When the MAX038’s frequency is controlled by a volt-

tor frequency. For example, using a 10k

Ω resistor for

width of the IIN control amplifier, which limits the modu-

lating signal’s highest frequency, is typically 2MHz.

IIN can be used as a summing point to add or subtract

currents from several sources. This allows the output

frequency to be a function of the sum of several vari-

due to the offset voltage of IIN.

Output frequency will be offset 1% from its final value

for 10 seconds after power-up.

FADJ Input

The output frequency can be modulated by FADJ,

which is intended principally for fine frequency control,

usually inside phase-locked loops. Once the funda-

changed further by setting FADJ to a voltage other than

0V. This voltage can vary from -2.4V to +2.4V, causing

the output frequency to vary from 1.7 to 0.30 times the

±2.4V can cause instability or cause the frequency

change to reverse slope.

The voltage on FADJ required to cause the output to

formula:

[5]

-2.4V and +2.4V.

corresponding to plus and minus deviation.

The voltage on FADJ for any frequency is given by the

formula:

[6]

where:

Likewise, for period calculations:

[7]

where:

by:

[8]

[9]

Programming FADJ

FADJ has a 250µA constant current sink to V- that must

be furnished by the voltage source. The source is usu-

ally an op-amp output, and the temperature coefficient

of the current sink becomes unimportant. For manual

adjustment of the deviation, a variable resistor can be

temperature coefficient becomes significant. Since

external resistors cannot match the internal tempera-

ture-coefficient curve, using external resistors to pro-

when the operator can correct for any errors. This

source.

and FADJ provides a convenient means of manually

setting the frequency deviation. The resistance value

[10]

(+58.3% deviation), the formula becomes:

= (4.5V) ÷ 250µA

= 18k

Ω

Disabling FADJ

The FADJ circuit adds a small temperature coefficient

to the output frequency. For critical open-loop applica-

tions, it can be turned off by connecting FADJ to GND

(not REF) through a 12k

Ω resistor (R1 in Figure 2). The

-250µA current sink at FADJ causes -3V to be devel-

oped across this resistor, producing two results. First,

the FADJ circuit remains in its linear region, but discon-

nects itself from the main oscillator, improving tempera-

ture stability. Second, the oscillator frequency doubles.

If FADJ is turned off in this manner, be sure to correct

equations 1-4 and 6-9 above, and 12 and 14 below by

the normal output frequency, it does not double the

upper frequency limit. Do not operate FADJ open cir-

cuit or with voltages more negative than -3.5V. Doing

so may cause transistor saturation inside the IC, lead-

ing to unwanted changes in frequency and duty cycle.

High-Frequency Waveform Generator

10

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