lm2575-5 ON Semiconductor, lm2575-5 Datasheet - Page 11

• Manufacturer Part Number: lm2575-5
• Description: 1.0 A, Adjustable Output Voltage, Step-down Switching Regulator
• Manufacturer: ON Semiconductor

Procedure (Fixed Output Voltage Version) (continued)
procedure and example is provided.
Procedure (Adjustable Output Version: LM2575−Adj)
5. Output Capacitor Selection (C
Given Parameters:
1. Programming Output Voltage
2. Input Capacitor Selection (C
3. Catch Diode Selection (D1)
A. Since the LM2575 is a forward−mode switching regulator
B. Due to the fact that the higher voltage electrolytic capacitors
V
V
I
To select the right programming resistor R1 and R2 value (see
Figure 14) use the following formula:
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).
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
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.
A. Since the diode maximum peak current exceeds the
B. The reverse voltage rating of the diode should be at least
Load(max)
out
in(max)
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.
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.
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.
1.25 times the maximum input voltage.
= Regulated Output Voltage
V out + V
= Maximum DC Input Voltage
= Maximum Load Current
in
and ground pin GND This capacitor should be
ref
R2 + R1
1 )
Procedure
Procedure
R2
R1
V out
V
in
ref
where V
)
out
)
1
ref
= 1.23 V
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In order to simplify the switching regulator design, a step−by−step design
11
5. Output Capacitor Selection (C
Given Parameters:
1. Programming Output Voltage (selecting R1 and R2)
2. Input Capacitor Selection (C
3. Catch Diode Selection (D1)
A. C
B. Capacitor voltage rating = 16 V.
V
V
I
Select R1 and R2:
A 100 mF 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 20 V 1N5820 or MBR320 Schottky diode or any
Load(max)
out
in(max)
R2 = 9.91 kW, choose a 9.88 k metal film resistor.
suggested fast recovery diode in the Table 4.
out
= 8.0 V
V out + 1.23 1 )
R2 + R1
= 100 mF to 470 mF standard aluminium electrolytic.
= 12 V
= 1.0 A
V out
V
ref
* 1
Example
Example
R2
R1 Select R1 = 1.8 kW
in
+ 1.8 k
)
out
)
1.23 V
8.0 V
* 1