Rainbow Electronics MAX8742 User Manual
Page 21
MAX8741/
M
AX8742
500kHz Multi-Output Power-Supply Controllers
with High Impedance in Shutdown
______________________________________________________________________________________
21
Inductor Value
The exact inductor value is not critical and can be
freely adjusted to make trade-offs between size, cost,
and efficiency. Lower inductor values minimize size
and cost but reduce efficiency due to higher peak-cur-
rent levels. The smallest inductor is achieved by lower-
ing the inductance until the circuit operates at the
border between continuous and discontinuous mode.
Further reducing the inductor value below this
crossover point results in discontinuous-conduction
operation even at full load. This helps lower output-filter
capacitance requirements, but efficiency suffers due to
high I
2
R losses. On the other hand, higher inductor val-
ues mean greater efficiency, but resistive losses due to
extra wire turns eventually exceed the benefit gained
from lower peak-current levels. Also, high inductor val-
ues can affect load-transient response (see the V
SAG
equation in the Low-Voltage Operation section). The
equations that follow are for continuous-conduction
operation, since the MAX8741/MAX8742 are intended
mainly for high-efficiency, battery-powered applica-
tions. Discontinuous conduction does not affect normal
idle-mode operation.
Three key inductor parameters must be specified: induc-
tance value (L), peak current (I
PEAK
), and DC resistance
(R
DC
). The following equation includes a constant (LIR),
which is the ratio of inductor peak-to-peak AC current to
DC load current. A higher LIR value allows smaller
inductance but results in higher losses and higher ripple.
A good compromise between size and losses is found at
a 30% ripple-current to load-current ratio (LIR = 0.3),
which corresponds to a peak-inductor current 1.15 times
higher than the DC load current:
where:
f = switching frequency, normally 333kHz or 500kHz
I
OUT
= maximum DC load current
LIR = ratio of AC to DC inductor current, typically 0.3;
should be >0.15
The nominal peak-inductor current at full load is 1.15
✕
I
OUT
if the above equation is used; otherwise, the peak
current can be calculated by:
The inductor’s DC resistance should be low enough that
R
DC
✕
I
PEAK
< 100mV, as it is a key parameter for effi-
ciency performance. If a standard off-the-shelf inductor is
not available, choose a core with an LI
2
rating greater
than L
✕
I
PEAK
2
and wind it with the largest diameter wire
that fits the winding area. Ferrite core material is strongly
preferred. Shielded-core geometries help keep noise,
EMI, and switching-waveform jitter low.
Current-Sense Resistor Value
The current-sense resistor value is calculated accord-
ing to the worst-case low current-limit threshold voltage
(from the Electrical Characteristics) and the peak
inductor current:
Use I
PEAK
from the second equation in the Inductor
Value section.
Use the calculated value of R
SENSE
to size the MOSFET
switches and specify inductor saturation-current ratings
according to the worst-case high current-limit threshold
voltage:
Low-inductance resistors, such as surface-mount
metal-film, are recommended.
Input-Capacitor Value
The input filter capacitor is usually selected according
to input ripple-current requirements and voltage rating,
rather than capacitor value. Ceramic capacitors or
Sanyo OS-CON capacitors are typically used to handle
the power-up surge currents, especially when connect-
ing to robust AC adapters or low-impedance batteries.
RMS input ripple current (I
RMS
) is determined by the
input voltage and load current, with the worst case
occurring at V
IN
= 2
✕
V
OUT
:
Therefore, when V
IN
is 2 x V
OUT
:
I
I
RMS
LOAD
=
2
I
I
V
V
V
V
RMS
LOAD
OUT
IN
OUT
IN
=
×
(
)
-
I
mV
R
PEAK MAX
SENSE
(
)
=
120
R
mV
I
SENSE
PEAK
=
80
I
I
V
V
V
f
L
V
PEAK
LOAD
OUT
IN MAX
OUT
IN MAX
=
+
(
)
Ч Ч
Ч
(
)
(
)
(
)
-
2
L
V
V
V
V
f
I
LIR
OUT
IN MAX
OUT
IN MAX
OUT
=
(
)
Ч Ч
Ч
(
)
(
)
-