UM3433 Datasheet, PDF (7/9 Pages) Union Semiconductor, Inc. – 600mA, 600kHz Step-Up DC-DC Converter

UM3433
Applications Information
Inductor Selection
The UM3433 can utilize small surface mount and chip inductors due to its 600kHz switching
frequency. Typically, a 4.7μH inductor is recommended for most applications. Larger values of
inductance will allow greater output current capability by reducing the inductor ripple current.
Increasing the inductance above 10μH will increase size while providing little improvement in
output current capability.
The approximate output current capability of the UM3433 versus inductance value is given in
the equation below.
where:
η = estimated efficiency
I_P = peak current limit value (0.6A)
V_IN = input (battery) voltage
D = steady-state duty ratio = (V_OUT – V_IN)/V_OUT
f = switching frequency (600kHz typical)
L = inductance value
The inductor current ripple is typically set for 20% to 40% of the maximum inductor current (I_P).
High frequency ferrite core inductor materials reduce frequency dependent power losses
compared to cheaper powdered iron types, improving efficiency. The inductor should have low
ESR (series resistance of the windings) to reduce the I²R power losses, and must be able to handle
the peak inductor current without saturating. Molded chokes and some chip inductors usually do
not have enough core to support the peak inductor currents of 850mA seen on the UM3433. To
minimize radiated noise, use a toroid, pot core or shielded bobbin inductor.
Output and Input Capacitor Selection
Low ESR (equivalent series resistance) capacitors should be used to minimize the output voltage
ripple. Multilayer ceramic capacitors are an excellent choice as they have extremely low ESR and
are available in small footprints. A4.7μF to 15μF output capacitor is sufficient for most
applications. Larger values up to 22μF may be used to obtain extremely low output voltage ripple
and improve transient response. An additional phase lead capacitor may be required with output
capacitors larger than 10μF to maintain acceptable phase margin. X5R and X7R dielectric
materials are preferred for their ability to maintain capacitance over wide voltage and temperature
ranges.
Low ESR input capacitors reduce input switching noise and reduce the peak current drawn from
the battery. It follows that ceramic capacitors are also a good choice for input decoupling and
should be located as close as possible to the device. A 10μF input capacitor is sufficient for
virtually any application. Larger values may be used without limitations.
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