With linear regulators, you measure dropout voltage, VIN – VOUT, at the minimum input voltage for which the IC sustains regulation. Low dropout means longer battery life, because the load circuit continues to operate while the battery discharges to a lower terminal voltage. The external transistor helps to form a linear-regulator circuit whose dropout voltage at 100-mA load current is only 10 mV (Figure 1). (The linear-regulator IC by itself specs a 100-mV dropout at 100 mA.) The external transistor also boosts the maximum available load current to 1 A.
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| Figure 1. | Unorthodox transistor connections to a low-dropout regulator allow you to squeeze a 100-mV dropout voltage down to 10 mV. |
Unorthodox connections enable the IC to drive Q1. Connecting Pin 3 to the transistor's base allows base current to flow through the internal switching MOSFET, out of Pin 4, and through R2 to ground. The MOSFET then regulates VOUT by controlling Q1’s base current. Because C2 sets a dominant pole that stabilizes the loop, you should choose a ceramic type or other low-ESR capacitor. C2 improves the phase margin by forming a pole-zero combination that increases the phase at crossover. Q1 saturates when the battery voltage drops low enough for VOUT to drop out of regulation, and R2 limits the base current for that condition to approximately 10 mA. Q1’s collector-emitter voltage at saturation, 10 mV with 1-mA base current and 100-mA collector current, sets the dropout voltage for these conditions.
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| Figure 2. | Dropout voltage for the circuit in Figure 1 varies from 10 mV at 100 mA to 90 mV at 1 A. |
The measured dropout voltage varies with load current (Figure 2). The circuit delivers as much as 1 A at 3.3 V. You can adjust the output from 5.5 V down to 1.25 V using the formula
with appropriate changes to the value of R2, using the formula
Small components allow the entire circuit to occupy less than 0.24 in.2 of board area. (IC1 comes in an SOT-23 package.)
