1.0A Step-Down Switching Regulator
TC2575
Procedure (Adjustable Output Version: TC2575-ADJ) (Continued)
Procedure
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 with stand 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.
Use the following formula to calculate the inductor
Volt x microsecond [V x
µs]
constant:
E x T = ( V
IN
– V
OUT
)
V
OUT
10
x
[V x
µsec]
V
IN
F[Hz]
6
Example
3. Catch Diode Selection (D1)
A.
For this example, a 3.0A current rating is adequate.
B.
Use a 20V IN5820 or MBR320 Schottky diode or any
suggested fast recovery diodes in Table 4.
4. Inductor Selection (L1)
A.
Calculate E x T [V x
µsec]
constant:
E x T = ( 12 – 8.0) x
8.0
1000
x
= 51[V x
µsec]
12
52
B.
Match the calculated E x T value with the corresponding
number on the vertical axis of the Inductor Value
Selection Guide shown in Figure 37. This E x T constant
is a measure of the energy handling capability of an
inductor and is dependent upon the type of core, the core
area, the number of turns, and the duty cycle.
C.
Next step is to identify the inductance region intersected
by the E x T value and the maximum load current value
on the horizontal axis shown in Figure 35.
D.
From the inductor code, identify the inductor value. Then
select an appropriate inductor from Table 1 or Table 2.
The inductor chosen must be rated for a switching
frequency of 52kHz and for a current rating of 1.15 x
I
LOAD
. The inductor current rating can also be deter-
mined by calculating the inductor peak current:
I
P (max)
= I
LOAD(max)
+
(V
IN
– V
OUT
) t
ON
2L
B.
E x T = 51 [V x
µsec]
C.
I
LOAD(max)
= 1.0A
Inductance Region = L220
D.
Proper inductor value = 220µH
Choose the inductor from Table 1 or Table 2
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.
TC2575-1 3/13/00
9
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
