Need a Stable Voltage or Current Source? Sometimes it Pays to be Discrete!

Voltage Regulation

Nowadays, switching regulators are widely used as power supplies but linear regulation is also required for applications where electromagnetic interference (EMI) and voltage ripple can be problematic, including application-specific chips (ASICs), microcontrollers, application processors and sensors. This is why linear regulators are now also included after switching regulators in almost every power supply block. Unlike in switching regulators, the difference between input and output power is dissipated in a pass device like a BJT (Figure 1). To minimize these power losses, linear regulators are mainly used in applications where the difference between input and output voltage is small, or where only small output currents are required. Figure 1 shows a basic linear regulator which employs feedback regulation and where the NPN pass transistor is driven by a PNP BJT.

The minimum voltage drop across the BJT output stage (Figure 1) is the sum of the emitter-collector saturation voltage of the PNP transistor and the voltage drop of the base emitter junction of the NPN:

V_OUT(min) = V_CEsat + V_BE

The output voltage is regulated by an error amplifier which tracks it using a resistor divider. This feedback voltage is compared to a reference voltage (V_REF) and the base drive of PNP-BJT is adjusted accordingly, which in turn adjusts the drive of the pass transistor. In this way, if the feedback voltage is above the reference voltage, the error amplifier lowers the base current, and vice versa.

Where lower accuracy voltage regulation and larger output currents (in the double-digit mA range) are required, a Zener diode, with V_Z larger or equal to the desired output voltage and the maximum base emitter voltage can be used (Figure 2).

A more precise low dropout (LDO) regulator uses a shunt regulator like the TL431 or TLVH431 to drive the pass BJT (Figure 3). With this approach, the minimum output voltage is then given by:

V_OUT(min) = V_ref

Nexperia has a large portfolio of low-V_CEsat BJTs which are ideal for use as pass devices in linear regulation applications. A comparison of features in some of these devices is shown in Table 1.

Products with “PA” in their name are available in a modern 2 mm × 2 mm leadless package, while products with “X” in their name come in a legacy SOT89 package (4.6 mm × 4.25 mm). The thermal performance of these devices for different load currents is shown in Figure 4.

Here, the output current is given by:

However, these basic circuits have the disadvantage that the stability of the output current is impacted by the negative temperature coefficient of V_BE, which is approximately −2 mV/K, meaning the change in output current over temperature is:

ΔI_OUT/ΔT = −2 mV/R_E 

This can be partially compensated by including a P-N or a Zener diode in the base bias circuit as shown in Figure 6. The benefit of using a Zener diode is that I_OUT becomes independent of VCC, increasing its immunity to supply voltage ripple.

Very precise current stabilization can be achieved by combining a BJT with a base drive controller to provide an accurate and thermally compensated control loop. A shunt regulator, like the TL431 or TLVH431 can be used to implement such a solution (Figure 7).

The shunt regulator controls the base voltage of the BJT so that the voltage drop at the sense resistor R_S is the same as the reference voltage (V_REF) of the controller IC so that the output current is then given by: