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Rev. B, Sep 2005
Applications Information (1)
● Recommended Component Placement.
Inductor Selection
The high frequency operation of the ML3440 allows the
use of small surface mount inductors. The inductor
current ripple is typically set to 20% to 40% of the
maximum inductor current. For a given ripple the
inductance terms are given as follows:
VIN (min)
2
· (VOUT - VIN (min)) 100
L >
f
Iout (max) %Ripple VOUT
2
H
VOUT (VIN (max)- VOUT) 100
L >
f
Iout (max) %Ripple VIN (max)
H
where f = operating frequency, Hz
%Ripple = allowable inductor current ripple,%
VIN (Min) = minimum input voltage, V
VIN (Max) = maximum input voltage, V
VOUT = output voltage, V
Iout (Max) = maximum output load current
For high efficiency, choose and inductor with a high
frequency core material, such as ferrite, to reduce core
loses. The inductor should have low ESR (equivalent
series resistance) to reduce the I
2R losses, and must be
able to handle the peak inductor current without
saturating. Molded chokes or chip inductors usually do
not have enough core to support the peak inductor
currents in the 1A to 2A region. To minimize radiated
noise, use a toroid, pot core or shielded bobbin inductor.
Output Capacitor Selection
The bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The steady state ripple
due to charge is given by:
Iout (max) ·( VOUT - VIN (min)) 100
%Ripple_Boost =
COUT VOUT
2 f
%
Iout (max) ·( VIN (max)- VOUT) 100
%Ripple_Buck =
COUT Vin(max) VOUT · f
%
where COUT=output filter capacitor, F
The output capacitance is usually many times larger in order to
handle the transient response of the converter. For a rule of
thumb, the ratio of the operating frequency to the unity-gain
bandwidth of the converter is the amount of the output
capacitance will have to increase from the above calculations
in order to maintain the desired transient response.
The other component of ripple is due to the ESR (equivalent
series resistance) of the output capacitor. Low ESR capacitors
should be used to minimize output voltage ripple.
Input Capacitor Selection
Since the VIN pin is the supply voltage for the IC It is
recommended to place at least a 4.7uF, low ESR bypass
capacitor.
Optional Schottky Diodes
To achieve a 1%-2% efficiency improvement above 50mW,
Schottky diodes can be added across synchronous switches B
(SW1 to GND) and D (SW2 to VOUT). The Schottky diodes will
provide a lower voltage drop during the break-before-make
time (typically 15ns) of the NMOS to PMOS transition. General
purpose diodes such as a 1N914 are not recommended due to
the slow recovery times and will compromise efficiency. If
desired a large Schottky diode, such as an MBRM120T3, can
be used from SW2 to VOUT. A small diode, such as ZHCS400
from Zetex or CMDSH2-3 from Central Semiconductor, can be
used from SW1 to GND.
Output Voltage>4.3V
A Schottky diode from SW to VOUT is required for output
voltages over 4.3V. The diode must be located as close to the
pins as possible in order to reduce the peak voltage on SW2
due to the parasitic lead and trace inductance.
ML3440