LM3460M5X-1.5/NOPB【PDF 8/9 页】1-OUTPUT THREE TERM VOLTAGE REFERENCE, 1

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App Circuit Technical Information

(Continued)

In the circuit of Figure 2, drive for Q1 is provided by R7, but

only when U1 sources current: The operation of the drive

scheme is as follows:

If the voltage at V

OUT is below 1.2V, no current flows from

the OUT pin of U1. Q1 is held OFF as the current flowing

down through R7 goes through D1 and R5 to ground.

IMPORTANT: Diode D1 is a 1N4001 because its V

F must be

much less than the V

BE of Q1 (a signal diode like 1N4148 will

not work here).

When U1 is not sourcing current, the voltage at the OUT pin

(and the cathode of D1) will be held at about 50 mV by the

R7/D1/R5 divider. The current flowing to ground through

these components is about 110 A.

Because D1 is a 1A power diode, the V

F across D1 at this

small value of current will be much less than the V

BE needed

to turn ON Q1 (so Q1 is held off by D1).

When U1 begins to source current (to cut off the pass

transistor and regulate V

OUT) it forces the voltage at the

cathode of D1 to rise.

This action causes the current that was flowing through D1

to flow into the base of Q1, turning it ON and taking drive

away from the base of Q2.

This action provides the negative feedback required to regu-

late V

OUT and allows the LM3460 to operate with only 1.2V

of total supply voltage across the device.

LM3460

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