1.0A Step-Down Switching Regulator
TC2575
Procedure (Fixed Output Voltage Version)
In order to simplify the switching regulator design, a step-by-step design procedure and some examples are provided.
Procedure
Given Parameters:
V
OUT
= Regulated Output Voltage (3.3V, 5.0V or 12V)
V
IN
(max) = Maximum DC Input Voltage
I
LOAD
(max) = Maximum Load Current
1. Controller IC Selection
According to the required input voltage, output voltage and
current select the appropriate type of the controller IC output
voltage version.
2. Input Capacitor Selection (C
IN
)
To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminum or tantalum
electrolytic bypass capacitor is needed between the input pin +V
IN
and ground pin GND. This capacitor should be located close to the
IC using short leads. This capacitor should have a low ESR
(Equivalent Series Resistance) value.
3. Catch Diode Selection (D1)
A.
Since the diode maximum peak current exceeds the
regulator maximum load current, the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design the diode should have a
current rating equal to the maximum current limit of the
TC2575 to be able to withstand a continuous output short.
B.
The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
4. Inductor Selection (L1)
A.
According to the required working conditions, select the
correct inductor value using the selection guide from
Figures 32 to 36.
B.
From the appropriate inductor selection guide, identify the
inductance region intersected by the Maximum Input Voltage
line and the Maximum Load Current line. Each region is
identified by an inductance value and an inductor code.
C.
Select an appropriate inductor from the several different
manufacturers part numbers listed in Table 1 or Table 2
When using Table 2 for selecting the right inductor, the
designer must realize that the inductor current rating must
be higher than the maximum peak current flowing through the
inductor. This maximum peak current can be calculated as
follows:
2L
where t
ON
is the "on" time of the power switch and
t
ON
=
V
OUT
1.0
x
Example
Given Parameters:
V
OUT
= 5.0V
V
IN
(max) = 20V
I
LOAD
(max) = 0.8A
1. Controller IC Selection
According to the required input voltage, output voltage,
current polarity and current value, use the TC2575 (5V)
controller IC.
2. Input Capacitor Selection (C
IN
)
A 47µF, 25V aluminium electrolytic capacitor
located near to the input and ground pins provides
sufficient bypassing.
3. Catch Diode Selection (D1)
A.
For this example the current rating of the diode is 1.0A
B.
Use a 30V 1N5818 Schottky diode, or any of the
suggested fast recovery diodes shown in Table 4.
4. Inductor Selection (L1)
A.
Use the inductor selection guide shown in Figure 32
to 36.
B.
From the selection guide, the inductance area
intersected by the 20V line and 0.8A line is L300.
C.
Inductor value required is 300µH. From Table 1, or
Table 2, choose an inductor from any of the listed
manufacturers.
I
P(max)
= I
LOAD (max)
+
(V
IN
– V
OUT
) t
ON
V
IN
f
OSC
For additional information about the inductor, see the inductor
section in the “EXTERNAL COMPONENTS” section of this
data sheet.
TC2575-1 3/13/00
7
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
