Applications Information

The LM3411 regulator/driver provides the reference and

feedback drive functions in a regulated power supply. It can

also be used together with many different types of regula-

tors, (both linear and switching) as well as other power semi-

conductor devices to add precision and improve regulation

specifications. Output voltage tolerances better than 0.5%

are possible without using trim pots or precision resistors.

One of the main applications of the LM3411 is to drive an

opto-isolator to provide feedback signal isolation in a switch-

ing regulator circuit. For low current applications, (up to 250

mA) the circuit shown in

Figure 3 provides good regulation

and complete input/output electrical isolation.

For an input voltage of 15V, this circuit can provide an output

of either 3.3V or 5V with a load current up to 250 mA with ex-

cellent regulation characteristics. With the part values

shown, this circuit operates at 80 kHz., and can be synchro-

nized to a clock or an additional LM3578. (See LM1578 data

sheet for additional information.)

An isolated DC/DC flyback converter capable of higher out-

put current is shown in

Figure 4. This circuit utilizes the

Pulse Width Modulation (PWM), power switch and protection

functions, while the LM3411 provides the voltage reference,

gain and opto coupler drive functions. In this circuit, the ref-

erence and error amplifier in the LM2577 are not used (note

that the feedback pin is grounded). The gain is provided by

the LM3411. Since the voltage reference is located on the

secondary side of the transformer, this circuit provides very

good regulation specifications.

The output of a switching regulator typically will contain a

small ripple voltage at the switching frequency and may also

contain voltage transients. These transient voltage spikes

can be sensed by the LM3411 and could give an incorrect

regulation voltage. An RC filter consisting of a 1

Ω resistor

and a 100 nF capacitor will filter these transients and mini-

mize this problem. The 1

Ω resistor should be located on the

ground side of the LM3411, and the capacitor should be

physically located near the package.

DS011987-15

FIGURE 3. Isolated 250 mA Flyback Switching Regulator

DS011987-16

FIGURE 4. Isolated 1.5A Flyback Switching Regulator Using a LM2577

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