LM3478EVAL National Semiconductor, LM3478EVAL Datasheet - Page 2

• Manufacturer Part Number: LM3478EVAL
• Description: BOARD EVALUATION LM3478
• Manufacturer: National Semiconductor

Specifications of LM3478EVAL

• Main Purpose: DC/DC, Step Up
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 12V
• Current - Output: 1.5A
• Voltage - Input: 4.5 ~ 5.5V
• Regulator Topology: Boost
• Board Type: Fully Populated
• Utilized Ic / Part: LM3478
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Frequency - Switching: -

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Performance
Benchmark data has been taken from the evaluation board
using the LM3478. Figure 2 shows an efficiency measure-
ment taken at the maximum load of 1.5A with Vin at 5V.
The open loop frequency response was also measured us-
ing the evaluation board as specified in the bill of materials.
The bode plot can be seen below in Figure 3.
The advantage of the evaluation board is the ability to ex-
amine performance tradeoffs through substitution of parts.
By careful selection of the components used, it is possible to
optimize the application circuit for a given parameter. For
instance, the FET footprint has been designed to accommo-
date either a SO-8 or SOT23-6 package. The selection of
FET would then be determined by the design constraints. An
example would be that a lower system cost could be ob-
tained by selection of a FET with a higher R
performance would be sacrificed through reduced efficiency.
FIGURE 3. Frequency Response
FIGURE 2. Efficiency vs Load
20023501
DS(ON)
20023503
, although
2
Current Limit
The purpose of the R
selection of the slope compensation needed for the required
application. The amount of slope compensation directly de-
termines the minimum inductance required for stability.
(Please see the LM3478 or LM3488 datasheet for adjust-
ment of slope compensation). In addition to slope compen-
sation R
current limit. Current limit is usually solely determined by the
value of the sense resistor Rsn. But in the LM3478 and
LM3488 an increase in R
crease by a slight amount. This can be advantageous in
several situations. Common sense resistor values are typi-
cally separated by large intervals, making the task of accu-
rately setting the current limit in any application difficult. As a
result current limit is often ignored during the design phase,
which can cause the application to suffer. An excessively
high current limit can result in startup problems if the
cycle-by-cycle current limit does not engage, limiting the
effect of the soft start feature. Or worse, current limit could
be set to low causing the output voltage to drop at the
maximum load. This is where the R
avoid these issues. By selecting a common value sense
resistor, current limit can be accurately set by calculating the
R
sense resistors that can be cost prohibitive and cause pro-
duction issues because of the difficulty in obtaining an ad-
equate supply. For a complete discussion on how to calcu-
late the R
the LM3478 or LM3488 datasheet.
Layout Fundamentals
Good layout for DC-DC converters can be implemented by
following a few simple design guidelines:
1. Place the power components (catch diode, inductor, and
2. Use wide traces between the power components and for
3. Connect the ground pins of the input and output filter
4. Arrange the power components so that the switching
5. Separate noise sensitive traces, such as the voltage
6. Ensure a good low-impedance ground for the converter
7. Place the supporting components for the converter IC,
8. Place noise sensitive circuitry such as radio or modem
SL
filter capacitors) close together. Make the traces be-
tween them as short and wide as possible.
power connections to the DC-DC converter circuit.
capacitors and catch diode as close as possible using
generous
pseudo-ground plane. Then, connect this to the ground
plane through several vias.
loops curl in the same direction.
feedback path, from noisy traces associated with the
power components.
IC.
such as compensation and frequency selection compo-
nents as close to the converter IC as possible, but away
from noisy traces and the power components. Make
their connections to the converter IC and its pseudo-
ground plane as short as possible.
blocks away from the DC-DC converter.
size needed. This eliminates the need to choose custom
SL
SL
also provides assistance in the adjustment of
value needed, refer to the current limit section in
component-side
SL
resistor is to provide flexibility in the
SL
causes the current limit to de-
SL
copper
resistor can be used to
fill
as
a