LM2575
Procedure (Fixed Output Voltage Version) (continued)In
order to simplify the switching regulator design, a step−by−step
design procedure and example is provided.
Procedure
5. Output Capacitor Selection (C
out
)
A.
Since the LM2575 is a forward−mode switching regulator
with voltage mode control, its open loop 2−pole−2−zero
frequency characteristic has the dominant pole−pair
determined by the output capacitor and inductor values. For
stable operation and an acceptable ripple voltage,
(approximately 1% of the output voltage) a value between
100
mF
and 470
mF
is recommended.
B.
Due to the fact that the higher voltage electrolytic capacitors
generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating at least 8V is appropriate, and a 10 V or
16 V rating is recommended.
Example
5. Output Capacitor Selection (C
out
)
A.
C
out
= 100
mF
to 470
mF
standard aluminium electrolytic.
B.
Capacitor voltage rating = 16 V.
Procedure (Adjustable Output Version: LM2575−Adj)
Procedure
Given Parameters:
V
out
= Regulated Output Voltage
V
in(max)
= Maximum DC Input Voltage
I
Load(max)
= Maximum Load Current
1. Programming Output Voltage
To select the right programming resistor R1 and R2 value (see
Figure 14) use the following formula:
V out
+
V
ref
1
)
R2
R1 where V
ref
= 1.23 V
Given Parameters:
V
out
= 8.0 V
V
in(max)
= 12 V
I
Load(max)
= 1.0 A
1. Programming Output Voltage
(selecting R1 and R2)
Select R1 and R2:
V out
+
1.23 1
)
R2
+
R1
V out
V
ref
R2
R1 Select R1 = 1.8 kW
+
1.8 k
8.0 V
*
1
1.23 V
Example
Resistor R1 can be between 1.0 k and 5.0 kW. (For best
temperature coefficient and stability with time, use 1% metal
film resistors).
V out
R2
+
R1
–1
V
ref
2. Input Capacitor Selection (C
in
)
To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium 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.
For additional information see input capacitor section in the
“External Components” section of this data sheet.
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
LM2575 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.
*
1
R2 = 9.91 kW, choose a 9.88 k metal film resistor.
2. Input Capacitor Selection (C
in
)
A 100
mF
aluminium electrolytic capacitor located near the
input and ground pin provides sufficient bypassing.
3. Catch Diode Selection (D1)
A.
For this example, a 3.0 A current rating is adequate.
B.
Use a 20 V 1N5820 or MBR320 Schottky diode or any
suggested fast recovery diode in the Table 4.
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