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LT3697IMSE#PBF

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For more information www.linear.com/LT3697
LT3697
16
3697f
applicaTions inForMaTion
Figure 3 shows the LT3697 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
LT3697 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 LT3697 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 LT3697 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.
VOLTAGE (V)
CURRENT (A)
5.00
5.25
5.50
3697 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
Figure 3. Effect of Cable Inductance on Load Step
Transient Response
Probing a Remote Output Correctly
Take care when probing the LT3697’s remote output to
obtain correct results. The whole point of cable drop com
-
pensation 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. Since the local
ground at the LT3697 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 LT8697
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 measure
-
ment. Figure 4
c shows this strange behavior. A 1A/µs,
0.5A load step is applied to the LT3697 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 LT3697 remote output.
Reducing Output Overshoot
A consequence of the use of cable drop compensation is that
the local output voltage at the LT3697 SYS pin is regulated
to a voltage that is higher than the remote output voltage at
the point of load. Several hundred mΩ of line impedance 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. Ensure that any components tied
to the LT3697 output can withstand this increased voltage.
The LT3697 has several features designed to mitigate
any
effects of higher output voltage due to cable drop
compensation. First, the LT3697 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 6.1V. For V
SYS
< 6.1V, the USB5V feedback input
runs the LT3697 control loop, and for V
SYS
> 6.1V, the
SYS feedback input runs the LT3697 control loop. This
6.1V upper limit on the maximum SYS voltage protects
components tied to the LT3697 output like a USB Switch
from an overvoltage condition, but reduces the possible
amount of cable drop compensation to 1.1V.
For more information www.linear.com/LT3697
LT3697
17
3697f
applicaTions inForMaTion
Additionally, the LT3697 can sink current from the output
with an included 180mA active load from SYS to GND.
This feature improves the step response for a load step
from high to low. Cable drop compensation adds voltage
to the output to compensate for voltage drop across the
line resistance at high load. Since most DC/DC convertors
Figure 4a. Incorrect Remote Output Probing. Do Not Use!
Figure 4b. Correct Remote Output Probing
Figure 4c. Effect of Probing Remote Output Incorrectly
+
LOAD
V
OUT
C
BUS
100µF
LONG CABLE
3697 F04a
PROBE
POINT
PROBE
GROUND
POINT
+
LOAD
V
OUT
C
BUS
100µF
LONG CABLE
3697 F04b
PROBE
POINT
PROBE
POINT
can only source current, a load step from high to near zero
current leaves the output voltage high and out of regulation.
The LT3697 fixes this problem by allowing the regulator
to sink current from the output when USB5V is too high
using this active load. Figure 5 shows the output voltage
of the front page application circuit with and without the
active load.
Figure 5. Load Step Response with and without the Active Load
The load step response from high current to zero without
the active load is extremely slow and is limited by the SYS
and BST pin bias currents. However, with the active load
enabled, the output slews quickly back into regulation. If
V
SYS
is above 7V, the active load is disabled.
Interfacing with a USB Switch
A USB or similar electronic switch can be tied between
the LT3697 output and the point of load. To improve load
regulation, tie the USB5V feedback input through R
CDC
to
the output of the USB Switch so the USB Switch imped-
ance is
removed from the DC load response. Ti
e the SYS
pin to the LT3697 side of the USB Switch input. The SYS
pin regulates to a maximum
of 6.1
V, so the USB Switch
should be chosen accordingly.
The LT3697 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 limits over all operating
conditions.
200µs/DIV
V
LOAD
INCORRECTLY
PROBED
V
LOAD
CORRECTLY
PROBED
I
LOAD
25mA/µs
4.25
VOLTAGE (V)
CURRENT (A)
5.75
5.50
5.25
5.00
4.75
4.50
1.0
7.0
6.0
5.0
4.0
3.0
2.0
3697 F04c
250µs/DIV
V
LOAD
WITH ACTIVE LOAD
V
IN
= 12V
R
CABLE
= 0.26Ω
C
LOAD
= 10µF
I
LOAD
25mA/µs
3.50
VOLTAGE (V)
CURRENT (A)
6.50
6.00
5.50
5.00
4.50
4.00
0.0
12.0
10.0
8.0
6.0
4.0
2.0
3697 F05
V
LOAD
WITHOUT ACTIVE LOAD
APRX 150ms TO SETTLE
For more information www.linear.com/LT3697
LT3697
18
3697f
applicaTions inForMaTion
The LT3697 has many of the features of USB Switches such
as programmable output current limit, filtered overcurrent
fault reporting and on/off functionality. In addition, unlike
many USB switches, the LT3697 output survives shorts
to 20V, enhancing system robustness. In many cases, a
USB Switch therefore is not necessary and the LT3697
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 6.1V 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. This SYS output will have neither cable
drop compensation nor output current limit, so the load
on the SYS pin should be designed to limit load current.
Also, an electronic switch may be necessary to prevent an
output overcurrent condition on the USB5V output from
bringing down the SYS
output. See the inductor selection
and
maximum output current discussion below to deter-
mine how
much total load current can be drawn from the
SYS and USB5V outputs for a given LT3697 application.
Setting the Current Limit
In
addition to regulating the output voltage, the LT3697
includes a current regulation loop for setting the average
output current limit as shown in the Typical Applications
section.
The LT3697 measures the voltage drop across an external
current sense resistor using the ISP and ISN pins. This
resistor should be connected in series with the load cur
-
rent after the output capacitor. The LT3697 control loop
modulates
the cycle-by-cycle switch current limit such
that the average voltage across the ISP–ISN pins does
not exceed its regulation point.
The LT3697 output current limit can be programmed by
tying a resistor from R
LIM
to ground. Program the current
limit using the following equation:
R
LIM
= (I
LIM
• R
SENSE
• 1.848) – 8.49
Where I
LIM
is the output current limit in amps, R
SENSE
is
the resistance in mΩ tied between the ISP and ISN pins,
and R
LIM
is the resistance in kΩ tied from the RLIM pin
to ground.
The preceding I
LIM
equation is valid for V
ISP
– V
ISN
< 60mV.
At 60mV V
SENSE
, the internal current limit loop takes over
output current regulation from the R
LIM
pin. The maximum
programmable output current is therefore found by the
following equation:
I
LIMMAX
=
60mV
R
SENSE
The internal 11µA pull-up on the R
LIM
pin allows this pin
to be floated if unused, in which case the I
LIMMAX
would
be the output current limit.
The LT3697’s output current limit loop cannot regulate
to zero output current even if the R
LIM
pin is grounded.
R
LIM
can program the output current down to 1/3 of the
maximum value, or V
SENSE
= 20mV.
The LT3697’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 output overcurrent
threshold of V
SENSE
= 70mV. To help mitigate this effect,
at low output voltage the LT3697 folds back the switching
frequency to 240kHz (at V
SYS
= 0V) to allow regulation
at very low duty cycle. Also, above V
IN
= 35V the LT3697
stops switching. For V
IN
< 35V, 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)
= 240kHz • t
ON(MIN)
• (V
IN
– V
SW
+ V
D
)
– V
D
– V
SENSE
– V
L
where t
ON(MIN)
is the minimum on-time (110ns at 25°C),
V
SW
is the internal switch drop of 1.6V without BST at
2A, V
D
is the Schottky catch diode forward drop, 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 LT3697 regulates the output current limit.
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LT3697IMSE#PBF

Mfr. #:
Buy LT3697IMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators USB 5V, 2.5A, 40V Input Step-Down Switching Regulator with Cable Drop Compensation
Lifecycle:
New from this manufacturer.
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