Choosing the Right Linear Regulator: Dropout Voltage and Ground Current Qi Deng Senior Applications Engineer, Analog and Interface Products Division Microchip Technology Inc. A linear regulator’s pass element establishes its dropout voltage and ground current, which determine the types of applications for which the regulator is suitable. Each of the five main linear regulators used today has a different pass element and unique characteristics that make it ideal or not so ideal for certain applications. For example, the standard NPN regulator uses a PNP transistor (Q1) and a NPN Darlington transistor pair (Q2 and Q3) as its pass element. Its dropout voltage is typically between 1.8V and 2.4V, given by: VDROP = VSAT (Q1) + VBE (Q2) + VBE (Q3)

Equation (1)

The standard NPN regulator has the following advantages: • Steady ground current roughly equal to the base current of the PNP transistor (the load current divided by the gain of the pass element). The ground current is typically several mA. • Unconditional stability without an output capacitor, because its pass element resembles a common collector configuration with very low output impedance. The standard NPN regulator is useful in applications where the input-to-output voltage differential is high. However, its high dropout voltage makes it undesirable for many embedded applications. The NPN pass transistor regulator uses a PNP transistor (Q1) and a NPN transistor (Q2) as its pass element. The dropout voltage is typically between 1.0V and 1.5V, given by: VDROP = VSAT (Q1) + VBE (Q2)

Equation (2)

The NPN pass transistor regulator has the following characteristics: • High gain NPN pass transistor resulting in a steady ground current of typically several mA. • It is in a common collector configuration with low output impedance, but nonetheless higher than that of the standard NPN regulator. Therefore, it requires an output capacitor with low capacitance for stability. The capacitor’s ESR (Equivalent Series Resistance) is not critical. The NPN pass transistor regulator is useful in many embedded applications because of its lower dropout voltage and ease-of-use. However, it does not have a low enough dropout voltage for battery-powered applications with lower voltage differential budgets.

The PNP pass transistor is a Low Dropout Regulator (LDO) with a PNP transistor (Q1) as its pass element. Its dropout voltage is usually between 0.3V and 0.7V, given by: VDROP = VSAT (Q1)

Equation (3)

The PNP pass transistor regulator has disadvantages: • Its PNP transistor has low gain, resulting in unsteady ground current of typically several mA. • Its high output impedance, due to its common emitter configuration, means that an output capacitor within a certain range of capacitance and ESR is required for stability. Because of its low dropout voltage, the PNP pass transistor regulator is useful for battery-powered embedded applications. However, its high ground current shortens battery life. The P-channel FET regulator uses a P-channel FET (Q1) as its pass element. dropout voltage is given by:

Its

VDROP = RDS(ON) (Q1) x IOUT Equation (4) *RDS(ON) (Q1) is the drain-to-source resistance of the FET when fully on, IOUT is the output current. The P-channel FET regulator’s dropout voltage is low, because the RDS(ON) is easily adjusted to a low value by sizing the FET. The P-channel FET regulator has the following characteristics: • Low ground current because of the low “gate current” of the P-channel FET. • Because of the P-channel FET’s relatively high gate capacitance, it requires an output capacitor with a certain range of capacitance and ESR for stability. Because of its low dropout voltage and ground current, the P-channel FET regulator is popular for many of today’s battery powered devices. The N-channel FET regulator uses an N-channel FET (Q1) as its pass element, with its dropout voltage given by: VDROP = RDS(ON) (Q1) x IOUT

Equation (5)

The N-channel FET regulator’s dropout voltage and ground current are low. It requires an output capacitor for stability, but the capacitance can be low, and the ESR is not critical. N-channel FET regulators require a charger pump to establish the gate bias voltage for the N-channel FET, resulting in more complex circuitry. Luckily, the size of an Nchannel FET is up to 50 percent smaller than that of a P-channel FET with the same load current. The N-channel FET regulator is ideal for applications that require low dropout voltage, low ground current and high load current.

Each type of linear regulator has its own advantages and disadvantages. Ultimately, it is up to the designer to determine whether a certain type of linear regulator is appropriate for the application based on its dropout voltage, ground current and stability compensation method requirements.

Regulator Type

Standard NPN (Darlington) 2VBE + VSAT, 1.5V - 2.4V typical

NPN Pass Transistor

VBE + VSAT, 1.0V - 1.5V typical Dropout Voltage Relatively steady, Ground Pin Current Steady, 5mA typical 5mA typical High Output Current High Need output capacitor Operation Stability Intrinsically stable Small capacitance, insensitive to ESR Output Capacitor Not needed AC powered with 3V and above voltage Embedded differential Applications applications

PNP Pass Transistor VSAT, 0.3V - 0.7V typical

P-channel FET

N-channel FET

RDS(ON) x IOUT, 0.05V - 0.7V typical

RDS(ON) x IOUT, 0.05V - 0.7V typical

Unsteady, 5mA typical Very low Medium Medium

Very low High

Need output capacitor Need output capacitor Certain range of Certain range of capacitance and ESR capacitance and ESR Battery powered applications that Battery powered require long battery life applications

Need output capacitor Certain range of capacitance and ESR High current applications requiring better efficiency.

Figure 1. Comparison of different types of linear regulators