Product Description

The LM3460 is a shunt regulator designed for use as a pre-

cision control element in a feedback loop. The regulated out-

put voltage is sensed between the IN pin and GROUND pin

of the LM3460.

The output of the LM3460 sources current whenever the

voltage at the IN pin reaches the regulated voltage.

This current is used to cut off the drive to the external pass

trnasistor, which provides the negative feedback to force the

output voltage to be the same value as V

If the voltage on the IN pin is forced above the V

the maximum voltage applied to the IN pin should not ex-

ceed 20V. In addition, an external resistor may be required

on the OUT pin to limit the maximum current to 20 mA.

Compensation

The inverting input of the error amplifier is brought out to sim-

plify closed-loop compensation. Typically, compensation is

provided by a single capacitor connected from the COM-

PENSATION pin to the OUT pin of the LM3460.

Applying a load pulse to the output of the regulator circuit

and observing the output voltage response is a good method

of verifying the stability of the control loop.

If excessive ringing on the output waveform is observed, this

usually indicates marginal stability resulting from insufficient

phase margin.

Test Circuit

The test circuit shown in

Figure 3 can be used to measure

various LM3460 parameters. Test conditions are set by forc-

ing the appropriate voltage at the V

lecting the appropriate R

cal Characteristics section. Use a DVM at the

″measure″ test

points to read the data.

Setting the Output Voltage

If a regulated voltage is desired which is not available as a

standard voltage, the output voltage may be adjusted by us-

ing an external resistive divider (see

Figure 4):

DS012603-9

V

0V to 200 mV for LM3460-1.2

FIGURE 3. Test Circuit

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