Datasheet

LM611

SNOSC08C –MAY 1998–REVISED MARCH 2013

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APPLICATION INFORMATION

VOLTAGE REFERENCE

Reference Biasing

The voltage reference is of a shunt regulator topology that models as a simple zener diode. With current I

flowing in the ‘forward' direction there is the familiar diode transfer function. I

flowing in the reverse direction

forces the reference voltage to be developed from cathode to anode. The applied voltage to the cathode may

range from a diode drop below V

−

to the reference voltage or to the avalanche voltage of the parallel protection

diode, nominally 7V. A 6.3V reference with V+ = 3V is allowed.

Figure 53. Voltages Associated with Reference

(Current Source I

is External)

The reference equivalent circuit reveals how V

is held at the constant 1.2V by feedback, and how the

FEEDBACK pin passes little current.

To generate the required reverse current, typically a resistor is connected from a supply voltage higher than the

reference voltage. Varying that voltage, and so varying I

, has small effect with the equivalent series resistance of

less than an ohm at the higher currents. Alternatively, an active current source, such as the LM134 series, may

generate I

Figure 54. Reference Equivalent Circuit

Figure 55. 1.2V Reference

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