Datasheet
Evaluates: MAX8643
MAX8643 Evaluation Kit
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When externally programming the MAX8643 (Figure
1a), install an 8.06kΩ resistor at R8. The output voltage
is then determined by:
For an output voltage of 0.6V, install an 8.06kΩ resistor
at R8 and do not install R9. Refer to the MAX8643 data
sheet for information on selecting output inductor,
capacitor, and compensation components to optimize
the circuit for different output voltages.
Evaluating Other Switching
Frequencies (FREQ)
The MAX8643 EV kit comes preset with a 1MHz switch-
ing frequency. Replace R7 to change the switching fre-
quency. R7 is calculated as:
where the switching frequency is in megahertz and
must be between 500kHz and 2MHz. Refer to the
MAX8643 data sheet for information on selecting output
inductor, capacitor, and compensation components to
optimize the circuit for different switching frequencies.
Using the REFIN Input
The MAX8643 features an external reference input
(REFIN). The IC regulates FB to the voltage applied to
REFIN. The internal soft-start is not available when
using an external reference. A method of soft-start
when using an external reference is shown by the use
of R3 and C7 in Figure 2. To use the REFIN input of the
EV kit, remove the shunt on jumper JU2. Connect an
external reference to the REFIN pad on the EV kit. If the
external reference produces step-voltage changes,
install C7. Refer to the MAX8643 data sheet for more
details.
Power Good (PWRGD)
PWRGD is an open-drain output that goes high im-
pedance when V
FB
is above 0.54V. PWRGD pulls low
when V
FB
is below 0.54V for at least 48 clock cycles.
PWRGD is low during shutdown. PWRGD is pulled up
to V
DD
through R5.
Jumper JU1 Function (Shutdown Mode)
The MAX8643 features a shutdown mode to minimize
the IC quiescent current. To shut down the IC, place a
shunt between pins 1-2 of JU1. For normal operation,
remove the shunt from JU1.
Rx
f
k
S
75263
1
005= −
⎛
⎝
⎜
⎞
⎠
⎟
..Ω
R
V
OUT
9
4836
06
=
− .
MAX8643
OUT
LX
V
OUT
C12
C2
L
FB
R6
C10
R4
C11
COMP
CTL1
CTL2
R8
8.06kΩ
R9
MAX8643
OUT
LX
V
OUT
C12
C2
L
Ri
8kΩ
FB
R6
C10
R4
C11
COMP
VOLTAGE
SELECT
CTL1
CTL2
Figure 1. Preset and Adjustable Output Configuration
1a 1b