0.5A Step-Down Switching Regulator
TC2574
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. 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:
PCB LAYOUT GUIIDELINES
As with any switching regulator, the layout of the printed
circuit board is very important. Rapidly switching currents
associated with wiring inductance, stray capacitance and
parasitic inductance of the printed circuit board traces can
generate voltage transients which can generate electro-
magnetic interferences (EMI) and affect the desired opera-
tion.AsindicatedintheFigure2,tominimizeinductanceand
ground loops, the length of the leads indicated by heavy
lines should be kept as short as possible.
For best results, single–point grounding (as indicated)
or ground plane construction should be used.
On the other hand, the PCB area connected to the
Pin 7 (emitter of the internal switch) of the TC2574 should
be kept to a minimum in order to minimize coupling to
sensitive circuitry.
(VOUT – VD ) tOFF
IL (OFF)
=
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:
tON
T
d =
,where T is the period of switching.
Another sensitive part of the circuit is the feedback. It is
important to keep the sensitive feedback wiring short. To
assure this, physically locate the programming resistors
nearto the regulator, when using the adjustable version of
the TC2574 regulator.
For the buck converter with ideal components, the duty
cycle can also be described as:
VOUT
d =
VIN
DESIGN PROCEDURE
Buck Converter Basics
Figure 4 shows the buck converter idealized waveforms
of the catch diode voltage and the inductor current.
The TC2574 is a “Buck” or Step–Down Converter which
is the most elementary forward–mode converter. Its basic
schematic can be seen in Figure 3.
V
(SW)
ON
The operation of this regulator topology has two distinct
time periods. The first one occurs when the series switch is
on,theinputvoltageisconnectedtotheinputoftheinductor.
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:
Power
Switch
Off
Power
Switch
Off
Power
Switch
On
Power
Switch
On
V
/(FWD)
D
Time
(VIN – VOUT ) tON
IL (ON)
=
L
I
PK
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.
I
(AV)
LOAD
Time
I
Power Diode
Switch
Power
Switch
MIN
Diode
L
Power Switch
Figure 4. Buck Converter Idealized Waveforms
+
C
+
V
R
OUT
IN
LOAD
D1
–
Figure 3. Basic Buck Converter
TC2574-1 1/6/00
6
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