Datasheet

LM2578A, LM3578A
www.ti.com SNVS767E –AUGUST 2000–REVISED APRIL 2013
Component values are selected as outlined for the buck regulator with a discontinuity factor of 10%, with the
addition of R4 and R5:
R4 = 10V
BE1
B
f
/I
p
R5 = (V
in
− V − V
BE1
− V
sat
) B
f
/(I
L(max, DC)
+ I
R4
)
where:
V
BE1
is the V
BE
of transistor Q1.
V
sat
is the saturation voltage of the LM2578A output transistor.
V is the current limit sense voltage.
B
f
is the forced current gain of transistor Q1 (B
f
= 30 for Figure 30).
I
R4
= V
BE1
/R4
I
p
= I
L(max, DC)
+ 0.5ΔI
L
V
in
= 15V R4 = 200Ω
V
o
= 5V R5 = 330Ω
V
ripple
= 50 mV C1 = 1820 pF
I
o
= 1.5A C2 = 330 μF
f
osc
= 50 kHz C3 = 20 pF
R1 = 40 kΩ L1 = 220 μH
R2 = 10 kΩ D1 = 1N5819
R3 = 0.05Ω Q1 = D45
Figure 30. Buck Converter with Boosted Output Current
BOOST REGULATOR
The boost regulator converts a low input voltage into a higher output voltage. The basic configuration is shown in
Figure 31. Energy is stored in the inductor while the transistor is on and then transferred with the input voltage to
the output capacitor for filtering when the transistor is off. Thus,
V
o
= V
in
+ V
in
(t
on
/t
off
).
Figure 31. Basic Boost Regulator
The circuit of Figure 32 converts a 5V supply into a 15V supply with 150 mA of output current, a load regulation
of 14 mV (30 mA to 140 mA), and a line regulation of 35 mV (4.5V ≤ V
in
≤ 8.5V).
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