LM3578A MWC National Semiconductor, LM3578A MWC Datasheet - Page 14

• Manufacturer Part Number: LM3578A MWC
• Description: Switching Regulator
• Manufacturer: National Semiconductor

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Typical Applications
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
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:
V
V
tor.
V
where:
d
sat
sat1
is the forward voltage drop of the diodes.
is the saturation voltage of the LM1578A output transis-
is the saturation voltage of transistor Q1.
o
− 1) R2 where R2 = 10 k
L1
(V
in
− V
sat
− V
sat1
(Continued)
) (t
on
V
V
V
I
I
f
R1 = 160 k
R2 = 10 k
R3 = 0.01
o
min
osc
in
o
ripple
/I
= 300 mA
= −15V
p
= 5V
= 60 mA
= 50 kHz
)
= 5 mV
FIGURE 21. Inverting Regulator
14
R4 = 190
R5 = 82
R6 = 220 k
C1 = 1820 pF
C2 = 1000 µF
C3 = 20 pF
C4 = 0.0022 µF
L1 = 150 µH
D1 = 1N5818
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
circuit provides a load regulation of
100% of full load) and a line regulation of
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
R2 = (V
R3 = (|V
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 . A value of 0.1
±
12V at
o
o
| +1V)/54.2 µA = 240 k
− 1V)/45.8 µA = 240 k
±
40 mA with better than 70% efficiency. The
DS008711-12
was used.
±
for R1;
150 mV (from 10% to
±
10 mV. Other no-