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UCC2626 Datasheet(PDF) 6 Page - Texas Instruments |
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UCC2626 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 15 page 6 UCC2626 UCC3626 RC networks generally work well and should be located as close to the IC as possible. Fig. 2 illustrates these techniques. Configuring the Oscillator The UCC3626 oscillator is designed to operate at fre- quencies up to 250kHz and provide a triangle waveform on CT with a peak to peak amplitude of 5V for improved noise immunity. The current used to program CT is de- rived off of the R_TACH resistor according to the follow- ing equation: I RTACH Amps OSC = 25 _ Oscillator frequency is set by R_TACH and CT according to the following relationship: Frequency RTACH CT Hz = • 25 . (_ ) Timing resistor values should be between 25k Ω and 500k Ω while capacitor values should fall between 100pF and 1 µF. Fig. 4 provides a graph of oscillator frequency for various combinations of timing components. As with any high frequency oscillator, timing components should be located as close to the IC pins as possible when lay- ing out the printed circuit board. It is also important to ref- erence the timing capacitor directly to the ground pin on the UCC3626 rather than daisy chaining it to another trace or the ground plane. This technique prevents switching current spikes in the local ground from causing jitter in the oscillator. Synchronizing the Oscillator A common system specification is for all oscillators in a design to be synchronized to a master clock. The UCC3626 provides a SYNCH input for exactly this pur- pose. The SYNCH input is designed to interface with a digital clock pulse generated by the master oscillator. A positive going edge on this input causes the UCC3626 oscillator to begin discharging. In order for the slave os- cillator to function properly it must be programmed for a frequency slightly lower than that of the master. Also, a resistor equal to RTACH must be placed in parallel with CT. Fig. 3 illustrates the waveforms for a slave oscillator programmed to 20kHz with a master frequency of 30kHz. The SYNCH pin should be grounded when not used. APPLICATION INFORMATION (cont.) 1.E+03 1.E+04 1.E+05 1.E+06 1.E-10 1.E-09 1.E-08 1.E-07 CT (F) R_TACH = 25k R_TACH = 500k R_TACH = 100k R_TACH = 250k Figure 4. PWM oscillator frequency vs. CT and R_TACH. SYNCH CT WITHOUT SYNCH WITH SYNCH Figure 3. Synchronized and unsynchronized oscillator waveforms. 1k Ω 1k Ω 1k Ω VREF VREF VREF HALLA HALLB HALLC HALLA HALLB HALLC 2.2nF 2.2nF 2.2nF 499 Ω 499 Ω 499 Ω Figure 2. Passive hall filtering technique. |
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