MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
18 ______________________________________________________________________________________
where f
S
is the switching frequency. Choose a standard-
value inductor close to the calculated value. The exact
inductor value is not critical and can be adjusted to
make trade-offs between size, cost, and efficiency.
Lower inductor values minimize size and cost, but they
also increase the output ripple and reduce the efficiency
due to higher peak currents. On the other hand, higher
inductor values increase efficiency, but eventually resis-
tive losses due to extra turns of wire exceed the benefit
gained from lower AC current levels. This is especially
true if the inductance is increased without also increas-
ing the physical size of the inductor. Find a low-loss
inductor with the lowest possible DC resistance that fits
the allotted dimensions. Ferrite cores are often the best
choice, although powdered iron is inexpensive and can
work well at 300kHz. The chosen inductor’s saturation
current rating must exceed the peak inductor current
determined as:
Setting the Current Limit
Valley Current Limit
The MAX8650 has an adjustable valley current limit,
configurable for foldback with automatic recovery, or a
constant-current limit with latchup. To set the current
limit for foldback mode, connect a resistor from ILIM2
to the output (R
FOBK
), and another resistor from ILIM2
to GND (R
ILIM2
). See Figure 7. The values of R
FOBK
and R
ILIM2
are calculated as follows:
1) First, select the percentage of foldback (P
FB
). This
percentage corresponds to the current limit when
V
OUT
equals zero, divided by the current limit when
V
OUT
equals its nominal voltage. A typical value of
P
FB
is in the 15% to 40% range. A lower value of
P
FB
yields lower short-circuit current. The following
equations are used to calculate R
FOBK
and R
ILIM2
:
where I
VALLEY
is the value of the inductor valley current
at maximum load (I
LOAD(MAX)
- 1/2 x I
P-P
), and
R
DS(ON)
is the maximum on-resistance of the low-side
MOSFET at the highest operating junction temperature.
2) If the resulting value of R
ILIM2
is negative, either
increase P
FB
or choose a low-side MOSFET with a
lower R
DS(ON)
. The latter is preferred as it increases
the efficiency and results in a lower short-cir-
cuit current.
To set the constant-current limit for the latchup
mode, only R
ILIM2
is used. The equation for R
ILIM2
below sets the current-limit threshold at 1.2 times the
maximum rated output current:
Similarly, I
VALLEY
is the value of the inductor valley
current at maximum load and R
DS(ON)
is the maxi-
mum on-resistance of the low-side MOSFET at the
highest operating junction temperature.
Peak Current Limit
The peak current-limit threshold (V
TH
) is set by a resis-
tor connected from ILIM1 to GND. V
TH
corresponds to
the peak voltage across the sensing element (inductor
or current-sense resistor), R
LIM1
. R
LIM1
is calculated
as follows:
This allows a maximum DC output current (I
LIM
) of:
where R
DC
is either the DC resistance of the inductor or
the value of the optional current-sense resistor.
To ensure maximum output current, use the minimum
value of V
TH
from each setting, and the maximum R
DC
values at the highest expected operating temperature.
