參數(shù)資料
型號(hào)： LM2574
廠商： ON SEMICONDUCTOR
英文描述： 0.5 A, Adjustable Output Voltage, Step-Down Switching Regulator(0.5A,可調(diào)輸出電壓,降壓型開(kāi)關(guān)穩(wěn)壓器)
中文描述： 0.5甲，可調(diào)輸出電壓，降壓型開(kāi)關(guān)穩(wěn)壓器（0.5A的，可調(diào)輸出電壓，降壓型開(kāi)關(guān)穩(wěn)壓器）
文件頁(yè)數(shù)： 11/26頁(yè)
文件大?。?/td> 295K
代理商： LM2574
LM2574, NCV2574
http://onsemi.com
11
Procedure
(Fixed Output Voltage Version) (continued)
In order to simplify the switching regulator design, a stepbystep
design procedure and example is provided.
Procedure
Example
5. Output Capacitor Selection (C
out
)
A.
Since the LM2574 is a forwardmode switching regulator
with voltage mode control, its open loop 2pole1zero
frequency characteristic has the dominant polepair
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 F and 470 F 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.
5. Output Capacitor Selection (C
out
)
A.
C
out
= 100 F to 470 F standard aluminium electrolytic.
B.
Capacitor voltage rating = 20 V.
Procedure
(Adjustable Output Version: LM2574ADJ)
Procedure
Example
Given Parameters:
V
out
= Regulated Output Voltage
V
in(max)
= Maximum DC Input Voltage
I
Load(max)
= Maximum Load Current
Given Parameters:
V
out
= 24 V
V
in(max)
= 40 V
I
Load(max)
= 0.4 A
1. Programming Output Voltage
To select the right programming resistor R1 and R2 value (see
Figure 2) use the following formula:
where V
ref
= 1.23 V
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).
Vout
Vref
Vout
Vref1.0
R2
R1
R2
R1
1.0
1. Programming Output Voltage
(
selecting R1 and R2)
Select R1 and R2 :
V
out
= 1.23
Select R1 = 1.0 k
R2 = 18.51 k , choose a 18.7 k metal film resistor.
1.0
R2
R1
R2
R1
Vout
Vref
1.0
1.0 k
10 V
1.23 V
1.0
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.
2. Input Capacitor Selection (C
in
)
A 22 F aluminium electrolytic capacitor located near the
input and ground pin provides sufficient bypassing.
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
LM2574 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.
3. Catch Diode Selection (D1)
A.
For this example, a 1.0 A current rating is adequate.
B.
Use a 50 V MBR150 Schottky diode or any suggested
fast recovery diodes in Table 1.
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