289

Figure 7. Generating Programmable DC Voltages.

+

–

DC

OUT

1/8 of SP9841

3

ICL8069

+

–

OP–90

1.22V

R1

10K

Ω

R2

4.75K

Ω

+1.8V

DC

OUT

At PRESET, DC

+

–

R3

1.2K

Ω

45µF*

(*Optional Noise Reduction)

Figure8showsanon–programmedstandaloneappli-

cation.BytyingPRESETLtoground,allchannelsare

permanently set to unity gain. While preset, the input

impedance at each input is set to 40kOhms nominal

(20kOhms minimum), which minimizes required

input current drive. The TL431 reference is pro-

grammedbyresistorsR

to provide at least 165

µA for each input driven, plus

0.5mAforthereferenceattheminimumsupplyvalue

to be considered. In the Figure, the 560 Ohms shown

will drive all eight inputs. The excellent capacitive

load capability of the output amplifiers handles any

value of capacitive bypass loads without oscillation;

however, to minimize ringing at powerup, load ca-

pacitance can be chosen to be greater than 0.1

µF or

less than 1,000pF.

of 3.75V while running from a 4.75V supply, this

behavior is not tested in device production. If

maximum linearity is required near the 3.66V

fullscale voltage, then output loading should be

kept under 1mA.

Figure 7 uses a 1.8V reference to provide an output

voltage range from near ground to almost 3.6V.

The external micropower reference uses a bandgap

in a bootstrapped configuration, which guarantees

excellent supply rejection. Voltage at V

1.223

R2

(R1

R2)

∗

+









V

cent current through the bandgap, I=V

op amp will easily drive one to all eight inputs.

Figure 8. Generating Up to Eight (8) 3.3V @ 10mA DC Supplies with Logic–Level Controlled Shutdown (Non–Programmed).

+

–

DC

OUT

1/8 of SP9841

PRESET

Eight independent 3.3V @ 10mA supplies with

logic–level controlled shutdown.

32K

Ω

+5V

+

–

2.5V

100K

Ω

TL431

560

Ω

AC04

DIG

IN

DIG

IN

DC

OUT

0

1

3.3V

0V