0.5A Step-Down Switching Regulator
TC2574
Procedure (Fixed Output Voltage Version)
In order to simplify the switching regulator design, a step-by-step design procedure and examples are provided.
Procedure
Given Parameters:
V
OUT
= Regulated Output Voltage (3.3V, 5.0V or 12V)
V
IN
(max) = Maximum 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
tantalum electrolytic bypass capacitor is needed between the
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.
For a robust design the diode should have a current rating
equal to the maximum current limit of the TC2574 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 39 to 41.
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 2. 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:
Example
Given Parameters:
V
OUT
= 5.0 V
V
IN
(max) = 15 V
I
LOAD
(max) = 0.4 A
1. Controller IC Selection
According to the required input voltage, output voltage,
polarity and current value, use the TC2574–5
controlller IC.
2. Input Capacitor Selection (C
IN
)
A 22µF, 25V aluminium electrolytic capacitor
located near to the input and ground pins provides input
sufficient bypassing.
3. Catch Diode Selection (D1)
A.
For this example the current rating of the diode is 1.0A
B.
Use a 20V 1N5817 Schottky diode, or any of the
suggested fast recovery diodes shown in Table 1.
4. Inductor Selection (L1)
A.
Use the inductor selection guide shown in Figure 38.
B.
From the selection guide, the inductance area
intersected by the 15V line and 0.4A line is 330.
C.
Inductor value required is 330µH. From Table 2,
choose an inductor from any of the listed
manufacturers.
I
P (max)
= I
LOAD (max)
+
(V
IN
– V
OUT
) t
ON
2L
where t
ON
is the "on" time of the power switch and
t
ON
=
V
OUT
1.0
x
V
IN
f
OSC
For additional information about the inductor, see the inductor
section in the “EXTERNAL COMPONENTS” section of this
data sheet.
TC2574-1 1/6/00
7
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
