Datasheet

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BYPASS
LM3481
LM3481
www.ti.com SNVS346E –NOVEMBER 2007–REVISED APRIL 2012
Figure 32. Reducing IC Input Noise
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:
Where
and D, the duty cycle is equal to (V
OUT
− V
IN
)/V
OUT
.
The ESR and ESL of the output 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 and polymer tantalum, Sanyo- OSCON, or multi-layer ceramic capacitors are recommended at the
output.
Driver Supply Capacitor Selection
A good quality ceramic bypass capacitor must be connected from the V
CC
pin to the PGND pin for proper
operation. This capacitor supplies the transient current required by the internal MOSFET driver, as well as
filtering the internal supply voltage for the controller. A value of between 0.47µF and 4.7µF is recommended.
Layout Guidelines
Good board layout is critical for switching controllers such as the LM3481. 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. Switch mode 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 effected by switching noise. This noise can cause duty cycle jitter which leads to
increased spectral noise. Although the LM3481 has 250 ns 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 33 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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