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LT1683

1683f

For continuous operation at IOUT(MIN) = IOUT(MAX)/4,

inductor ripple (the same as output ripple):

∆=

•

The duty cycle for nominal input is:

NOM

OUT

IN NOM

−

()

47 5

35 3

••

•.

()

Then

kHz

() −

()

=µ

50 5

1 2 35 3

100

.•

•

. %

•

Off-the-shelf components can be used for this inductor.

Say we choose a 22

µH inductor then ripple current at

maximum input (DC = 29.1%) is:

∆=

() −

()

kHz

50 5

1 2 29 1

100

103

.•

•

. %

•

The maximum inductor current is:

L MAX

()

103

252

Primary inductance should be greater than:

LPRI = 5 • 22µH • 6.12 = 4.1mH

The secondary inductance would then be:

4.1mH/6.12 = 110

µH

The magnetizing ripple current is approximately:

∆=

kHz

MAG

4 1

100

•.

•

Peak switch current is:

SW PEAK

()

•.

2 51

494

Note that you can discern your magnetizing ripple by

looking at the reflected inductance ripple and subtracting

it from the switch current ripple.

∆IMAG = N • IL – ∆ISW

The max ripple current on the switch is:

∆=

SW MAX

()

103

0 25

Knowing the peak switch current we can go back and

iterate with a more accurate switch on voltage. We would

have to know the RON of the FET. In our case our assump-

tions of a 0.5V switch on voltage is valid for

RON + RSENSE < 1Ω.

Capacitors

Correct choice of input and output capacitors is very

important to low noise switcher performance. Push-pull

topologies and other low noise topologies will in general

have continuous currents, which reduce the requirements

for capacitance. However, noise depends more on the ESR

of the capacitors. In addition lower ESR can also improve

efficiency.

Input capacitors must also withstand surges that occur

during the switching of some types of loads. Some solid

tantalum capacitors can fail under these surge conditions.

Design Note 95 offers more information but the following

is a brief summary of capacitor types and attributes.

Aluminum Electrolytic: Low cost and higher voltage. They

can be used in this application but in general you will need

higher capacitance to achieve low ESR. Additional

nonelectrolytic capacitors may be required to achieve

better performance.

Specialty Polymer Aluminum: Panasonic has come out

with their series CD capacitors. While they are only avail-

able for voltages below 16V, they have very low ESR and

good surge capability.

APPLICATIO S I FOR ATIO

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