LT1533CS datasheet(8/20 Pages) LINER

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LT1533

APPLICATIONS INFORMATION

It is beyond the scope of this data sheet to get into EMI

fundamentals. AN70 contains much information concern-

ing noise in switching regulators and should be consulted.

Oscillator Frequency

The oscillator determines the switching frequency and

therefore the fundamental positioning of all harmonics.

The use of good quality external components is important

to ensure oscillator frequency stability. The oscillator is a

sawtooth design. A current defined by external resistor RT

is used to charge and discharge the capacitor CT. The

discharge rate is approximately ten times the charge rate.

By allowing the user to have control over both compo-

nents, trimming of oscillator frequency can be more easily

achieved.

The external capacitance CT is chosen by:

CT(nF) = 2180/[fOSC(kHz) • RT(kΩ)]

where fOSC is the desired oscillator frequency in kHz.

For RT equal to 16.9k, this simplifies to:

CT(nF) = 129/fOSC(kHz),

e.g., CT = 1.29nF for fOSC = 100kHz

Nominally RT should be 16.9k. Since it sets up current, its

temperature coefficient should be selected to compliment

the capacitor. Ideally, both should have low temperature

coefficients.

When the DUTY pin is high or floating, the outputs will be

turned off during the discharge time of the oscillator. Due

to slew rate control, turning off the outputs does not

produce immediate transitions. Turn-off will require the

current to ramp down and the switch voltage to ramp up.

If the DUTY pin is grounded, then the outputs will turn on

or off starting with the clock discharge.

If the FB pin is below 0.4V the oscillator discharge time will

increase, causing the oscillation frequency to decrease by

approximately 6:1. This feature helps minimize power

dissipation during start-up and short-circuit conditions.

Oscillator frequency is important for noise reduction in

two ways: 1) the lower the oscillator frequency the lower

the harmonics of waveforms are, making it easier to filter

them, 2) the oscillator will control the placement of output

frequency harmonics which can aid in specific problems

where you might be trying to avoid a certain frequency

bandwidth that is used for detection elsewhere.

Oscillator Sync

If a more precise frequency is desired (e.g., to accurately

place harmonics) the oscillator can be synchronized to an

external clock. Set the RC timing components for an

oscillator frequency 10% lower than the desired sync

frequency.

Drive the SYNC pin with a square wave (with greater than

1.4V amplitude). The rising edge of the sync square wave

will initiate clock discharge. The sync pulse should have a

minimum pulse width of 0.5

µs.

Be careful in sync’ing to frequencies much different from

the part since the internal oscillator charge slope deter-

mines slope compensation. It would be possible to get into

subharmonic oscillation if the sync doesn’t allow for the

charge cycle of the capacitor to initiate slope compensa-

tion. In general, this will not be a problem until the sync

frequency is greater than 1.5 times the oscillator free-run

frequency.

Slew Rate Setting

Setting the voltage and current slew rates is easy. External

resistors to ground on the RVSL and RCSL pins determine

the slew rates. Determining what slew rate to use is more

difficult. There are several ways to approach the problem.

First, start by putting a 50k resistor pot with a 3.9k series

resistance on each pin. In general, the next step will be to

monitor the noise that you are concerned with. Be careful

with measurement technique (consult AN70). Keep probe

ground leads very short.

Usually it will be desirable to keep the voltage and current

slew resistors approximately the same. There are circum-

stances where a better optimization can be found by

adjusting each separately, but as these values are sepa-

rated further, a loss of independence of control will occur.

Starting from the lowest resistor setting adjust the pots

until the noise level meets your guidelines. Note that

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