English
Language : 

LM3578A Datasheet, PDF (11/20 Pages) National Semiconductor (TI) – Switching Regulator
◁
Applications Information (Continued)
1.5V to 2.0V. The signal source must be capable of 1.)
driving capacitive loads and 2.) delivering up to 500 µA for
each LM2578A.
Capacitors C1 thru CN are to be selected for a 20% slower
frequency than the synchronization frequency.
Vout = D x Vin = Vin x (ton)/(ton + toff).
FIGURE 13. Synchronizing Devices
00871125
Typical Applications
The LM2578A may be operated in either the continuous or
the discontinuous conduction mode. The following applica-
tions (except for the Buck-Boost Regulator) are designed for
continuous conduction operation. That is, the inductor cur-
rent is not allowed to fall to zero. This mode of operation has
higher efficiency and lower EMI characteristics than the dis-
continuous mode.
BUCK REGULATOR
The buck configuration is used to step an input voltage down
to a lower level. Transistor Q1 in Figure 14 chops the input
DC voltage into a squarewave. This squarewave is then
converted back into a DC voltage of lower magnitude by the
low pass filter consisting of L1 and C1. The duty cycle, D, of
the squarewave relates the output voltage to the input volt-
age by the following equation:
00871105
FIGURE 14. Basic Buck Regulator
Figure 15 is a 15V to 5V buck regulator with an output
current, Io, of 350 mA. The circuit becomes discontinuous at
20% of Io(max), has 10 mV of output voltage ripple, an effi-
ciency of 75%, a load regulation of 30 mV (70 mA to 350 mA)
and a line regulation of 10 mV (12 ≤ Vin ≤ 18V).
Component values are selected as follows:
R1 = (Vo − 1) x R2 where R2 = 10 kΩ
R3 = V/Isw(max)
R3 = 0.15Ω
where:
V is the current limit sense voltage, 0.11V
Isw(max) is the maximum allowable current thru the output
transistor.
L1 is the inductor and may be found from the inductance
calculation chart (Figure 16) as follows:
Given
Vin = 15V
Vo = 5V
Io(max) = 350 mA
fOSC = 50 kHz
Discontinuous at 20% of Io(max).
Note that since the circuit will become discontinuous at 20%
of Io(max), the load current must not be allowed to fall below
70 mA.
11
www.national.com
 ▷