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

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

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

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.