LM2576
Procedure (Fixed Output Voltage Version) (continued)In order to simplify the switching regulator design, a step−by−step
design procedure and some examples are provided.
Procedure
Example
5. Output Capacitor Selection (C )
out
5. Output Capacitor Selection (C
)
out
A. Since the LM2576 is a forward−mode switching regulator
A. C = 680 mF to 2000 mF standard aluminium electrolytic.
out
with voltage mode control, its open loop 2−pole−1−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
680 mF and 2000 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 8.0 V is appropriate, and a 10 V or
16 V rating is recommended.
B. Capacitor voltage rating = 20 V.
Procedure (Adjustable Output Version: LM2576−ADJ)
Procedure
Example
Given Parameters:
Given Parameters:
V
out
= Regulated Output Voltage
V
out
= 8.0 V
V
= Maximum DC Input Voltage
V
= 25 V
in(max)
in(max)
I
= Maximum Load Current
I
= 2.5 A
Load(max)
Load(max)
1. Programming Output Voltage
1. Programming Output Voltage (selecting R1 and R2)
To select the right programming resistor R1 and R2 value (see
Figure 2) use the following formula:
Select R1 and R2:
R2
R1
+ 1.23ǒ1.0 )
Ǔ
V
Select R1 = 1.8 kW
out
R2
R1
ref ǒ1.0 )
Ǔ
V
+ V
where V = 1.23 V
ref
out
V
out
8.0 V
+ 1.8 kǒ
* 1.0Ǔ
1.23 V
R2 + R1ǒ Ǔ
* 1.0
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
ref
R2 = 9.91 kW, choose a 9.88 k metal film resistor.
V
out
R2 + R1ǒ Ǔ
– 1.0
V
ref
2. Input Capacitor Selection (C )
2. Input Capacitor Selection (C )
in
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
A 100 mF, 150 V aluminium electrolytic capacitor located near
the input and ground pin provides sufficient bypassing.
input pin +V and ground pin GND This capacitor should be
in
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
“Application Hints” section of this data sheet.
3. Catch Diode Selection (D1)
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
LM2576 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.
A. For this example, a 3.0 A current rating is adequate.
B. Use a 30 V 1N5821 Schottky diode or any suggested fast
recovery diode in the Table 1.
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