LM3478MMX National Semiconductor, LM3478MMX Datasheet - Page 19

• Manufacturer Part Number: LM3478MMX
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,TSSOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM3478MMX

• Rohs Compliant: NO

Designing SEPIC Using LM3478
Peak current in the inductor, to ensure the inductor does not
saturate:
I
the current sense resistor.
The value of L1 can be increased above the minimum rec-
ommended to reduce input ripple and output ripple. How-
ever, once D
output ripple is minimal.
By increasing the value of L2 above the minimum recom-
mended, ∆
output ripple voltage:
where ESR is the effective series resistance of the output
capacitor.
If L1 and L2 are wound on the same core, then L1 = L2 = L.
All the equations above will hold true if the inductance is
replaced by 2L. A good choice for transformer with equal
turns is Coiltronics CTX series Octopack.
SENSE RESISTOR SELECTION
The peak current through the switch, I
justed using the current sense resistor, R
certain output current. Resistor R
the formula:
Sepic Capacitor Selection
The selection of SEPIC capacitor, CS, depends on the rms
current. The rms current of the SEPIC capacitor is given by:
The SEPIC capacitor must be rated for a large ACrms cur-
rent relative to the output power. This property makes the
SEPIC much better suited to lower power applications where
the rms current through the capacitor is relatively small
(relative to capacitor technology). The voltage rating of the
SEPIC capacitor must be greater than the maximum input
voltage. Tantalum capacitors are the best choice for SMT,
L1PK
(Continued)
must be lower than the maximum current rating set by
IL2
IL1
can be reduced, which in turn will reduce the
is less than 20% of I
SEN
can be selected using
L1AVE
SW(PEAK)
SEN
, the benefit to
, to provide a
can be ad-
19
having high rms current ratings relative to size. Ceramic
capacitors could be used, but the low C values will tend to
cause larger changes in voltage across the capacitor due to
the large currents. High C value ceramics are expensive.
Electrolytics work well for through hole applications where
the size required to meet the rms current rating can be
accommodated. There is an energy balance between CS
and L1, which can be used to determine the value of the
capacitor. The basic energy balance equation is:
Where
is the ripple voltage across the SEPIC capacitor, and
is the ripple current through the inductor L1. The energy
balance equation can be solved to provide a minimum value
for C
Input Capacitor Selection
Similar to a boost converter, the SEPIC has an inductor at
the input. Hence, the input current waveform is continuous
and triangular. The inductor ensures that the input capacitor
sees fairly low ripple currents. However, as the input capaci-
tor gets smaller, the input ripple goes up. The rms current in
the input capacitor is given by:
The input capacitor should be capable of handling the rms
current. Although the input capacitor is not as critical in a
boost application, low values can cause impedance interac-
tions. Therefore a good quality capacitor should be chosen
in the range of 100µF to 200µF. If a value lower than 100µF
is used than problems with impedance interactions or
switching noise can affect the LM3478. To improve perfor-
mance, especially with V
to use a 20Ω resistor at the input to provide a RC filter. The
resistor is placed in series with the V
capacitor attached to the V
0.1µF or 1µF ceramic capacitor is necessary in this configu-
ration. The bulk input capacitor and inductor will connect on
the other side of the resistor with the input power supply.
Output Capacitor Selection
The ESR and ESL of the output capacitor directly control the
output ripple. Use low capacitors with low ESR and ESL at
S
:
IN
below 8 volts, it is recommended
IN
pin directly (see Figure 11). A
IN
pin with only a bypass
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