LM3478 MDC National Semiconductor, LM3478 MDC Datasheet - Page 19

• Manufacturer Part Number: LM3478 MDC
• Description: LM3478/LM3478Q High Efficiency Low-Side N-Channel Controller for Switching Regulator; ; Container: Tray
• Manufacturer: National Semiconductor

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
Other Application Circuit
S
:
FIGURE 14. Typical Flyback Circuit
19
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.
IN
IN
below 8 volts, it is recommended to use a
pin directly (see Figure 11). A 0.1µF or 1µF
IN
pin with only a bypass capacitor
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