MAX8686 Maxim Integrated Products, MAX8686 Datasheet - Page 17

• Manufacturer Part Number: MAX8686
• Description: DC-DC Converter Delivers Up To 25A Per Phase
• Manufacturer: Maxim Integrated Products

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To set the output voltage for the MAX8686, connect
REFIN to the center of an external resistor-divider from
PHASE/REFO to GND (R3 and R4 of Figures 2, 3, or 4).
The sum of R3 and R4 should exceed 165kΩ.
Preselect R4 and calculate R3 using the following
equation:
where V
comes from the reference voltage (V
resistor-divider should be placed as close as possible to
REFIN. If an external reference is used, see the
Reference Output (PHASE/REFO)/Reference Input
(REFIN) section for more details.
The output inductor is selected based on the desired
amount of inductor ripple current. A larger inductance
value minimizes output ripple current and increases
efficiency but slows down the output-inductor-current
slew rate during a load transient. LIR is the ratio of rip-
ple current to the total current per phase. For the best
tradeoff of efficiency and transient response, an LIR of
30% to 60% is recommended (LIR = 0.3 to 0.6).
Choose a higher LIR when more phases are used to
take advantage of ripple-current cancellation. The
inductor value is determined from:
where f
I
duty ratio (V
V
should have low DC resistance, and the saturation cur-
rent should be greater than the peak inductor current,
I
When the DC resistance (R
used for current sensing, the DC resistance should be
selected to ensure a sufficient current-sense signal for
OUT_MAX
PEAK
OUT
DC-DC Converter Delivers Up to 25A Per Phase
. I
is the output voltage. The selected inductor
PEAK
SW
OUT
is the maximum-rated output current, D is the
is the per-phase switching frequency,
OUT
I
is found from:
PEAK
is the desired output voltage and 3.3V
L
/V
≥
R
______________________________________________________________________________________
3
LIR f
IN
=
=
), N is the number of phases, and
I
Setting the Output Voltage
V
OUT MAX
R
OUT
×
4
SW
Design Procedures
×
N
_
× −
⎛
⎜
⎝
DC
×
V
(
1
3 3
OUT
I
OUT MAX
) of the output inductor is
.
×
Inductor Selection
D
)
⎛
⎝ ⎜
−
_
1
×
1
+
N
⎞
⎟
⎠
LIR
PHASE/REFO
2
⎞
⎠ ⎟
Single/Multiphase, Step-Down,
). The
robust current-mode control. The following equation
can be used as a guideline.
where R
or sense resistor at the highest operating temperature.
It is also important to choose lower LIR to keep the cur-
rent-sense signal below 45mV, which is the maximum
current limit:
If this condition is not met, then the LIR must be adjust-
ed or the input signal to the current-sense amplifier
must be scaled down with a resistor-divider.
To set the switching frequency, connect a capacitor
from FREQ to GFREQ. Calculate the capacitor value in
microfarads from the following equation:
where f
hertz and C
The operating frequency range is from 300kHz to
1MHz, so the capacitance at FREQ should be between
600pF and 180pF. Parasitic capacitance from device
pads and PCB layout should be deducted from the
above calculation especially at high switching frequen-
cies. In the estimation of parasitic capacitance, 15pF
per phase should be used.
For most applications where the duty cycle is less than
40%, set EN/SLOPE = 1.25V. For applications with a
duty cycle greater than 40%, set the slope compensa-
tion with a resistor (R
Calculate the R
where R
V
f
SW
R
IN_MIN
SLOPE
is the switching frequency.
SW
DC
=
is the minimum operating input voltage, and
DC
1 22
is the sense resistance value of the inductor
is the desired switching frequency in kilo-
FREQ
.
I
I
OUT MAX
OUT MAX
Setting the Switching Frequency
is the DC resistance of the inductor,
Setting the Slope Compensation
f
SLOPE
C
SW x L
x
N
N
_
FREQ
_
is the total capacitance in picofarads.
10
7
SLOPE
R
using the following formula:
=
DC
×
⎡
⎢
⎣
1
LIR R
5
+
) from EN/SLOPE to GND.
x
x V
LIR
×
10
2
(
2 7
OUT
.
−
DC
⎤
⎥
⎦
5
x R
x f
−
≥ 10
−
30
DC
SW
0 1
x SW
.
mV
≤
f
8 8 2
45
x V
mV
IN MIN
_
17
)