NCP1547 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1547

1.5 A, 340 kHz, Buck Regulator with Synchronization Capability

ON Semiconductor 

NCP1547

Figure 15 to Figure 18 show the output ripple of a 5.0 V

to 3.3 V/500 mA regulator using 22 mH inductor and various

capacitor types. At the switching frequency, the low ESR

and ESL make the ceramic capacitors behave capacitively

as shown in Figure 15. Additional paralleled ceramic

capacitors will further reduce the ripple voltage, but

inevitably increase the cost. “POSCAP”, manufactured by

SANYO, is a solid electrolytic capacitor. The anode is

sintered tantalum and the cathode is a highly conductive

polymerized organic semiconductor. TPC series, featuring

low ESR and low profile, is used in the measurement of

Figure 16. It is shown that POSCAP presents a good balance

of capacitance and ESR, compared with a ceramic capacitor.

In this application, the low ESR generates less than 5.0 mV

of ripple and the ESL is almost unnoticeable. The ESL of the

through−hole OS−CON capacitor give rise to the inductive

impedance. It is evident from Figure 17 which shows the

step rise of the output ripple on the switch turn−on and large

spike on the switch turn−off. The ESL prevents the output

capacitor from quickly charging up the parasitic capacitor of

the inductor when the switch node is pulled below ground

through the catch diode conduction. This results in the spike

associated with the falling edge of the switch node. The D

package tantalum capacitor used in Figure 18 has the same

footprint as the POSCAP, but doubles the height. The ESR

of the tantalum capacitor is apparently higher than the

POSCAP. The electrolytic and tantalum capacitors provide

a low−cost solution with compromised performance. The

reliability of the tantalum capacitor is not a serious concern

for output filtering because the output capacitor is usually

free of surge current and voltage.

Diode Selection

The diode in the buck converter provides the inductor

current path when the power switch turns off. The peak

reverse voltage is equal to the maximum input voltage. The

peak conducting current is clamped by the current limit of

the IC. The average current can be calculated from:

ID(AVG)

+

IO(VIN *

VIN

VO)

The worse case of the diode average current occurs during

maximum load current and maximum input voltage. For the

diode to survive the short circuit condition, the current rating

of the diode should be equal to the Foldback Current Limit.

See Table 1 for Schottky diodes from ON Semiconductor

which are suggested for use with the NCP1547 regulator.

Inductor Selection

When choosing inductors, one might have to consider

maximum load current, core and copper losses, component

height, output ripple, EMI, saturation and cost. Lower

inductor values are chosen to reduce the physical size of the

inductor. Higher value cuts down the ripple current, core

losses and allows more output current. For most

applications, the inductor value falls in the range between

2.2 mH and 22 mH. The saturation current ratings of the

inductor shall not exceed the IL(PK), calculated according to

IL(PK)

+

IO

)

VO(VIN * VO)

2(fS)(L)(VIN)

The DC current through the inductor is equal to the load

current. The worse case occurs during maximum load

current. Check the vendor’s spec to adjust the inductor value

under current loading. Inductors can lose over 50% of

inductance when it nears saturation.

The core materials have a significant effect on inductor

performance. The ferrite core has benefits of small physical

size, and very low power dissipation. But be careful not to

operate these inductors too far beyond their maximum

ratings for peak current, as this will saturate the core.

Powered Iron cores are low cost and have a more gradual

saturation curve. The cores with an open magnetic path, such

as rod or barrel, tend to generate high magnetic field

radiation. However, they are usually cheap and small. The

cores providing a close magnetic loop, such as pot−core and

toroid, generate low electro−magnetic interference (EMI).

There are many magnetic component vendors providing

standard product lines suitable for the NCP1547. Table 2

lists three vendors, their products and contact information.

Table 1.

Part Number

VBREAKDOWN (V)

IAVERAGE (A)

V(F) (V) @ IAVERAGE

Package

1N5817

20

1.0

0.45

Axial Lead

1N5818

30

1.0

0.55

Axial Lead

1N5819

40

1.0

0.6

Axial Lead

MBR0520

20

0.5

0.385

SOD−123

MBR0530

30

0.5

0.43

SOD−123

MBR0540

40

0.5

0.53

SOD−123

MBRS120

20

1.0

0.55

SMB

MBRS130

30

1.0

0.395

SMB

MBRS140

40

1.0

0.6

SMB

http://onsemi.com

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