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M3406 Datasheet PDF : 9 Pages
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AEROSEMI
M3406 800mA ,1.5MHz Synchronous Step-Down Converter
APPLICATIONS INFORMATION
Setting the Output Voltage
The internal reference VREF is 0.6V (Typical).The
output voltage is divided by a resistor, R1 and R2
to the FB pin. The output voltage is given by:
VOUT
=
0.6 × 1+
R1
R2
Inductor Selection
For most designs, the M3406 operates with
inductors of 1µH to 4.7µH. Low inductance
values are physically smaller but require faster
switching, which results in some efficiency loss.
The inductor value can be derived from the
following equation:
( ) L = VOUT × VIN − VOUT
VIN × ΔIL × fOSC
Where ΔIL is inductor Ripple Current. Large
value inductors result in lower ripple current and
small value inductors result in high ripple current.
For optimum voltage-positioning load transients,
choose an inductor with DC series resistance in
the 50mΩ to 150mΩ range.
Input Capacitor Selection
The input capacitor reduces the surge current
drawn from the input and switching noise from
the device. The input capacitor impedance at
the switching frequency should be less than
input source impedance to prevent high
frequency switching current passing to the input.
A low ESR input capacitor sized for maximum
RMS current must be used. Ceramic
capacitors with X5R or X7R dielectrics are highly
recommended because of their low ESR and
small temperature coefficients. A 4.7µF
ceramic capacitor for most applications is
sufficient. A large value may be used for
improved input voltage filtering.
Output Capacitor Selection
The output capacitor is required to keep the
output voltage ripple small and to ensure
regulation loop stability. The output capacitor
must have low impedance at the switching
frequency. Ceramic capacitors with X5R or X7R
dielectrics are recommended due to their low
ESR and high ripple current ratings. The output
ripple ΔVOUT is determined by:
ΔVOUT
≤
VOUT × (VIN − VOUT
VIN × fOSC × L
)
×
ESR
+
1
8 × fOSC × COUT
A 10µF ceramic can satisfy most applications.
PCB Layout Recommendations
When laying out the printed circuit board, the
following checking should be used to ensure
proper operation of the M3406. Check the
following in your layout:
The power traces, consisting of the GND
trace, the SW trace and the VIN trace should
be kept short, direct and wide
Does the (+) plates of CIN connect to VIN as
closely as possible. This capacitor provides
the AC current to the internal power
MOSFETs.
Keep the switching node, SW, away from
the sensitive VOUT node.
Keep the (-) plates of CIN and COUT as close
as possible
VIA TO VIN
PIN1
GND
COUT
M3406
L
VOUT
SW
VIA TO VOUT
R1
C1
R2
CIN
VIN
Figure 3. M3406 Suggested Layout
M3406 Rev1.0
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