Datasheet

MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
______________________________________________________________________________________ 17
Design Procedure
Setting the Output Voltage
To set the output voltage for the MAX8650, connect FB
to the center of an external resistor-divider from the out-
put to GND (R9 and R10 of Figure 3). Select R9
between 8kΩ and 24kΩ, and then calculate R10 with
the following equation:
where V
FB
= 0.7V. R9 and R10 should be placed as
close to the IC as possible.
Setting the Output Overvoltage
Protection Threshold
To set the overvoltage threshold voltage for the
MAX8650, connect OVP to the center of an external
resistor-divider from the output to GND (R11 and R12 of
Figure 3). Select R11 between 8kΩ and 24kΩ, then cal-
culate R12 with the following equation:
where V
OVP
= 0.8V when using the internal reference.
When using an external reference, V
OVP
is 115% of
V
REFIN
.
Setting the Slope Compensation
For most applications where the duty cycle is less than
50%, connect SCOMP to GND to set the slope com-
pensation to the default of 125mV/T, where T is the
oscillator period (T = 1 / f
S
).
For a slope compensation of 250mV/T, connect
SCOMP to AVL.
For applications with a duty cycle greater than 50%, set
the SCOMP voltage with a resistor voltage-divider from
AVL to GND (R3 and R4 in Figure 6). First, use the fol-
lowing equation to find the SCOMP voltage:
where R
L
is the DC resistance of the inductor, and f
S
is
the switching frequency.
Next, select a value for R3, typically 10kΩ, and solve
for R4 as follows:
This sets the slope-compensation voltage rate to
V
SCOMP
/ (10 x T).
Inductor Selection
There are several parameters that must be examined
when determining which inductor is to be used. Input
voltage, output voltage, load current, switching fre-
quency, and LIR. LIR is the ratio of inductor-current rip-
ple to maximum DC load current. A higher LIR value
allows for a smaller inductor, but results in higher loss-
es and higher output ripple. A good compromise
between size and efficiency is an LIR of 0.3. Once all
the parameters are chosen, the inductor value is deter-
mined as follows:
L
VVV
V f I LIR
OUT IN OUT
IN S LOAD MAX
=
×−
×× ×
()
()
R
VV R
V
SCOMP
SCOMP
4
53
=
−
()
×
V
VR
fL
SCOMP
OUT L
S
=
××
×
60
RR
V
V
OUT
OVP
12 11 1=× −
⎛
⎝
⎜
⎞
⎠
⎟
RR
V
V
OUT
FB
10 9 1=× −
⎛
⎝
⎜
⎞
⎠
⎟
INDUCTOR CURRENT
I
LOAD
I
VALLEY
TIME
I
PEAK
Figure 5. Inductor-Current Waveform
MAX8650
AVL
R3
R4
SCOMP
Figure 6. Resistor-Divider for Setting the Slope Compensation