Datasheet

MAX8686
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
______________________________________________________________________________________ 17
Design Procedures
Setting the Output Voltage
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
OUT
is the desired output voltage and 3.3V
comes from the reference voltage (V
PHASE/REFO
). The
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.
Inductor Selection
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
SW
is the per-phase switching frequency,
I
OUT_MAX
is the maximum-rated output current, D is the
duty ratio (V
OUT
/V
IN
), N is the number of phases, and
V
OUT
is the output voltage. The selected inductor
should have low DC resistance, and the saturation cur-
rent should be greater than the peak inductor current,
I
PEAK
. I
PEAK
is found from:
When the DC resistance (R
DC
) of the output inductor is
used for current sensing, the DC resistance should be
selected to ensure a sufficient current-sense signal for
robust current-mode control. The following equation
can be used as a guideline.
where R
DC
is the sense resistance value of the inductor
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.
Setting the Switching Frequency
To set the switching frequency, connect a capacitor
from FREQ to GFREQ. Calculate the capacitor value
from the following equation:
where f
SW
is the desired switching frequency in kilo-
hertz and C
FREQ
is the total capacitance in picofarads.
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. GFREQ may be connected
to GND (quiet ground).
Setting the Slope Compensation
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
SLOPE
) from EN/SLOPE to GND.
Calculate the R
SLOPE
using the following formula:
where R
DC
is the DC resistance of the inductor,
V
IN_MIN
is the minimum operating input voltage, and
f
SW
is the switching frequency.
R
xR
f
xV
SLOPE
DC
SW
xL
OUT
.
.= −
122 10
01
7
882
_
xV
IN MIN
()
C
xf
xf
FREQ
x
SW
SW
.
=
−510
27
5
30
I
N
LIR
xR mV
OUT MAX
DC
_
1
2
45+
⎡
⎣
⎢
⎤
⎦
⎥
≤
I
N
LIR R mV
OUT MAX
DC
_
×× ≥ 10
I
I
N
LIR
PEAK
OUT MAX
=×+
⎛
⎝
⎜
⎞
⎠
⎟
_
1
2
L
VDN
LIR f I
OUT
SW OUT MAX
≥
× − ×
××
()
_
1
RR
V
OUT
34
33
1=× −
⎛
⎝
⎜
⎞
⎠
⎟
.