LM3411 Datasheet, PDF (10/13 Pages) National Semiconductor (TI) – Precision Secondary Regulator/Driver

Applications Information (Continued)
DS011987-19
FIGURE 7. Precision 5V 1A Low Dropout Regulator
DS011987-20
FIGURE 8. 3.3V 0.5A Low Dropout Regulator
The circuit in Figure 8 shows a 3.3V low dropout regulator
using the LM3411-3.3 and several discrete components.
This circuit is capable of excellent performance with both the
dropout voltage and the ground pin current specifications im-
proved over the LM2941/LM3411 circuit.
The standard LM317 three terminal adjustable regulator cir-
cuit can greatly benefit by adding a LM3411. Performance is
increased and features are added. The circuit shown in Fig-
ure 9 provides much improved line and load regulation,
lower temperature drift, and full remote output voltage sens-
ing on both the high and low side. In addition, a precise cur-
rent limit or constant current feature is simple to add.
Current limit protection in most IC regulators is mainly to pro-
tect the IC from gross over-current conditions which could
otherwise fuse bonding wires or blow IC metalization, there-
fore not much precision is needed for the actual current limit
values. Current limit tolerances can sometimes vary from
±10% to as high as +300% over manufacturing and tem-
perature variations. Often critical circuitry requires a much
tighter control over the amount of current the power supply
can deliver. For example, a power supply may be needed
that can deliver 100% of its design current, but can still limit
the maximum current to 110% to protect critical circuitry from
high current fault conditions.
The circuit in Figure 9 can provide a current limit accuracy
that is better than ±4%, over all possible variations, in addi-
tion to having excellent line, load and temperature
specifications.
DS011987-21
FIGURE 9. Precision Positive Voltage Regulator with Accurate Current Limit
Like the positive regulators, the performance of negative ad-
justable regulators can also be improved by adding the
LM3411. Output voltages of either 3.3V or 5V at currents up
to 1.5A (3A when using a LM333) are possible. Adding two
resistors to the circuit in Figure 10 adds the precision current
limit feature as shown in Figure 11. Current limit tolerances
of ±4% over manufacturing and temperature variations are
possible with this circuit.
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