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LT8697HUDD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30
LT8697
16
8697fb
For more information www.linear.com/LT8697
APPLICATIONS INFORMATION
Figure 3 shows the LT8697 load step transient response
to a 50mA/µs, 0.5A load step. Tw o 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.
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.
Figure 3. Effect of Cable Inductance on Load Step
Transient Response
VOLTAGE (V)
CURRENT (A)
5.00
5.25
5.50
8697 F03
4.75
4.50
4.25
3
4
5
2
1
0
100µs/DIV
V
LOAD
THROUGH
0.2Ω
I
LOAD
50mA/µs
V
LOAD
THROUGH
0.2Ω CABLE
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.
Figure 4. Effect of Probing Remote Output Incorrectly
VOLTAGE (V)
CURRENT (A)
5.0
5.3
5.6
8697 F04
4.7
4.4
4.1
3
4
5
2
1
0
100µs/DIV
V
LOAD
INCORRECTLY
PROBED
I
LOAD
1A/µs
V
LOAD
CORRECTLY
PROBED
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.
LT8697
17
8697fb
For more information www.linear.com/LT8697
APPLICATIONS INFORMATION
Ensure that any components tied to the LT8697 output
can withstand this increased voltage.
The LT8697 has several features designed to mitigate
any effects of higher output voltage due to cable drop
compensation. First, the LT8697 error amplifier, in addi
-
tion to
regulating the voltage on the USB5V pin to 5V for
the
primary output, also regulates the SYS pin voltage to
less than 5.8V. For V
SYS
< 5.8V, the USB5V feedback input
runs the LT8697 control loop, and for V
SYS
> 5.8V, the
SYS feedback input runs the LT8697 control loop. This
5.8V upper limit on the maximum SYS voltage protects
components tied to the LT8697 output, such as a USB
device or a USB Switch, from an overvoltage condition,
but limits the possible amount of cable drop compensa
-
tion to 0.8V.
Additionally, the LT8697 can sink current from the output
and return the charge to the input when in forced continu
-
ous mode (FCM). This feature improves the step response
for a load step from high to low. Cable drop compensa-
tion adds
voltage to the output to compensate for voltage
drop
across the line resistance at high load. Since most
DC/DC convertors
can only source current, a load step
from high to near zero current leaves the output voltage
high and out of regulation.
The LT8697 fixes this problem by allowing the regulator
to sink current from the output when USB5V is too high
using FCM. Figure 5 shows the output voltage of the front
page application circuit with and without FCM.
Figure 5. Load Step Response with and
without Forced Continuous Mode
The load step response from high current to zero without
the FCM is extremely slow and is limited by the SYS pin
bias current. However, with FCM enabled, the output slews
quickly back into regulation. If V
IN
is above 29V or V
SYS
is above 7.5V, FCM is disabled.
Interfacing with a USB Switch
A USB or similar electronic switch can be tied between
the LT8697 output and the point of load. The switch on
resistance can be included in the cable drop compensation
calculation. Alternately, to improve load regulation, tie the
USB5V feedback input through R
CDC
to the output of the
USB Switch so the USB Switch impedance is removed from
the DC load response. Tie the output to the USB Switch
input. The
SYS pin regulates to a maximum of 5.8V, so
the USB Switch should be chosen accordingly.
The LT8697 has output current limit. Many USB Switches
implement current limit as well. For well controlled and
predicable behavior, ensure that only one chip sets the
output current limit, and the other chip has current limit
that exceeds the desired current limit over all operating
conditions.
The LT8697 has many of the features of USB Switches:
programmable output current limit, filtered fault report
-
ing and on/off functionality. In addition, unlike many
USB
Switches, the LT8697 output can survive shorts to
30V, enhancing system robustness. Therefore, in many
cases a USB Switch is not necessary and the LT8697 can
provide both the functionality of a voltage regulator and
a USB Switch.
Using SYS as a Secondary Output
For some applications, the SYS pin can be used as a sec
-
ondary voltage
output in addition to the primary voltage
output regulated by the USB5V pin. The SYS pin voltage
varies between 5V and 5.8V depending on the load cur
-
rent if cable drop compensation is used on the primary
output. A 3.3V low dropout regulator can be tied to SYS to
provide
a secondary regulated output such as to power a
USB μcontroller. The SYS output will not have cable drop
compensation, but will rise above 5V depending on the
USB output load current. The load on the SYS pin should
VOLTAGE (V)
CURRENT (A)
5.00
5.25
5.50
8697 F05
4.75
4.50
4.25
4
6
8
2
0
–2
400µs/DIV
V
LOAD
WITHOUT
FCM
V
LOAD
WITH FCM
I
LOAD
25mA/µs
LT8697
18
8697fb
For more information www.linear.com/LT8697
APPLICATIONS INFORMATION
be designed to limit load current. Also, an electronic switch
may be necessary to prevent an output overcurrent condi-
tion on
the USB5V output from bringing down the SYS
output.
See the Inductor Selection and Maximum Output
Current discussion below to determine how much total
load current can be drawn from the outputs for a given
LT8697 application.
Setting the Current Limit
In addition to regulating the output voltage, the LT8697
includes a current regulation loop for setting the average
output current limit. The LT8697 measures the voltage drop
across an external current sense resistor R
SENSE
using
the ISP and ISN pins. This resistor should be connected
in series with the load current after the output capacitor.
The current loop modulates the cycle-by-cycle top switch
switch current limit such that the average voltage across
the ISP–ISN pins does not exceed its regulation point.
The LT8697 current limit can be programmed by forcing
a voltage on the ICTRL pin between 0V and 1V. Program
the current limit using the following equation:
I
LIM
=
V
CTRL
R
SENSE
• 20.3
The preceding I
LIM
equation is valid for V
ISP
– V
ISN
<
48mV. At 48mV V
SENSE
, the internal current limit loop
takes over output current regulation from the ICTRL pin.
The maximum programmable output current (I
LIM(MAX)
)
is therefore found by the following equation:
I
LIM(MAX)
=
48mV
R
SENSE
The internal 2μA pull-up on the ICTRL pin allows this pin
to be floated if unused, in which case the I
LIM(MAX)
would
be the output current limit.
When in forced continuous mode, the LT8697’s ability to
regulate the output current is limited by its t
ON(MIN)
. In this
scenario, at very low output voltage the output current can
exceed the programmed output current limit and is limited
by the bottom switch current limit of 4.5A plus 1/2 the
ripple current. To help mitigate this effect, at low output
voltage the LT8697 folds back the switching frequency
by 10:1 to allow regulation at very low duty cycle. Also,
above V
IN
= 29V the LT8697 disables forced continuous
mode so the part can pulse skip to maintain regulation at
any low V
OUT
to V
IN
ratio. For V
IN
< 29V, use the following
equation to find the minimum output voltage (V
OUT(MIN)
)
where the LT8697 can regulate the output current limit:
V
OUT(MIN)
= 0.1 • f
SW
• t
ON(MIN)
• (V
IN
– V
SW(TOP)
+
V
SW(BOT)
) – V
SW(BOT)
– V
SENSE
– V
L
where f
SW
is the switching frequency, t
ON(MIN)
is the
minimum on-time, V
SW(TOP)
and V
SW(BOT)
are the in-
ternal switch
drops (~0.3V and ~0.15V) respectively at
maximum load), V
SENSE
is voltage across the R
SENSE
at
the programmed output current and V
L
is the resistive
drop across the inductor ESR at the programmed output
current. If the calculated V
OUT(MIN)
is negative or is less
than the IR drop across the resistive short on the output
at the programmed current limit, then the LT8697 can
regulate the output current limit.
In practical applications, the resistances of the cable,
inductor and sense resistor are more than adequate to
allow the LT8697 to regulate to the output current limit for
any switching frequency and input voltage. For a 400kHz
application in a worst-case condition, the programmed
output current can be regulated into V
OUT
= 0V for any
input voltage up to 42V. For a 2MHz application in a worst-
case condition, the programmed output current can be
regulated into V
OUT
= 0.3V or higher. Refer to Figure 6
to see how the front page application circuit responds to
a short directly on the regulator output without a cable.
I
OUT
(A)
1
V
OUT
(V)
0.4
0.6
3
8697 F06
0.2
0
1.5
2
2.5
1.0
0.8
V
CTRL
= OPEN, V
IN
= 27V
V
CTRL
= 0.5V, V
IN
= 16V
V
CTRL
= OPEN, V
IN
= 16V
V
CTRL
= 0.5V, V
IN
= 27V
Figure 6. Output Current Regulation
vs V
OUT
at f
SW
= 2MHz, R
SENSE
= 18mΩ
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LT8697HUDD#PBF

Mfr. #:
Buy LT8697HUDD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators USB 5V 2.5A Output, 42V Inpuut Synchronous Step-Down Regulator with Cable Drop Compensation
Lifecycle:
New from this manufacturer.
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