Voltage References Basics

Voltage reference is a reference voltage Vref stable against process, voltage and temperature. Voltage reference is a basic block used in analog/mixed signal Integrated Circuit(IC) and radio frequency ICs. Voltage reference is used in DC-DC converter, power management circuits, ADC and DAC. So there are more than one voltage reference designed in an analog/mixed signal IC. The design of voltage reference is very important because the accuracy of the voltage reference decides the accuracy and performance of the whole system.

For example, in a voltage regulator the line regulation is one of the important specifications. Line regulation refers to variation in output voltage with the variation in input supply voltages. In a voltage regulator, a voltage reference is generated and is multiplied by a specific ratio to get the regulated output voltage. So if the voltage reference doesn’t vary with the input supply voltage, then the regulated output voltage doesn’t vary with supply voltage. So the accuracy of the output voltage is decided by the accuracy of the reference voltage.

For the figure shown below, 

If VREF changes, then VOUT changes by  1+ R1/R2, so there is a gain from VREF to VOUT. If VOUT specification is 2.5V ± 1%. Then ∆VOUT = ±25mV. If VREF=1.2V and changes by 100ppm/C i.e., For change in temperature of 100C, Vref=1.2±1.2×100×1e-6×100=1.2±0.012V. Let  1+ R1/R2=2.1, then VOUT changes by 2.5±0.0252. From the above example, the accuracy of the VREF decides the accuracy of the regulator output voltage VOUT.

Low power Voltage Reference

Voltage Reference is a circuit which generated a stable reference voltage across Process, Voltage and Temperature. Generally, Analog circuits such as DC-DC converters, power management circuits, Analog to Digital Converters require a stable voltage reference circuit with a tight specification for voltage variation in the range of 4ppm/C. Most of the voltage references used in analog circuits are Bandgap voltage reference circuits. Bandgap Voltage reference circuits generate a stable reference voltage of 1.2V. Since Silicon Bandgap energy is 1.12eV, so the circuit is called Bandgap reference. 

Bandgap Reference

Bandgap reference can be generated by adding positive temperature coefficient voltage with the negative temperature coefficient voltage to cancel the effect of temperature. In CMOS technology, negative tempco voltage can be generated with a PNP/NPN transistor VBE voltage and positive tempco voltage can be generated using the difference of two VBE voltages(VBE1-VBE2)

VREF= VBE2 + (VBE1-VBE2)

VBE1 = VT*ln(Jc/Jc0),

where  VT is the Volt equivalent of temperature,

Jc is the collector current density,

Jc0 is the reverse saturation current density

Basic bandgap reference voltage circuit is shown in the figure below:

Figure 1: Bandgap Voltage Reference

Q1 is PNP transistor and Q2 are N such PNP transistors

In the above circuit, I1 = (VEB1-VEB2)/R1 = VT * ln(N)

So, VREF = VEB2 + (VEB1-VEB2)/R1* R2

Reference Voltage, VREF = VEB2 + VT * ln(N) * R2/R1

The above Bandgap voltage reference circuit requires an opamp which requires and additional current, so for low power voltage reference circuit(current less than 100nA),  the circuit given below can be used. The reference voltage can be designed with negative tempco of Vbe of PNP transistor and positive tempco of Vt(thermal voltage). The transistors in this circuit should be in subthreshold region. The challenge in this design is to get a reference voltage less than 1V with low current across process

Figure 2: Low power Voltage Reference