LT1683IG View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1683IG

Slew Rate Controlled Ultralow Noise Push-Pull DC/DC Controller

Linear Technology 

LT1683

APPLICATIONS INFORMATION

So, given the turns ratio, primary inductance and cur-

rent, the transformer can be designed. The design of the

transformer will require analyzing the power losses of the

transformer and making necessary adjustments.

Most transformer companies can assist you with designing

an optimal solution. For instance Midcom, Inc. (1-800-

643-2661). Linear Technology’s application group can

also help.

As an example say we are designing a 48V ±20% to 5V

100kHz converter with 2A output and 500mA ripple. Then

starting with a guess for the on voltage of the MOSFET

plus sense resistor of 0.5V and VF of 0.5V:

N

=

88%

•

5 + 0.5

(48 • 80%

−

0.5)

=

1

6.1

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

inductor ripple (the same as output ripple):

∆IL

=

2

•

2A

4

=

1A

The duty cycle for nominal input is:

( ) DCNOM = 2 •N

VOUT + VF

VIN(NOM) −ISW

• RON

=

5.5

2

6.1

•

47.5

=

35.3%

Then:

L = (5+ 0.5) •(1− 2 • 35.3%) = 16µH

1A • 100kHz

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

Say we choose a 22µH inductor, then ripple current at

maximum input (DC = 29.1%) is:

∆IL

=

(5

+

0.5) •(1− 2 • 29.1%)

22µF • 100kHz

=

1.03A

The maximum inductor current is:

IL(MAX )

=

2A

+

1.03A

2

=

2.52A

Primary inductance should be greater than:

LPRI = 5 • 22µH • 6.12 = 4.1mH

The secondary inductance would then be:

4.1mH/6.12 = 110µH

The magnetizing ripple current is approximately:

∆IMAG =

1

5.5

= 81mA

• 4.1mH • 100kHz

6.1

Peak switch current is:

ISW (PEAK )

=

1•

6.1

2.51A

+

81mA

=

494mA

Note that you can discern your magnetizing ripple by

looking at the reflected inductance ripple and subtracting

it from the switch current ripple.

∆IMAG = ∆ISW – N • IL

The max ripple current on the switch is:

∆ISW(MAX

)

=

1.03A

6.1

+

81mA

=

0.25A

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

assumptions 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

1683fd

19

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