LM3478_09 Datasheet, PDF (17/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

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FIGURE 12. Current Flow In A Boost Application
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 Figure 13. Note that the output volt-
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:
In the above equation, V_Q is the on-state voltage of the MOS-
FET, Q, and V_DIODE is the forward voltage drop of the diode.
FIGURE 13. Typical SEPIC Converter
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POWER MOSFET SELECTION
As in a boost converter, parameters governing the selection
of the MOSFET are the minimum threshold voltage, V_TH
(MIN)^{, the on-resistance, R}DS(ON)^{, the total gate charge, Q}g^{, the}
reverse transfer capacitance, C_RSS, and the maximum drain
to source voltage, V_DS(MAX). The peak switch voltage in a
SEPIC is given by:
V_SW(PEAK) = V_IN + V_OUT + V_DIODE
The selected MOSFET should satisfy the condition:
V_DS(MAX) > V_SW(PEAK)
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