Typical Applications (Continued)
DS008711-12
=
=
R4 190Ω
Vin 5V
=
=
R5 82Ω
Vo −15V
=
=
R6 220 kΩ
Vripple 5 mV
=
=
C1 1820 pF
Io 300 mA
=
=
C2 1000 µF
Imin 60 mA
=
=
C3 20 pF
fosc 50 kHz
=
=
C4 0.0022 µF
R1 160 kΩ
=
=
L1 150 µH
R2 10 kΩ
=
=
D1 1N5818
R3 0.01Ω
FIGURE 21. Inverting Regulator
BUCK-BOOST REGULATOR
RS-232 LINE DRIVER POWER SUPPLY
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.
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.
=
=
R1 (Vo − 1) R2 where R2 10 kΩ
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.
=
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.
The feedback resistors, R2 and R3, may be selected as fol-
lows by assuming a value of 10 kΩ for R1;
=
=
R2 (Vo − 1V)/45.8 µA 240 kΩ
=
=
R3 (|Vo| +1V)/54.2 µA 240 kΩ
where:
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.
Vd is the forward voltage drop of the diodes.
Vsat is the saturation voltage of the LM1578A output transis-
tor.
Vsat1 is the saturation voltage of transistor Q1.
L1 ≥ (Vin − Vsat − Vsat1) (ton/Ip)
where:
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 0.15Ω. A value of 0.1Ω was used.
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