Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX8529: inductance value (L),
peak-inductor current (I
PEAK
), and DC resistance (R
DC
).
The following equation assumes a constant ratio of
inductor peak-to-peak AC current to DC average current
(LIR). For LIR values too high, the RMS currents are
high, and therefore I
2
R losses are high. Large induc-
tances must be used to achieve very low LIR values.
Typically inductance is proportional to resistance (for a
given package type), which again makes I
2
R losses
high for very low LIR values. A good compromise
between size and loss is a 30% peak-to-peak ripple cur-
rent to average-current ratio (LIR = 0.3). The switching
frequency, input voltage, output voltage, and selected
LIR determine the inductor value as follows:
where V
IN
, V
OUT
, and I
OUT
are typical values (so that effi-
ciency is optimum for typical conditions). The switching
frequency is set by R
OSC
(see the Setting the Switching
Frequency section). The exact inductor value is not
critical and can be adjusted in order to make trade-offs
among size, cost, and efficiency. Lower inductor values
minimize size and cost, but also improve transient
response and reduce efficiency due to higher peak cur-
rents. On the other hand, higher inductance increases
efficiency by reducing the RMS current. However, resis-
tive losses due to extra wire turns can exceed the benefit
gained from lower AC current levels, especially when the
inductance is increased without also allowing larger
inductor dimensions.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. The
inductor’s saturation rating must exceed the peak-
inductor current at the maximum defined load current
(I
LOAD(MAX)
):
Setting the Valley Current Limit
The minimum current-limit threshold must be high
enough to support the maximum expected load current
with the worst-case low-side MOSFET on-resistance
value since the low-side MOSFET’s on-resistance is used
as the current-sense element. The inductor’s valley cur-
rent occurs at I
LOAD(MAX)
minus half of the ripple cur-
rent. The current-sense threshold voltage (V
ITH
) should
be greater than the voltage on the low-side MOSFET
during the ripple-current valley:
where R
DS(ON)
is the on-resistance of the low-side
MOSFET (N
L
). Use the maximum value for R
DS(ON)
from the low-side MOSFET’s data sheet, an additional
margin to account for R
DS(ON)
rise with temperature is
also recommended. A good general rule is to allow
0.5% additional resistance for each °C of the MOSFET
junction temperature rise.
Connect ILIM_ to V
L
for the default 100mV (typ) current-
limit threshold. For an adjustable threshold, connect a
resistor (R
ILIM
_) from ILIM_ to GND. The relationship
between the current-limit threshold (V
ITH
_) and R
ILIM
_ is:
where R
ILIM
_ is in Ω and V
ITH
_ is in V.
An R
ILIM
resistance range of 100kΩ to 600kΩ corre-
sponds to a current-limit threshold of 50mV to 300mV.
When adjusting the current limit, 1% tolerance resistors
minimize error in the current-limit threshold.
For foldback current limit, a resistor (R
FBI
) is added
from ILIM pin to output. The value of R
ILIM
and R
FBI
can then be calculated as follows:
First select the percentage of foldback, P
FB
, from 15%
to 30%, then:
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents
as defined by the following equation:
I
RMS
has a maximum value when the input voltage equals
twice the output voltage (V
IN
= 2V
OUT
), so I
RMS(MAX)
=
I
LOAD
/ 2. For most applications, nontantalum capacitors
