LM611IM datasheet(14/28 Pages) TI

14 / 28 page

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 Ir

flowing in the ‘forward' direction there is the familiar diode transfer function. Ir 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 Ir is External)

The reference equivalent circuit reveals how Vr 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 Ir, 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 Ir.

Figure 54. Reference Equivalent Circuit

Figure 55. 1.2V Reference

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