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Rev. E, Sep 2005
ML3406
Application Information (1)
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1µH to 4.7µH. Its value is chosen based on
the desired ripple current. Large value inductors lower
ripple current and small value inductors result in higher
ripple currents. Higher VIN or VOUT also increases the ripple
current as shown in equation 1. A reasonable starting point
for setting ripple current is
IL=240mA (40% of 600mA)
1
VOUT
IL =
(f) (L)
VOUT
( 1-
VIN
)
(1)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
inductor
should
be
enough
for
most
applications
(600mA+120mA). For better efficiency, choose a low
DC-resistance inductor.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher
IL) will cause this
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Output
Voltage
Programming
(ML3406
Only)
In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
R2
vOUT = 0.6V ( 1+
R1
)
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 5.
Figure 5
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle Vout / Vin. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
CIN required
[ VOUT (VIN-VOUT) ]
1/2
IRMS = IOMAX
VIN
This formula has a maximum at VIN = 2VOUT, where IRMS =
IOUT / 2. This simple worst-case condition is commonly used
for design because even significant deviations do not offer
much relief. Note that the capacitor manufacturer’s ripple
current ratings are often based on 2000 hours of life. This
makes it advisable to further derate the capacitor, or
choose a capacitor rated at a higher temperature than
required. Always consult the manufacturer if there is any
question.
The selection of COUT is driven by the required effective
series resistance (ESR).
Typically, once the ESR requirement for COUT has been
met, the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement. The output ripple
VOUT is
determined by:
1
VOUT =
IL ( ESR+
8f COUT
)
Where f=operating frequency, COUT =output capacitance
and
IL=ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since
IL increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
ML3406