Evaluates: MAX77857 (WLP)
MAX77857 Evaluation Kit
www.analog.com
Analog Devices | 7
Output Voltage Configuration
By default, the EV kit is configured to use internal feedback resistors, with a 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
external feedback configuration, replace the feedback resistor R
TOP
(R10) and R
BOT
(R11) with appropriate value
resistors (use resistors with 1% tolerance 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 chose 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:
𝑅
𝐵𝑂𝑇
=
𝑅
𝑇𝑂𝑃
× 𝑉
𝑅𝐸𝐹
𝑉
𝑂𝑈𝑇
− 𝑉
𝑅𝐸𝐹
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 of 15V. The recommended external feedback resistor values for common output voltages are
listed in
After startup, the output voltage can be adjusted dynamically using the I
2
C serial interface. See the
EV Kit Software
section for more information. When using internal feedback, output voltage ranges are 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:
𝑉
𝑂𝑈𝑇
=
𝑉
𝑅𝐸𝐹
𝑅
𝐵𝑂𝑇
× (𝑅
𝐵𝑂𝑇
+ 𝑅
𝑇𝑂𝑃
)
Output voltage step size can be calculated with the following equation:
𝑉
𝑂𝑈𝑇_𝑆𝑇𝐸𝑃
=
4.9𝑚𝑉
𝑅
𝐵𝑂𝑇
× (𝑅
𝐵𝑂𝑇
+ 𝑅
𝑇𝑂𝑃
)
Programmable output voltage ranges and output voltage step sizes for each recommended feedback resistor pairs are
listed in
Table 3. Feedback Resistor Value Recommendations
DEFAULT
V
REF
(V)
R
TOP
R10 (kΩ)
R
BOT
R11 (kΩ)
STARTUP
V
OUT
(V)
PROGRAMMABLE
V
OUT
RANGE (V)
V
OUT
STEP SIZE
(mV)
0.333
160
20
3
3.0 to 9.0
44.1
178
20
3.3
3.0 to 9.9
48.5
Internal feedback resistors*
5
4.5 to 15
73.5
312
12
9
8.1 to 15
132.3
232
6.65
12
10.7 to 15
175.8
234
5.3
15
13.5 to 15
221.2
*Default EV kit configuration
High-Temperature Testing
The MAX77857 is rated for operation under junction temperatures up to +125°C. Note that not all components on the EV
kit are rated for temperatures this high. Some ceramic capacitors experience extra leakage when put under temperatures
higher than they are rated for and supply current readings for the IC might be larger than expected. The
MAXUSB_INTERFACE# is also not rated for +125°C. Double-check the components on the EV kit if testing at +125°C
ambient or junction temperatures. Consider replacing these components if IC operation at +125°C ambient or junction
temperature is an important use case.
List of capacitors not rated for +125°C: C10, C11 (OUT capacitors)
