Analog Devices │
4
MAX77831 Evaluation Kit
Evaluates: MAX77831
9) Turn off the 1.8V V
IO
power supply and input power
supply connected in step 4 and step 5.
10) Disconnect the 1.8V V
IO
power supply connected in
step 4 from the EV kit. The MAXUSB_INTERFACE#
has an on-board LDO to supply 1.8V to V
IO
.
11)
Ensure the SW1 and SW2 switches on the
MAXUSB_INTERFACE# are set to the ON position.
This enables I
2
C mode on the MAXUSB_INTERFACE#.
12)
Important:
Ensure the VL Jumper on the MAX
-
USB_INTERFACE# (J5) is set to 1.8V. This sets
the MAXUSB_INTERFACE# V
IO
voltage. Setting
this incorrectly to 3.3V could potentially damage the
MAX77831 IC.
13)
Connect the MAXUSB_INTERFACE# to the
MAX77831 EV kit. Connect the MAXUSB_INTER
-
FACE# to your PC’s USB port using a USB Type-A to
Micro-USB Cable.
14) Turn on the input power supply.
15) Open the GUI and click the
Device
button in the menu
bar. Click the
Connect
button in the
Device
button’s
drop-down list. Wait for the device to respond, and
in the
Synchronize
window, press the
Connect and
Read
button.
16) Drag the slide bar in the
section to change the output voltage and click Write.
17)
Confirm with a DVM that the software instruction to
change output voltage was successful. If so, the I
2
C
serial interface is confirmed as working.
This concludes the Quick Start procedure. Users are now
encouraged to further explore the device and its register
settings with the GUI software. For more information on
the GUI, see the
EV Kit Hardware
The MAX77831 EV kit demonstrates the MAX77831
buck-boost regulator. It regulates output from input voltage
ranges from 2.5V to 16V. The programmable output range
is from 4.5V to 15V with internal feedback resistors, or from
3V to 15V with external feedback resistors. The EV kit is
suited with a general DC input.
associated functions that are available on the EV kit.
R
SEL
Configuration Resistor
The MAX77831 includes an SEL pin to set the I
2
C slave
address, switching current limit, and feedback configura
-
tion. A resistor with a tolerance of 1% (or better) should be
chosen for R9, which is the SEL pin configuration resistor
R
SEL
. The default R
SEL
value installed on the EV kit is
0Ω. See
for the nominal R
SEL
values and their
corresponding settings. The switching current limit is also
adjustable dynamically through the I
2
C serial interface
when the MAX77831 is enabled. See the
section for more information.
Output Voltage Configuration
By default, the EV kit is configured to use internal feed
-
back resistors, with 5V default startup output voltage and
an output voltage range from 4.5V to 15V. To achieve a
different default startup output voltage other than 5V, or to
achieve a lower minimum output voltage range down to
3V, external feedback resistors are required. To change
the EV kit to the external feedback configuration, replace
the feedback resistors R
TOP
(R10) and R
BOT
(R11) with
appropriate value resistors (use resistors with 1% toler-
ance or better) and adjust R
SEL
(R9) accordingly to select
the external feedback option. It is also recommended to
install a 10pF feed-forward capacitor on C9 when using
external feedback.
To select appropriate external feedback resistor values,
first choose R
TOP
(R10) to be between 150kΩ and
330kΩ. Then calculate the value of R
BOT
(R11) for a
desired startup output voltage with the following equation:
BOT
REF
OUT
OUT
REF
R
V
V
V
V
×
−
=
where V
REF
is 0.333V and V
OUT
is the desired startup
output voltage. Note that the output voltage cannot
exceed the maximum output voltage 15V. The recom
-
mended external feedback resistor values for common
After startup, the output voltage can be adjusted dynami
-
cally using the I
2
C serial interface. See the
section for more information. When using inter
-
nal feedback, output voltage ranges between 4.5V and
15V in 73.5mV steps. When using external feedback,
output voltage range and step size vary based on the
external feedback resistor values. To calculate output
voltage range, use the following equation and plug in the
minimum V
REF
of 0.299V and maximum V
REF
of 1V:
REF
OUT
BOT
TOP
BOT
V
V
(R
R
)
R
+
=
×
Output voltage step size can be calculated with the fol
-
lowing equation:
BOT
TOP
O
OUT_STEP
B T
V
(R
R
)
R
4.9mV
+
=
×
Programmable output voltage ranges and output volt
-
age step sizes for each recommended external feedback
