LM3578AM datasheet(17/20 Pages) NSC

17 / 20 page

Typical Applications (Continued)

where:

RS-232 LINE DRIVER POWER SUPPLY

The power supply, shown in Figure 23, operates from an

input voltage as low as 4.2V (5V nominal), and delivers an

output of ±12V at ±40 mA with better than 70% efficiency.

The circuit provides a load regulation of ±150 mV (from 10%

to 100% of full load) and a line regulation of ±10 mV. Other

notable features include a cycle-by-cycle current limit and an

output voltage ripple of less than 40 mVp-p.

A unique feature of this circuit is its use of feedback from

both outputs. This dual feedback configuration results in a

sharing of the output voltage regulation by each output so

that neither side becomes unbalanced as in single feedback

systems. In addition, since both sides are regulated, it is not

necessary to use a linear regulator for output regulation.

The feedback resistors, R2 and R3, may be selected as

follows by assuming a value of 10 k

Ω for R1;

R2=(V

o − 1V)/45.8 µA = 240 k

Ω

R3 = (|V

o| +1V)/54.2 µA = 240 k

Ω

Actually, the currents used to program the values for the

feedback resistors may vary from 40 µA to 60 µA, as long as

their sum is equal to the 100 µA necessary to establish the

1V threshold across R1. Ideally, these currents should be

equal (50 µA each) for optimal control. However, as was

done here, they may be mismatched in order to use standard

resistor values. This results in a slight mismatch of regulation

between the two outputs.

The current limit resistor, R4, is selected by dividing the

current limit threshold voltage by the maximum peak current

level in the output switch. For our purposes R4 = 110 mV/

750 mA = 0.15

Ω. A value of 0.1Ω was used.

Capacitor C1 sets the oscillator frequency and is selected

from Figure 1.

Capacitor C2 serves as a compensation capacitor for syn-

chronous operation and a value of 10 to 50 pF should be

sufficient for most applications.

00871113

≤ V

≤ 15V

R5 = 270

o = 12V

C1 = 1820 pF

o = 100 mA

C2 = 220 µF

ripple =50mV

C3=20pF

osc = 50 kHz

C4 = 0.0022 µF

R1 = 110k

L1 = 220 µH

R2 = 10k

D1, D2 = 1N5819

R3 = 0.15

Q1 = D44

R4 = 220k

FIGURE 22. Buck-Boost Regulator

00871114

in = 5V

R4 = 0.15

Ω

o ±12V

C1 = 820 pF

o = ±40 mA

C2 = 10 pF

osc = 80 kHz

C3 = 220 µF

R1=10k

Ω

D1, D2, D3 = 1N5819

R2 = 240 k

Ω

T1 = PE-64287

R3 = 240 k

Ω

FIGURE 23. RS-232 Line Driver Power Supply

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