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Described is a circuit which uses variable impedance for controlling output current. With the variable impedance method, current limiting is a function of output voltage.
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Current Limiting Achieved by Variable Impedance
Described is a circuit which uses variable impedance for controlling output
current. With the variable impedance method, current limiting is a function of
The figure shows a circuit in which the output current is a function of the
output voltage. Resistors ROUT1 and ROUT2 establish a voltage threshold.
This voltage threshold controls the operating region of Q2 with respect to the
output voltage. By adjusting the operating region of Q2, the effective emitter
impedance of the reference current source, Q1 is varied. The variation of the
effective emitter impedance controls the current source that controls the base
drive of the output PNP transistor Q4. This control of base drive of Q4 enables
output current limiting. Thus, the output current limiting is directly a function of
output voltage. Furthermore, output current limiting is achieved by a logarithmic
gain feedback loop. The feedback loop includes resistors RDIF, R1, R2, ROUT1,
ROUT2, diodes D1 - D3 and transistors Q2-Q5 .
The logarithmic gain feedback loop is dominated by the equation of the base-
emitter diode of Q5. This exponential "knee" of the output current limiting curve,
not shown, provides significant changes in output current for small changes in