.0
Capacitance across the remote load to ground downstream
of R_SENSE forms a zero in the LT8697’s feedback loop
due to cable drop compensation. C_CDC reduces the cable
drop compensation gain at high frequency. The 1nF C_CDC
capacitor tied across the 10k R_CDC is required for stability
4.8
0
0.5 1 1.5 2 2.5 3
LOAD CURRENT (A)
8697 F01
Figure 1. Cable Drop Compensation Load Line
8697p
12
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