William H. Morong
Electronic Design
A current-limiting regulator prevents excessive load current, but it may not prevent excessive regulator dissipation. Many integrated regulators do protect themselves from excessive dissipation and many provide a current limit, yet their current limits may not be at the desired current or may not be sufficiently predictable. This regulator provides predictable output-current and regulator-power limits, as well as undervoltage protection.
Between the regulator’s input rail and ground is a current source (see Figure 1) comprising diode-connected Q3, R4, Q10, R12, R7, D1, R8, and R13. Current through R13 and R8 causes the base of Q10 to rise, raising its emitter, until the junction of R12 and R7 raises D1’s reference terminal to 1.24 V. D1 then draws current, limiting further rise and setting the current in Q10 to about 6.8 mA, which is drawn through Q3 and R4.
 |
||
| Figure 1. | This circuit protects itself and its load by limiting both dissipation and current to levels desired by the designer. |
|
Until the input reaches about 6.5 V, the drop across R13 is too small to energize the current source comprising Q12, Q13, and current-setting resistor R9. That current source remains off, keeping the entire regulator off and providing undervoltage lockout. Once a desired voltage is reached, Q13’s current is about 6 mA.
Q13’s current drives the base of pass-transistor Q6, which draws its collector current through diode-connected Q5. The current exiting through Q6’s emitter passes through resistor R5 to provide current to the load through the output terminal.
R10 and R11 divide the output voltage to D2’s reference terminal. When the divided output rises above 1.24 V, D2 draws current from Q13, denying Q6 additional base drive and preventing an increase of the output. The effect is voltage regulation at about 5 V. R14 current limits D2. C1 decouples the load.
Q8 senses the voltage across R5. When that voltage reaches Q8’s VBE, the transistor diverts current from Q13 to limit Q6’s base and emitter current, current-limiting the regulator. Q5’s VBE approximates the logarithm of the output current.
Q9 and pull-down resistor R3 form an emitter follower, the output of which is one diode-drop below the emitter of Q6, compensating the drop of diode-connected Q1. Therefore, the voltage across and current through R1 and Q1 are approximately proportional to the voltage across Q6. Consequently, Q1’s VBE approximates the logarithm of the pass-transistor voltage, and the voltage between the input terminal and Q1’s base approximates the sum of the logarithms of the pass-transistor voltage and current.
The current drawn by Q10 dominates the current in Q3, causing its VBE to approximate the logarithm of a constant. Since Q3’s current density is less than that of Q5, Q10’s current also passes through R4 to provide a small scaling voltage. Otherwise, Q3 might need to be operated at a wastefully high current. Designers can adjust R4 to provide the desired scaling.
The difference between Q5’s plus Q1’s VBEs and the sum of Q3’s VBE and R4’s drop are applied to the VBE of Q2. Q2’s current approximates the antilog of its VBE. So, Q2’s current approximates the antilog of the difference of the sum of the logs of the pass-transistor’s voltage and current and the log of a constant. As a result, current through Q2 and R2 is proportional to the product of pass-transistor voltage and current – that is, pass-transistor power.
If the voltage across R2 rises to that of Q4’s VBE, Q4 turns on, diverting current from Q13 to deny the pass-transistors additional base current, limiting regulator dissipation. C3 frequency compensates the power-limiting loop.
The circuit requires a good thermal connection between Q3 and Q5. Since both transistors have large collector pads, this is easily accomplished by placing their collector pads side by side on a shared thermal pad.
Q6 should have an appropriate heatsink thermally isolated from Q1, Q2, Q3, and Q5, which together form a simple analog power computer that is temperature sensitive. If some thermal flux to these transistors is inevitable, Q2 and Q3 should be heated most, since heating them more than Q1 and Q5 lowers the power limit as temperature rises.
Using a large transistor for Q5 keeps its current density low enough to obtain approximately logarithmic behavior. Q3 was chosen to match, and to solder easily near, Q5. With the values shown, the power limit (See Table 1) is about 2 W, and the current limit is about 600 mA.
| Figure 1. | This circuit protects itself and its load by limiting both dissipation and current to levels desired by the designer. |
||||||||||||||||||||||||
|
|||||||||||||||||||||||||
The circuit naturally folds back current once the power limit is reached. The resulting drop in output voltage increases pass-transistor voltage, so current drops. In some cases, the current limit may be redundant, depending on the product of undervoltage lockout and maximum current.
