LT1533CS datasheet(11/20 Pages) LINER

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LT1533

APPLICATIONS INFORMATION

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.

OUT

MAX

IN MIN

•−

()

DCMAX is the maximum duty cycle of each driver with

respect to the entire cycle which consists of two periods

(Q1A on and Q1B on). So the effective duty cycle is

2 • DCMAX. The controller, in general, determines maxi-

mum duty cycle. A 44% maximum duty cycle is a guaran-

teed value for this part.

Figure 3

VC PIN

1533 F03

RVC

CVC

0.01

µF

CVC2

4.7nF

To prevent irregular switching, VC pin ripple should be

kept below 50mVP-P. Worst-case VC pin ripple occurs at

maximum output load current and will also be increased if

poor quality (high ESR) output capacitors are used. The

addition of a 0.0047

µF capacitor on the VC pin reduces

switching frequency ripple to only a few millivolts. A low

value for RVC will also reduce VC pin ripple, but loop phase

margin may be inadequate.

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.

Figure 4

VIN

VSEC

DS1

1 : N

Q1B

Q1A

VOUT

DS2

1533 F04

The following definitions will be used:

VIN = Input supply voltage

VSW = Switch-on voltage

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

Some Common Turns Ratios

VIN

VOUT

±10%

3.6

±10%

4.4

±10%

3.3

1.1

Remember to add sufficient margin in the turns ratio to

account for IR drops in the transformer windings, worst-

case diode forward drop (VF) and switch-on voltage (VSW).

There are a number of ways to choose the inductance

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

selected such that the converter is continuous at

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

you are operating at low currents such that the IC and

components can handle higher peak currents, then use a

higher number.

Continuous operation occurs when the current in the

inductor never goes to zero. Discontinuous operation

occurs when the inductor current drops to zero before the

start of the next cycle and can occur with small inductors

and light loads. There is nothing inherently bad about

discontinuous operation, however, the converter control

and operation is somewhat different. The inductor is

smaller for discontinuous operation but the peak currents

in the switch, the transformer, the diodes, inductor and

capacitor will be higher. But for low power situations these

may not present a big constraint.

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