LM3578AM datasheet(14/18 Pages) NSC

14 / 18 page

Typical Applications (Continued)

BUCK-BOOST REGULATOR

The Buck-Boost Regulator, shown in

Figure 22, may step a

voltage up or down, depending upon whether or not the de-

sired output voltage is greater or less than the input voltage.

In this case, the output voltage is 12V with an input voltage

from 9V to 15V. The circuit exhibits an efficiency of 75%, with

a load regulation of 60 mV (10 mA to 100 mA) and a line

regulation of 52 mV.

R1 = (V

o − 1) R2 where R2 = 10 kΩ

R3 = V/0. 75A

R4, C1, C3 and C4 are defined in the “Boost Regulator” sec-

tion.

D1 and D2 are Schottky type diodes such as the 1N5818 or

1N5819.

where:

d is the forward voltage drop of the diodes.

sat is the saturation voltage of the LM1578A output transis-

tor.

sat1 is the saturation voltage of transistor Q1.

≥ (V

in −Vsat −Vsat1)(ton/Ip)

where:

RS-232 LINE DRIVER POWER SUPPLY

The power supply, shown in

Figure 23, operates from an in-

put voltage as low as 4.2V (5V nominal), and delivers an out-

put 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 no-

table 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 fol-

lows by assuming a value of 10 k

Ω for R1;

R2 = (V

o − 1V)/45.8 µA = 240 kΩ

R3 = (|V

o| +1V)/54.2 µA = 240 kΩ

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 cur-

rent 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.

DS008711-12

Vin = 5V

R4 = 190

Ω

Vo = −15V

R5 = 82

Ω

Vripple = 5mV

R6 = 220 k

Ω

Io = 300 mA

C1 = 1820 pF

Imin = 60 mA

C2 = 1000 µF

fosc = 50 kHz

C3 = 20 pF

R1 = 160 k

Ω

C4 = 0.0022 µF

R2 = 10 k

Ω

L1 = 150 µH

R3 = 0.01

Ω

D1 = 1N5818

FIGURE 21. Inverting Regulator

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