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LT1683

1683f

APPLICATIO S I FOR ATIO

The soft-start current will be initiated as soon as the part

turns on. Soft-start will be reinititated after a short-circuit

fault.

Thermal Considerations

Most of the IC power dissipation is derived from the VIN

pin. The VIN current depends on a number of factors

including: oscillator frequency; loads on V5; slew settings;

gate charge current. Additional power is dissipated if V5

sinks current and during the MOSFET gate discharge.

The power dissipation in the IC will be the sum of:

1) The RMS VIN current times VIN

2) V5 RMS sink current times 5V

3) The gate drive’s RMS discharge current times voltage

Because of the strong VIN component it is advantageous

to operate the LT1683 at as low a VIN as possible.

It is always recommended that package temperature be

measured in each application. The part has an internal

thermal shutdown to minimize the chance of IC destruc-

tion but this should not replace careful thermal design.

The thermal shutdown feature does not protect the exter-

nal MOSFETs. A separate analysis must be done for those

devices to insure that they are operating within safe limits.

Once IC power dissipation, PDIS, is determined die junc-

tion temperature is then computed as:

TJ = TAMB + PDIS • θJA

where TAMB is ambient temperature and θJAis the package

thermal resistance. For the 20-pin SSOP,

θJA is 100°C/W.

Magnetics

Design of magnetics is dependent on topology. The fol-

lowing details the design of the magnetics for a push-pull

converter. In this converter the transformer usually stores

little energy. The following equations should be consid-

ered as the starting point to building a prototype.

The following definitions will be used:

VIN = Input supply voltage

RON = Switch-on resistance

ISW = Maximum switch current

VOUT = Desired output voltage

IOUT = Output current

f = Oscillator frequency

VF = Forward drop of the rectifier

Duty cycle is the major defining equation for this topology.

Note that the output L and C basically filter the chopped

voltage so duty cycle controls output voltage. N is the

turns ratio of the transformer. The turns ratio must be

large enough to ensure that the transformer can put out a

voltage equal to the output voltage plus the diode under

minimum input conditions. Note the transformer operates

at half the oscillator frequency (f).

OUT

MAX

IN MIN

SENSE

()

−+

()

[]

2•

()

DCMAX is the maximum duty cycle of each driver with

respect to the entire cycle, which consists of two periods

(A on and B on). So the effective duty cycle is 2 • DCMAX.

The controller, in general, determines maximum duty

cycle. A 44% maximum duty cycle is a guaranteed value

for this part.

Remember to add sufficient margin in the turns ratio to

account for IR drops in the transformer windings, worst-

case diode forward drops and switch on voltage. Also at

very slow slew rates the effective DC may be reduced.

There are a number of ways to choose the inductance

value for L. We suggest as a starting point that L be

selected such that the converter is continuous at

IOUT(MAX)/4. If your minimum IOUT is higher than this or

your components can handle higher peak currents then

use a higher number.

Figure 7. Push-Pull Topology

1:N

LPRI

CROUT

RSENSE

VOUT

VIN

1683 F07

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