AN3867

AN3867
Maxim > Design Support > Technical Documents > Application Notes > Circuit Protection > APP 3867
Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 3867
Keywords: shunt regulator, current limit, power amplifier, adjustable shunt regulator
APPLICATION NOTE 3867
Shunt Regulator Improves PA's Current-Limit
Accuracy
Jul 26, 2006
Abstract: Substituting a shunt regulator (U1) for the usual transistor in this power-amplifier circuit
improves the current-limit accuracy.
Adding current-limiting circuitry to an emitter follower protects both the pass transistor and downstream
circuitry from excessive current damage. The classic way to implement such current limiting is to add a
ballast resistor between the pass transistor's emitter and the circuit output, and then monitor the resistor
drop with a small-signal transistor. See R4 and Q2 in the power amplifier (or linear regulator) shown in
Figure 1.
Figure 1. A small-signal transistor (Q2) provides an output current limit for this power amplifier.
Unfortunately, the base-emitter voltage of the small-signal transistor sets the current-limit threshold for
this circuit. That VBE has a well-known temperature coefficient of -2mV/°C, which causes a substantial
change in current limit across the operating temperature range.
An adjustable shunt regulator (U1 in Figure 2) is preferable to the small-signal transistor for sensing
current. This IC is chosen for its low-input threshold (0.6V), which is lower than that of common shunt
regulators (1.25V to 2.5V). In addition, the IC's separate power-supply input allows it to maintain
accuracy as the internal output transistor approaches saturation.
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Figure 2. Substituting a shunt regulator (U1) for Q2 in the Figure 1 circuit improves the current-limit
accuracy.
Figure 3 compares current-limit accuracies of the small-signal-transistor version of Figure 1 with the
shunt-regulator version of Figure 2. The transistor version exhibits a 25% change in current-limit
threshold over the operating temperature range, while the shunt-regulator provides better than 2%
accuracy over that range (neglecting the temperature coefficients of the sense resistors).
Figure 3. Current-limit accuracy versus temperature for the circuits of Figure 1 (top trace) and Figure 2
(bottom trace).
A similar article appeared as a Design Idea in the February 2, 2006 edition of EDN.
Related Parts
MAX8515
Wide-Input 0.6V Shunt Regulators for Isolated DC-DC
Converters
Free Samples Page 2 of 3
More Information
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Application Note 3867: http://www.maximintegrated.com/an3867
APPLICATION NOTE 3867, AN3867, AN 3867, APP3867, Appnote3867, Appnote 3867
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