Datasheet

LM3481

SNVS346E –NOVEMBER 2007–REVISED APRIL 2012

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Figure 33. 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 Note 1204 (SNVA042). For more information about layout in switch

mode power supplies please refer to Application Note 1229 (SNVA054).

Compensation

For detailed explanation on how to select the right compensation components to attach to the compensation pin

for a boost topology please see Application Note 1286 (SNVA067). When calculating the Error Amplifier DC gain,

, R

OUT

= 152 kΩ for the LM3481.

DESIGNING SEPIC USING LM3481

Since the LM3481 controls a low-side N-Channel MOSFET, it can also be used in SEPIC (Single Ended Primary

Inductance Converter) applications. An example of SEPIC using the LM3481 is shown in Figure 34. As shown in

Figure 34, 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 transformer 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 transformer. The input ripple can

be reduced along with size by using the coupled windings of transformer 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 SEPIC over a boost converter is the inherent input to output isolation. The capacitor C

isolates the input from the output and provides protection against 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

is the on-state voltage of the MOSFET, Q1, and V

DIODE

is the forward voltage drop of

the diode.

Product Folder Links: LM3481