LM3411 Datasheet, PDF (8/13 Pages) National Semiconductor (TI) – Precision Secondary Regulator/Driver

Applications Information
DS011987-15
FIGURE 3. Isolated 250 mA Flyback Switching Regulator
DS011987-16
FIGURE 4. Isolated 1.5A Flyback Switching Regulator Using a LM2577
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
LM2577 SIMPLE SWITCHER™ voltage regulator for 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 1Ω resistor
and a 100 nF capacitor will filter these transients and mini-
mize this problem. The 1Ω resistor should be located on the
ground side of the LM3411, and the capacitor should be
physically located near the package.
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