lm2574dw ON Semiconductor, lm2574dw Datasheet - Page 11

• Manufacturer Part Number: lm2574dw
• Description: 0.5 A, Adjustable Output Voltage, Stepdown Switching Regulator
• Manufacturer: ON Semiconductor

design procedure and example is provided.
Procedure (Adjustable Output Version: LM2574−ADJ)
Procedure (Fixed Output Voltage Version) (continued) In order to simplify the switching regulator design, a step−by−step
5. Output Capacitor Selection (C
Given Parameters:
1. Programming Output Voltage
2. Input Capacitor Selection (C
3. Catch Diode Selection (D1)
A. Since the LM2574 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 2) use the following formula:
Resistor R1 can be between 1.0 kW 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−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
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 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
LM2574 to be able to withstand a continuous output short.
1.25 times the maximum input voltage.
= Regulated Output Voltage
V out
= Maximum DC Input Voltage
= Maximum Load Current
in
+
and ground pin Gnd. This capacitor should be
V
R2 + R1
ref
1.0 )
Procedure
Procedure
V out
V
R2
R1
ref
in
)
out
* 1.0
)
where V
ref
= 1.23 V
LM2574, NCV2574
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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 = 20 V.
V
V
I
Select R1 and R2 :
R2 = 18.51 kW, choose a 18.7 kW metal film resistor.
A 22 mF aluminium electrolytic capacitor located near the
input and ground pin provides sufficient bypassing.
A. For this example, a 1.0 A current rating is adequate.
B. Use a 50 V MBR150 Schottky diode or any suggested
Load(max)
R2 + R1
out
in(max)
V
fast recovery diodes in Table 1.
out
out
= 24 V
= 1.23
= 100 mF to 470 mF standard aluminium electrolytic.
= 40 V
= 0.4 A
V out
V
ref
1.0 )
* 1.0
R2
R1
Example
Example
+ 1.0 k
Select R1 = 1.0 kW
in
)
out
)
1.23 V
10 V
* 1.0