LM2576-12 MOTOROLA [Motorola, Inc], LM2576-12 Datasheet - Page 11

• Manufacturer Part Number: LM2576-12
• Description: Easy Switcher 3.0A Step-Down Voltage Regulator
• Manufacturer: MOTOROLA [Motorola, Inc]

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 (Adjustable Output Version: LM2576–ADJ)
MOTOROLA ANALOG IC DEVICE DATA
5. Output Capacitor Selection (C out )
Given Parameters:
1. Programming Output Voltage
2. Input Capacitor Selection (C in )
3. Catch Diode Selection (D1)
A. Since the LM2576 is a forward–mode switching regulator
B. Due to the fact that the higher voltage electrolytic capacitors
V out = Regulated Output Voltage
V in(max) = Maximum DC Input Voltage
I Load(max) = Maximum Load Current
To select the right programming resistor R1 and R2 value (see
Figure 2) use the following formula:
Resistor R1 can be between 1.0 k and 5.0 k . (For best
temperature coefficient and stability with time, use 1% metal
film resistors).
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
“Application Hints” section of this data sheet.
A. Since the diode maximum peak current exceeds the
B. The reverse voltage rating of the diode should be at least
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 F and 2000 F is recommended.
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.
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.
1.25 times the maximum input voltage.
V out
+
V
ref
R2
1.0
+
Procedure
Procedure
R1
)
R2
R1
V out
V
ref
where V ref = 1.23 V
– 1.0
LM2576
5. Output Capacitor Selection (C out )
Given Parameters:
1. Programming Output Voltage (selecting R1 and R2)
2. Input Capacitor Selection (C in )
3. Catch Diode Selection (D1)
A. C out = 680 F to 2000 F standard aluminium electrolytic.
B. Capacitor voltage rating = 20 V.
V out = 8.0 V
V in(max) = 25 V
I Load(max) = 2.5 A
Select R1 and R2:
A 100 F, 150 V aluminium electrolytic capacitor located near
the input and ground pin provides sufficient bypassing.
A. For this example, a 3.0 A current rating is adequate.
B. Use a 30 V 1N5821 Schottky diode or any
R2 = 9.91 k , choose a 9.88 k metal film resistor.
suggested fast recovery diode in the Table 1.
V out
R2
+
+
R1
1.23 1.0
V out
V
ref
*
)
Example
Example
1.0
R2
R1
+
Select R1 = 1.8 k
1.8 k
1.23 V
8.0 V
*
1.0
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