LM3478EVAL National Semiconductor, LM3478EVAL Datasheet - Page 20

• 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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I
the current sense resistor.
The value of L1 can be increased above the minimum rec-
ommended to reduce input ripple and output ripple. However,
once D
is minimal.
By increasing the value of L2 above the minimum recom-
mended, Δ
output ripple voltage:
where ESR is the effective series resistance of the output ca-
pacitor.
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 re-
placed by 2L.
SENSE RESISTOR SELECTION
The peak current through the switch, I
ed using the current sense resistor, R
output current. Resistor R
mula:
Sepic Capacitor Selection
The selection of the 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 current
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 ca-
pacitor must be greater than the maximum input voltage.
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
L1PK
must be lower than the maximum current rating set by
IL1
is less than 20% of I
IL2
can be reduced, which in turn will reduce the
SEN
L1AVE
can be selected using the for-
, the benefit to output ripple
SEN
SW(PEAK)
, to provide a certain
can be adjust-
20
is the ripple voltage across the SEPIC capacitor, and
is the ripple current through the inductor L1. The energy bal-
ance 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 capacitor 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 performance, es-
pecially with V
20Ω resistor at the input to provide a RC filter. The resistor is
placed in series with the V
attached to the V
ceramic capacitor is necessary in this configuration. The bulk
input capacitor and inductor will connect on the other side of
the resistor with the input power supply.
Output Capacitor Selection
The output capacitor of the SEPIC sees very large ripple cur-
rents (similar to the output capacitor of a boost converter). The
rms current through the output capacitor is given by:
The ESR and ESL of the output capacitor directly control the
output ripple. Use low 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 ca-
pacitors are recommended at the output for low ripple.
S
:
IN
IN
below 8 volts, it is recommended to use a
pin directly (see
IN
pin with only a bypass capacitor
Figure
11). A 0.1µF or 1µF