LT8697 Datasheet, PDF (15/28 Pages) Linear Dimensions Semiconductor – USB 5V 2.5A Output, 42V Input Synchronous Buck with Cable Drop Compensation

LT8697
Applications Information
Figure 3 shows the LT8697 load step transient response
to a 50mA/µs, 0.5A load step. Two cable impedances are
compared: resistive only and then resistive plus inductive.
First, a surface mount 0.2Ω resistor is tied between the
LT8697 output and the load step generator. This resistor
stands in for a purely resistive “cable”. Second, actual AWG
20 twisted-pair cabling 3 meters long with 0.2Ω of total
resistance and about 2.3µH of inductance is connected
between the LT8697 output and the load step generator.
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 LT8697 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. Faster load step rate exacerbates
the effect of inductance on load step response.
5.50
5
5.25
V_LOAD
4
THROUGH
0.2Ω
5.00
3
V_LOAD
THROUGH
4.75
0.2Ω CABLE
2
I_LOAD
50mA/µs
4.50
1
4.25
100µs/DIV
0
8697 F03
Figure 3. Effect of Cable Inductance on Load Step
Transient Response
Since the local ground at the LT8697 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.
Do not simultaneously tie an oscilloscope’s probe ground
leads to both the local LT8697 ground and the remote
point of load ground. Doing so will result in high current
flow in the probe ground lines and a strange and incor-
rect measurement. Figure 4 shows this behavior. A 1A/µs,
0.5A load step is applied to the LT8697 output through
3 meters of AWG 20 twisted-pair cable. On one curve,
the resultant output voltage is measured correctly using
a differential 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
showing increased overshoot and ringing. Do not add your
oscilloscope, lab bench, and input power supply ground
lines into your measurement of the LT8697 remote output.
5.6
5
V_LOAD
5.3
INCORRECTLY
4
PROBED
5.0
3
V_LOAD
4.7
CORRECTLY
2
PROBED
4.4
I_LOAD
1A/µs
1
4.1
0
100µs/DIV
8697 F04
Figure 4. Effect of Probing Remote Output Incorrectly
Probing a Remote Output Correctly
Take care when probing the LT8697’s remote output to
obtain correct results. The whole point of cable drop
compensation is that 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 the same current as the output.
Reducing Output Overshoot
A consequence of the use of cable drop compensation
is that the local output voltage at the LT8697 SYS pin is
regulated to a voltage that is higher than the remote out-
put voltage at the point of load. Several hundred mΩ of
cable resistance can separate these two outputs, so at 2A
of load current, the SYS pin voltage may be significantly
higher than the nominal 5V output at the point of load.
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8697p
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