LM3478MA_15 Datasheet, PDF (20/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
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Figure 34. 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 AN-1204. For more information about layout in switch mode power supplies
please refer to AN-1229.
COMPENSATION
For detailed explanation on how to select the right compensation components to attach to the compensation pin
for a boost topology please see AN-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
Inductance Converter) applications. An example of a SEPIC using the LM3478 is shown in Figure 35. Note that
the output voltage 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 applied across the inductor throughout the switching cycle. Using two discrete
inductors allows use of catalog magnetics, as opposed to a custom inductor. The input ripple can be reduced
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 advantage 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:
(34)
In the above equation, V_Q is the on-state voltage of the MOSFET, Q, and V_DIODE is the forward voltage drop of
the diode.
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