LT8697
applicaTions inForMaTion
Ensure that any components tied to the LT8697 output
can withstand this increased voltage.
The load step response from high current to zero without
the FCM is extremely slow and is limited by the SYS pin
biascurrent.However,withFCMenabled,theoutputslews
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
quickly back into regulation. If V is above 29V or V
IN
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
resistancecanbeincludedinthecabledropcompensation
calculation. Alternately, to improve load regulation, tie the
less than 5.8V. For V < 5.8V, the USB5V feedback input
SYS
runs the LT8697 control loop, and for V
> 5.8V, the
SYS
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.
USB5V feedback input through R
to the output of the
CDC
USBSwitchsotheUSBSwitchimpedanceisremovedfrom
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.
Additionally, the LT8697 can sink current from the output
and return the charge to the input when in forced continu-
ousmode(FCM). Thisfeature improvesthestepresponse
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 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.
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.
5.50
5.25
5.00
4.75
4.50
4.25
8
V
LOAD
WITHOUT
FCM
Using SYS as a Secondary Output
6
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
4
V
LOAD
WITH FCM
2
I
LOAD
25mA/µs
0
–2
400µs/DIV
8697 F05
ꢁigure 5. Load Step Response with and
without ꢁorced Continuous Mode
8697p
16
For more information www.linear.com/LT8697
LINEAR_DIMENSIONS [ Linear Dimensions ]
