MAX8741EAI+T Maxim Integrated, MAX8741EAI+T Datasheet - Page 21

• Manufacturer Part Number: MAX8741EAI+T
• Description: Current & Power Monitors & Regulators 500kHz Multi-Out Pwr Supply Controller
• Manufacturer: Maxim Integrated

Specifications of MAX8741EAI+T

• Rohs: yes
• Product: Power Monitors
• Supply Voltage - Max: 30 V
• Supply Voltage - Min: 4.2 V
• Input Voltage Range: 4.2 V to 30 V

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
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
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
(R
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
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
current can be calculated by:
OUT
OUT
DC
500kHz Multi-Output Power-Supply Controllers
). The following equation includes a constant (LIR),
= maximum DC load current
if the above equation is used; otherwise, the peak
2
I
R losses. On the other hand, higher inductor val-
PEAK
=
L
I
LOAD
=
V
______________________________________________________________________________________
IN MAX
V
OUT
(
+
(
(
V
V
)
OUT IN MAX
IN MAX
× ×
2
(
f I
× × ×
(
f L V
V
PEAK
OUT
)
-
with High Impedance in Shutdown
V
(
OUT
×
), and DC resistance
LIR
Inductor Value
IN MAX
)
)
(
-
V
OUT
)
)
)
SAG
✕
The inductor’s DC resistance should be low enough that
R
ciency performance. If a standard off-the-shelf inductor is
not available, choose a core with an LI
than L
that fits the winding area. Ferrite core material is strongly
preferred. Shielded-core geometries help keep noise,
EMI, and switching-waveform jitter low.
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
Value section.
Use the calculated value of R
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.
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
input voltage and load current, with the worst case
occurring at V
Therefore, when V
DC
✕
PEAK
✕
I
PEAK
I
PEAK
I
from the second equation in the Inductor
RMS
< 100mV, as it is a key parameter for effi-
IN
2
I
and wind it with the largest diameter wire
PEAK MAX
=
= 2
Current-Sense Resistor Value
I
R
LOAD
IN
SENSE
I
✕
is 2 x V
RMS
(
V
×
OUT
=
)
=
V
=
Input-Capacitor Value
I
:
OUT
OUT
LOAD
RMS
80
I
PEAK
R
SENSE
120
2
SENSE
mV
(
:
V
V
) is determined by the
mV
IN
IN
-
to size the MOSFET
V
OUT
2
)
rating greater
21