8

SCD5031 Rev B

Pre

lim

in

ar

y

OSCILLATOR

The chip uses two precision current mirrors that alternately charge and discharge an external capacitor to generate an

extremely linear sawtooth oscillator waveform. At the start of each cycle, the charging current, set by the choice of

resistor at the Rset pin, is 1:1 mirrored over to the Cset pin where it charges an external capacitor. When the capacitor

to discharge the external capacitor. The discharge current is set at roughly five times the charging current to result in fast

discharge and minimal Dead Time. When the voltage reaches the comparator’s lower threshold (0.9V), the comparator

switches back to the charging mirror, powers down the discharge mirror, and the whole process repeats.

The frequency is set by choosing Rset and Cset such that:

Suggested Ranges for Cset and Rset are:

50K ohms < Rset < 300K

10pf < Cset < 600pF

Rt

Ct

Rset

Cset

GND

6

7

12

F

OSC

1

0.84

R

SET

×

C

SET

×

------------------------------------------------

≈

20KHz

F

OSC

1MHz

≤≤

FIGURE 3 – Timing Resistance vs Frequency

47pF

100pF

200pF

390pF

200pF

20pF

10pF

10K

100K

Frequency Hz

Ω

160K

140K

120K

100K

80K

60K

40K

1M

Frequency Hz

300K

280K

260K

240K

220K

200K

180K

320K