LM3478MAX/NOPB National Semiconductor, LM3478MAX/NOPB Datasheet - Page 18

• Manufacturer Part Number: LM3478MAX/NOPB
• Description: IC CTRLR SW REG N-CH 8SOIC
• Manufacturer: National Semiconductor
• Series: -r

Specifications of LM3478MAX/NOPB

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 100%
• Voltage - Supply: 2.97 V ~ 40 V
• Buck: No
• Boost: Yes
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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OUTPUT CAPACITOR SELECTION
The output capacitor in a boost converter provides all the out-
put current when the inductor is charging. As a result it sees
very large ripple currents. The output capacitor should be ca-
pable of handling the maximum rms current. The rms current
in the output capacitor is:
Where
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 tanta-
lums, 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 out-
put 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 fol-
lowed 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 prob-
lems 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 12 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.
18
The PGND and AGND pins have to be connected to the same
ground very close to the IC. To avoid ground loop currents,
attach all the grounds of the system only at one point.
A ceramic input capacitor should be connected as close as
possible to the Vin pin and grounded close to the GND pin.
For a layout example please see Application Note1204. For
more information about layout in switch mode power supplies
please refer to Application Note 1229.
COMPENSATION
For detailed explanation on how to select the right compen-
sation components to attach to the compensation pin for a
boost topology please see Application Note 1286.
Designing SEPIC Using the LM3478
Since the LM3478 controls a low-side N-Channel MOSFET,
it can also be used in SEPIC (Single Ended Primary Induc-
tance Converter) applications. An example of a SEPIC using
the LM3478 is shown in
age can be higher or lower than the input voltage. The SEPIC
uses two inductors to step-up or step-down the input voltage.
The inductors L1 and L2 can be two discrete inductors or two
windings of a coupled inductor since equal voltages are ap-
plied across the inductor throughout the switching cycle. Us-
ing two discrete inductors allows use of catalog magnetics, as
opposed to a custom inductor. The input ripple can be re-
duced along with size by using the coupled windings for L1
and L2.
Due to the presence of the inductor L1 at the input, the SEPIC
inherits all the benefits of a boost converter. One main ad-
vantage of a SEPIC over a boost converter is the inherent
input to output isolation. The capacitor CS isolates the input
from the output and provides protection against a shorted or
malfunctioning load. Hence, the SEPIC is useful for replacing
boost circuits when true shutdown is required. This means
that the output voltage falls to 0V when the switch is turned
off. In a boost converter, the output can only fall to the input
voltage minus a diode drop.
The duty cycle of a SEPIC is given by:
FIGURE 12. Current Flow In A Boost Application
Figure
13. Note that the output volt-
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