Datasheet

LM2578A, LM3578A
SNVS767E –AUGUST 2000–REVISED APRIL 2013 www.ti.com
where:
V
d
is the forward voltage drop of the diodes.
V
sat
is the saturation voltage of the LM2578A output transistor.
V
sat1
is the saturation voltage of transistor Q1.
L1 ≥ (V
in
− V
sat
− V
sat1
) (t
on
/I
p
) (1)
where:
RS-232 LINE DRIVER POWER SUPPLY
The power supply, shown in Figure 36, 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 kΩ 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 current 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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