TC2575 Datasheet, PDF (6/25 Pages) TelCom Semiconductor, Inc – 1.0A Step-Down Switching Regulator

1.0A Step-Down Switching Regulator
TC2575
PCB LAYOUT GUIDELINES
As in 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. As indicated in the Figure 2, to minimize inductance
and 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 2 (emitter of the internal switch) of the TC2575 should be
kept to a minimum in order to minimize coupling to sensitive
circuitry.
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
near to the regulator, when using the adjustable version of
the TC2575 regulator.
DESIGN PROCEDURE
Buck Converter Basics
The TC2575 is a “Buck” or Step–Down Converter which
is the most elementary forward–mode converter. Its basic
schematic can be seen in Figure 3.
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:
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:
I_L
(OFF)
=
(V_OUT
– V_D
L
)
t_OFF
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 , where T is the period of switching.
T
For the buck converter with ideal components, the duty
cycle can also be described as:
d = ^VOUT
V_IN
Figure 4 shows the buck converter idealized waveforms
of the catch diode voltage and the inductor current.
VON (SW)
Power
Switch
Off
Power
Switch
On
Power Power
Switch Switch
Off
On
Time
I_L
(ON)
=
(V_IN
–
V_OUT
L
)
t_ON
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
+
V_IN
D1
–
C_OUT+
V_OUT
R_LOAD
VD/(FWD)
IPK
IMIN Power Diode Power
Diode Switch
Switch
ILOAD (AV)
Time
Figure 4. Buck Converter Idealized Waveforms
Figure 3. Basic Buck Converter
TC2575-1 3/13/00
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