MD3221N 데이터 시트보기 (PDF) - Unspecified

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MD3221N

DC-DC Converter Power IC

Unspecified 

If a higher inductance value is selected the output

ripple voltage is reduced, however as the current rating

drops the size of the required inductor is increased.

On the other hand, if a lower inductance value is

selected the size of the required inductor is reduced

while peak current is increased, thus slightly

increasing the loss when a load is applied.

While the MD3221N incorporates an over-current

protection function, the inductor selected must be such

that magnetic saturation does not occur at the over-

current detection point.

As a magnetic field is generated in the vicinity of the

inductor, component layout and pattern design must

be such as to ensure that they do not affect the

control circuit. Toroidal or closed magnetic circuit pot

type inductors are recommended for applications likely

to be affected by magnetic field leakage and

radiated noise.

adjustment will prove difficult if capacitance is low.

Aluminum electrolytic condensers and functional

high-polymer electrolytic condensers are of

sufficiently high capacitance so that it need not be

considered when selecting on the basis of the

equation above. The use of such condensers, rather

than ceramic or film condensers, is therefore

recommended. On the other hand, concurrent use of

ceramic or film condensers is effective in eliminating

high frequency noise.

◆Selecting the Input Condenser (C1)

As a large ripple current flows in the input condenser

it is necessary to consider the allowable value for ripple

current. Select an input condenser with an allowable

ripple current exceeding the value calculated in the

equation below.

◆Selecting the Output Condenser (C5)

An output condenser is required to reduce output ripple

and thus obtain a stable DC voltage. The converter

output ripple voltage is determined by ΔI and

condenser impedance. The output condenser is

selected in relation to the impedance. Select the output

condenser based on the desired output ripple

voltage using the following equation.

Irip ＞ − D（1−D）× IO［A］

D＝

VO

Vi

Irip ：Allowable ripple curren［t A］

VO ：Output voltage［V］

V i ：Input voltage［V］

D ：Duty ratio

I O ：Output curren［t A］

Vrip

Z C＜ − ΔI ［Ω］

Zc ：Condenser impedance［Ω］

Vrip：Output ripple voltage［V］

ΔI ：30% of maximum output curren［t A］

The desired ripple voltage is obtained if a condenser

with an impedance less than that calculated with the

above equation is selected from the catalog.

In addition to a low impedance, the output

condenser must also have a reasonable capacitance.

Control will readily become unstable and amp gain

D is the ratio between the ON interval and the

OFF interval, and Irip is at a maximum when this

value is 0.5. The capacitance of the input

condenser need not be particularly high, however

care is required if a ceramic or film condenser is

used. As large ripple voltages are generated in the

input condenser during charging and discharge, a

particularly low input voltage may result in

operation becoming unstable. As this ripple

voltage is returned to the input line other circuits

powered from the same line may be affected.

Interference may occur, particularly when using

multiple DC-DC converters, and in such cases an

inductor of a few μH is inserted in the stage before

the input condenser to eliminate the problem.

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