LM611AMJSLASH883 datasheet(12/17 Pages) NSC

12 / 17 page

Application Information (Continued)

Connecting a resistor across Cathode-to-FEEDBACK cre-

ates a 0 TC current source, but a range of TCs may be syn-

thesized.

I = Vr/R1 = 1.24/R1

Hysteresis

The reference voltage depends, slightly, on the thermal his-

tory

the

die.

Competitive

micro-power

products

vary — always check the data sheet for any given device. Do

not assume that no specification means no hysteresis.

OPERATIONAL AMPLIFIER

The amp or the reference may be biased in any way with no

effect on the other, except when a substrate diode conducts

(see Guaranteed Electrical Characteristics Note 1). The amp

may have inputs outside the common-mode range, may be

operated as a comparator, or have all terminals floating with

no effect on the reference (tying inverting input to output and

non-inverting input to V

−

on unused amp is preferred).

Choosing operating points that cause oscillation, such as

driving too large a capacitive load, is best avoided.

Op Amp Output Stage

The op amp, like the LM124 series, has a flexible and rela-

tively wide-swing output stage. There are simple rules to op-

timize output swing, reduce cross-over distortion, and opti-

mize capacitive drive capability:

Output Swing: Unloaded, the 42 µA pull-down will bring

the output within 300 mV of V

− over the military tempera-

ture range. If more than 42 µA is required, a resistor from

output to V

− will help. Swing across any load may be im-

proved slightly if the load can be tied to V

+, at the cost of

poorer sinking open-loop voltage gain.

Cross-over Distortion: The LM611 has lower cross-over

distortion (a 1 V

BE deadband versus 3 VBE for the

LM124), and increased slew rate as shown in the char-

acteristic curves. A resistor pull-up or pull-down will force

class-A operation with only the PNP or NPN output tran-

sistor conducting, eliminating cross-over distortion.

Capacitive Drive: Limited by the output pole caused by

the output resistance driving capacitive loads, a

pull-down resistor conducting 1 mA or more reduces the

output stage NPN r

e until the output resistance is that of

the current limit 25

Ω. 200 pF may then be driven without

oscillation.

Op Amp Input Stage

The lateral PNP input transistors, unlike those of most op

amps, have BV

EBO equal to the absolute maximum supply

voltage. Also, they have no diode clamps to the positive sup-

ply nor across the inputs. These features make the inputs

look like high impedances to input sources producing large

differential and common-mode voltages.

DS009221-21

FIGURE 8. Diode in Series with R1 Causes

Voltage Across R1 and R2 to be Proportional

to Absolute Temperature (PTAT)

DS009221-22

FIGURE 9. Current Source is Programmed by R1

DS009221-23

FIGURE 10. Proportional-to-Absolute-

Temperature Current Source

DS009221-24

FIGURE 11. Negative −TC Current Source

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