+5 volt Reference Current Limiting

+5 volt Reference Current Limiting

A few short comings in the precision +5 volt reference have surfaced. The first is the lack of current limiting. If a short should develop on the reference somewhere the current boost transistor would dissipate too much power and destroy itself. As it turns out it is relative easy to add current limiting to the existing circuit. An LED across the R3 would divert base current from the pass transistor when the voltage across the emitter resistor reached approximately .7 volts. This voltage drop along with the forward bias voltage on the transistor would start to turn on the LED and divert base current thus turning off the transistor. The problem that arose next was was to activate the current limiting action at a reasonable level, say 100 ma, the voltage at the input to the LT1021 reference IC would drop too low and the regulator would start to drop out. Reducing the emitter resistor then the power dissipation in the pass transistor would be too high under a shorted condition and we were right back to our starting point with a damaged transistor. A fold-back circuit was needed. Several designs were tried with the below version working the best. This is still prototyped on the PCB and hasn't been committed to yet but it looks like it may be the final solution on this revision of the PCB.

Original +5 volt precision reference circuit.

The turn on relationship between the +14 volt VBATT input and the +8VF supply.

50ma load   R4 = 10 ohms

The turn on relationship between the +8VF supply and the output of the 5 volt reference.

50ma load   R4 = 10 ohms

Here is the relationship between the +8VF supply and the emitter of the current boost transistor. Was looking at the area where it was current limiting. A little instability as it comes out of current limit.

50ma load   R4 = 10 ohms

Here we are looking at the input of the LT1021 reference IC. The current limit period and the instability are clear.

Modified with current limiting diode and fold back.

R4 = 5 ohms

Looking at the turn on rate with the fold-back circuit installed. The circuit didn't have any affect on turn on.

105ma load  R4 = 5 ohms

Looking at the E-B junction of the fold-back transistor to assure that it didn't affect turn on.

105ma load  R4 = 5 ohms

Voltage across R3 (220 ohm) shows a brief period of current limiting during initial turn on. It is clear the the fold-back transistor is not turned on during this period.

105ma load  R4 = 5 ohms

Turn on one last time with (top) and without (bottom) fold-back installed.