LM2594 데이터 시트보기 (PDF) - ON Semiconductor
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LM2594 Datasheet PDF : 25 Pages
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LM2594
PCB LAYOUT GUIDELINES
As in any switching regulator, the layout of the printed
On the other hand, the PCB area connected to the Pin 2
circuit board is very important. Rapidly switching currents (emitter of the internal switch) of the LM2594 should be
associated with wiring inductance, stray capacitance and kept to a minimum in order to minimize coupling to sensitive
parasitic inductance of the printed circuit board traces can circuitry.
generate voltage transients which can generate
Another sensitive part of the circuit is the feedback. It is
electromagnetic interferences (EMI) and affect the desired important to keep the sensitive feedback wiring short. To
operation. As indicated in the Figure 16, to minimize assure this, physically locate the programming resistors near
inductance and ground loops, the length of the leads to the regulator, when using the adjustable version of the
indicated by heavy lines should be kept as short as possible. LM2594 regulator.
For best results, single−point grounding (as indicated) or
ground plane construction should be used.
DESIGN PROCEDURE
Buck Converter Basics
The LM2594 is a “Buck” or Step−Down Converter which
is the most elementary forward−mode converter. Its basic
schematic can be seen in Figure 17.
The operation of this regulator topology has two distinct
time periods. The first one occurs when the series switch is
on, the input voltage is connected to the input of the inductor.
The output of the inductor is the output voltage, and the
rectifier (or catch diode) is reverse biased. During this
period, since there is a constant voltage source connected
across the inductor, the inductor current begins to linearly
ramp upwards, as described by the following equation:
ǒVIN * VOUTǓton
IL(on) +
L
During this “on” period, energy is stored within the core
material in the form of magnetic flux. If the inductor is
properly designed, there is sufficient energy stored to carry
the requirements of the load during the “off” period.
Power
Switch
L
This period ends when the power switch is once again
turned on. Regulation of the converter is accomplished by
varying the duty cycle of the power switch. It is possible to
describe the duty cycle as follows:
d
+
ton
T
,
where
T
is
the
period
of
switching.
For the buck converter with ideal components, the duty
cycle can also be described as:
d
+
Vout
Vin
Figure 18 shows the buck converter, idealized waveforms
of the catch diode voltage and the inductor current.
Von(SW)
Power
Switch
Off
VD(FWD)
Power
Switch
On
Power
Switch
Off
Power
Switch
On
Vin
D
Cout
RLoad
Time
Figure 17. Basic Buck Converter
The next period is the “off” period of the power switch.
When the power switch turns off, the voltage across the
inductor reverses its polarity and is clamped at one diode
voltage drop below ground by the catch diode. The current
now flows through the catch diode thus maintaining the load
current loop. This removes the stored energy from the
inductor. The inductor current during this time is:
ǒVOUT * VDǓtoff
IL(off) +
L
Ipk
ILoad(AV)
Imin
Diode
Power
Switch
Diode
Power
Switch
Time
Figure 18. Buck Converter Idealized Waveforms
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