Low-noise synthesizers require quiet power supplies for their oscillators and other critical circuitry. They typically require more than 5 mA of current, however, making a voltage reference alone inadequate. Three-terminal low-dropout regulators, on the other hand, exhibit far too much noise for this application.
Voltage references and linear regulators have a lot in common. Regulators are functionally equivalent to references but have greater output current. In applications that require higher output current, references can be configured as regulators while maintaining their beneficial characteristics. These include lower noise, higher stability, tighter tolerance, better line regulation, and higher efficiency.
In addition, some references can provide features that don’t exist in linear regulators. For example, the ADR390/1 has a sense pin that can be used for Kelvin connections. This is an effective way to reduce the effects of stray resistance. It’s especially useful in high-current applications or when the load is located far from the source.
Stray resistance also can degrade performance significantly. A copper trace with a stray resistance of 1 Ω develops an error of 1 mV/mA and introduces a TC of 4 V/mA/°C. For a 2.5-V reference, this corresponds to an accuracy degradation of 0.04% and a TC of 1.6 ppm/°C.
Making a linear regulator from a reference is very simple. Most references can be combined with a current source. This allows the reference to control the voltage while the external current source provides the current. An external boost transistor can be added to the ADR391 (Fig. 1). The transistor sources the required current without dissipating excessive power in the IC. Only the capabilities of the boost transistor limit the maximum load current.
R1 provides supply current to the reference. It also delivers enough bias current to turn on the transistor. The voltage at the input of the reference is equal to VIN minus the drop across resistor R2 (5 Ω) minus the VBE of the transistor. For the ADR390/1 to operate correctly, this point should be at least 2.8 V.
The emitter current is approximately equal to the load current (load current = IC = \[β/(β+1)*IE\]. As the load current increases, the drop across the stray resistance, RS, increases as well. The ADR390/1 senses the drop through its sense pin and corrects the output to be 2.5 V, as specified. When the voltage drop across R1 (220 Ω) is more than about 0.7 V, the LED turns on, indicating an overcurrent condition.
If an automatic shutdown feature is required, the circuit shown in Figure 2 can be used. In this arrangement, an instrumentation amplifier, the AD627, senses the drop across R3 (5 Ω). It then automatically shuts down for the reference when an overcurrent condition is detected.
The current limit is sensitive to the temperature variation of the diode’s forward voltage drop and the pnp’s VBE, and will decrease with increasing temperature. When higher output currents are needed, the boost transistor can be replaced by a power pnp Darlington or by a power enhancement PMOS FET. In these types of applications, heatsinking may be required.