The ability of a
voltage reference or regulator to maintain a constant output under varying
external conditions is characterized in terms of performance parameters such as
line and load regulation, and the thermal coefficient. In the case of voltage
references, output noise and long-term stability are also significant.
Line and Load
Regulation
Line regulation, also
called input, or supply regulation, gives a measure of the circuit’s ability to
maintain the specified output under varying input conditions.
where ∆Vo is
the output change resulting from a change ∆Vi at the input. Line regulation
is expressed in millivolts or microvolts per volt.
Load regulation gives a
measure of the circuit’s ability to maintain the specified output voltage under
varying load conditions.
The temperature
coefficient of output voltage gives a measure of the circuit’s ability to
maintain the specified output voltage Vo under varying thermal conditions.
It is usually expresses
in mV/⁰C.
Besides line and load
regulation, thermal stability is the most critical performance requirement of
voltage references due to the tendency of IC components to be strongly affected
by change in temperature. Consider the silicon pn junction, which forms the
basis of diodes and BJTs. Its forward-bias voltage VD and current ID
are related as
where VT is
the thermal voltage and Is the saturation current.
where k =1.38×10-23
J/K is known as Boltzmann’s constant, q = 1.602 x 10-19 C, T is
absolute temperature.
TC = VT/T =
k/q = 0.086 mV/⁰C
Temperature coefficient
for diode forward voltage drop VD is -2.1 mV/ ⁰C.
Two types of voltage
references are popular - Zener voltage
reference and Bandgap reference.
Zener Voltage Reference
A
zener diode is widely used as a voltage reference. While true zener breakdown
occurs below 5V, avalanche breakdown occurs at higher voltages and has a
positive temperature coefficient. Zener breakdown truly has negative
temperature coefficient but nowadays avalanche breakdwon is referred to as
zener breakdown only.
A
combination of normal PN diode with negative temperature coefficient is used to
compensate for temperature instability as shown below.
However
to get maximum temperature compensation, odd values of output voltage is
produces like 4.7, 5.3, 6.2V etc.
As
Zener to avalance transition occur at 5.6 volt (and we use avalanche diode as
voltage reference), a input voltage higher than 6.2V (5.6+VD) is
required.
Bandgap
voltage reference
For circuits which are
powered by low voltage supply, bandgap reference offers an exciting choice.
They can offer much lower voltage output as they utilize PN junction bandgap voltage
which is 1.2V for Silicon.
The value of VT
is 0.086mV/⁰C
and it has positive temperature coefficient. This voltage can be used to
compensate temperature related variation of VBE by amplifying the
value to +2mV/⁰C.
This is the scheme
behind Bandgap reference.
The actual circuit of a bandgap reference may look as below.
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