LM3411_15 Datasheet, PDF (11/24 Pages) Texas Instruments – LM3411 Precision Secondary Regulator/Driver

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LM3411
SNVS113E – DECEMBER 1999 – REVISED APRIL 2013
Figure 23. 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 regulators, (both linear and switching) as well as other
power semiconductor 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
switching regulator circuit. For low current applications, (up to 250 mA) the circuit shown in Figure 22 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 excellent regulation characteristics. With the part values shown, this circuit operates at 80 kHz., and can
be synchronized to a clock or an additional LM3578. (See LM1578 data sheet for additional information.)
An isolated DC/DC flyback converter capable of higher output current is shown in Figure 23. 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 reference 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 minimize 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.
Figure 24. Precision 1A Buck Regulator
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