NCV8843 데이터 시트보기 (PDF) - ON Semiconductor

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NCV8843

Buck Regulator - Synchronous 1.5 A, 340 kHz

ON Semiconductor 

NCV8843

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 diode average current occurs during

maximum load current and maximum input voltage. Diode

power dissipation can be estimated by (Iavg x Vf) x

(100−duty cycle) / Average power, ambient temperature and

thermal characteristics must all be considered when

selecting a diode. For the diode to survive a short circuit

condition, the current rating of the diode should be equal to

the Foldback Current Limit.

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 NCV8843. Table 1

lists three vendors, their products and contact information.

Table 1.

Vendor

Coiltronics

Coilcraft

TDK

Product Family

UNI−Pac1/2: SMT, barrel

THIN−PAC: SMT, toroid, low profile

CTX: Leaded, toroid

DO1608: SMT, barrel

DS/DT 1608: SMT, barrel, magnetically shielded

DO3316: SMT, barrel

DS/DT 3316: SMT, barrel, magnetically shielded

DO3308: SMT, barrel, low profile

SLF10145, SLF12555, VLF10040

Web Site

www.coiltronics.com

www.coilcraft.com

www.tdk.com

http://onsemi.com

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