LM3478MA_15 Datasheet, PDF (19/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

LM3478MA
www.ti.com
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
OUTPUT CAPACITOR SELECTION
The output capacitor in a boost converter provides all the output current when the inductor is charging. As a
result it sees very large ripple currents. The output capacitor should be capable of handling the maximum RMS
current. The RMS current in the output capacitor is:
(32)
Where
(33)
The ESR and ESL of the capacitor directly control the output ripple. Use capacitors with low ESR and ESL at the
output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer electrolytic,
polymer tantalum, or multi-layer ceramic capacitors are recommended at the output.
For applications that require very low output voltage ripple, a second stage LC filter often is a good solution. Most
of the time it is lower cost to use a small second Inductor in the power path and an additional final output
capacitor than to reduce the output voltage ripple by purely increasing the output capacitor without an additional
LC filter.
LAYOUT GUIDELINES
Good board layout is critical for switching controllers. First the ground plane area must be sufficient for thermal
dissipation purposes and second, appropriate guidelines must be followed to reduce the effects of switching
noise. Switching converters are very fast switching devices. In such devices, the rapid increase of input current
combined with the parasitic trace inductance generates unwanted Ldi/dt noise spikes. The magnitude of this
noise tends to increase as the output current increases. This parasitic spike noise may turn into electromagnetic
interference (EMI), and can also cause problems in device performance. Therefore, care must be taken in layout
to minimize the effect of this switching noise. The current sensing circuit in current mode devices can be easily
affected by switching noise. This noise can cause duty cycle jittering which leads to increased spectral noise.
Although the LM3478 has 325ns blanking time at the beginning of every cycle to ignore this noise, some noise
may remain after the blanking time.
The most important layout rule is to keep the AC current loops as small as possible. Figure 34 shows the current
flow of a boost converter. The top schematic shows a dotted line which represents the current flow during on-
state and the middle schematic shows the current flow during off-state. The bottom schematic shows the currents
we refer to as AC currents. They are the most critical ones since current is changing in very short time periods.
The dotted lined traces of the bottom schematic are the once to make as short as possible.
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