Analog Devices MAX77857 Manual page 7

Evaluates: MAX77857 (WLP)
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
resistors (use resistors with 1% tolerance or better) and adjust R
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
calculate the value of R
BOT
Where V is 0.333V and V
REF
maximum output voltage of 15V. The recommended external feedback resistor values for common output voltages are
listed in Table 3.
After startup, the output voltage can be adjusted dynamically using the I
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
maximum V of 1V:
REF
Output voltage step size can be calculated with the following equation:
Programmable output voltage ranges and output voltage step sizes for each recommended feedback resistor pairs are
listed in Table 3.
Table 3. Feedback Resistor Value Recommendations
DEFAULT
R10 (kΩ)
V (V)
REF
Internal feedback resistors*
0.333
*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)
www.analog.com
(R11) for a desired startup output voltage with the following equation:
is the desired startup output voltage. Note that the output voltage cannot exceed the
OUT
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R R
TOP BOT
R11 (kΩ)
160 20
178 20
312 12
232 6.65
234 5.3
(R10) and R
TOP
(R9) accordingly to select the external feedback
SEL
TOP
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STARTUP
V (V)
OUT
3
3.3
5
9
12
15
MAX77857 Evaluation Kit
(R11) with appropriate value
BOT
(R10) to be between 150kΩ and 330kΩ. Then
2
C serial interface. See the
)
PROGRAMMABLE
V RANGE (V)
OUT
3.0 to 9.0
3.0 to 9.9
4.5 to 15
8.1 to 15
10.7 to 15
13.5 to 15
EV Kit Software
of 0.299V and
REF
V STEP SIZE
OUT
(mV)
44.1
48.5
73.5
132.3
175.8
221.2
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