Datasheet LT1533 (Analog Devices) - 8

LT1533
U U W U APPLICATIONS INFORMATION
It is beyond the scope of this data sheet to get into EMI them, 2) the oscillator will control the placement of output fundamentals. AN70 contains much information concern- frequency harmonics which can aid in specific problems ing noise in switching regulators and should be consulted. where you might be trying to avoid a certain frequency bandwidth that is used for detection elsewhere.
Oscillator Frequency Oscillator Sync
The oscillator determines the switching frequency and therefore the fundamental positioning of all harmonics. If a more precise frequency is desired (e.g., to accurately The use of good quality external components is important place harmonics) the oscillator can be synchronized to an to ensure oscillator frequency stability. The oscillator is a external clock. Set the RC timing components for an sawtooth design. A current defined by external resistor RT oscillator frequency 10% lower than the desired sync is used to charge and discharge the capacitor CT. The frequency. discharge rate is approximately ten times the charge rate. Drive the SYNC pin with a square wave (with greater than By allowing the user to have control over both compo- 1.4V amplitude). The rising edge of the sync square wave nents, trimming of oscillator frequency can be more easily will initiate clock discharge. The sync pulse should have a achieved. minimum pulse width of 0.5µs. The external capacitance CT is chosen by: Be careful in sync’ing to frequencies much different from the part since the internal oscillator charge slope deter- CT(nF) = 2180/[fOSC(kHz) • RT(kΩ)] mines slope compensation. It would be possible to get into where fOSC is the desired oscillator frequency in kHz. subharmonic oscillation if the sync doesn’t allow for the For R charge cycle of the capacitor to initiate slope compensa- T equal to 16.9k, this simplifies to: tion. In general, this will not be a problem until the sync CT(nF) = 129/fOSC(kHz), frequency is greater than 1.5 times the oscillator free-run e.g., CT = 1.29nF for fOSC = 100kHz frequency. Nominally RT should be 16.9k. Since it sets up current, its temperature coefficient should be selected to compliment
Slew Rate Setting
the capacitor. Ideally, both should have low temperature Setting the voltage and current slew rates is easy. External coefficients. resistors to ground on the RVSL and RCSL pins determine When the DUTY pin is high or floating, the outputs will be the slew rates. Determining what slew rate to use is more turned off during the discharge time of the oscillator. Due difficult. There are several ways to approach the problem. to slew rate control, turning off the outputs does not First, start by putting a 50k resistor pot with a 3.9k series produce immediate transitions. Turn-off will require the resistance on each pin. In general, the next step will be to current to ramp down and the switch voltage to ramp up. monitor the noise that you are concerned with. Be careful If the DUTY pin is grounded, then the outputs will turn on with measurement technique (consult AN70). Keep probe or off starting with the clock discharge. ground leads very short. If the FB pin is below 0.4V the oscillator discharge time will Usually it will be desirable to keep the voltage and current increase, causing the oscillation frequency to decrease by slew resistors approximately the same. There are circum- approximately 6:1. This feature helps minimize power stances where a better optimization can be found by dissipation during start-up and short-circuit conditions. adjusting each separately, but as these values are sepa- Oscillator frequency is important for noise reduction in rated further, a loss of independence of control will occur. two ways: 1) the lower the oscillator frequency the lower Starting from the lowest resistor setting adjust the pots the harmonics of waveforms are, making it easier to filter until the noise level meets your guidelines. Note that 8