Texas Instruments | AN-173 IC Zener Eases Reference Design (Rev. B) | Application notes | Texas Instruments AN-173 IC Zener Eases Reference Design (Rev. B) Application notes

Texas Instruments AN-173 IC Zener Eases Reference Design (Rev. B) Application notes
Application Report
SNOA590B – November 1976 – Revised May 2013
AN-173 IC Zener Eases Reference Design
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ABSTRACT
A new IC zener with low dynamic impedance and wide operating current range significantly simplifies
reference or regulator circuit design. The low dynamic impedance provides better regulation against
operating current changes, easing the requirements of the biasing supply. Further, the temperature
coefficient is independent of operating current, so that the LM129 can be used at any convenient current
level. Other characteristics such as temperature coefficient, noise and long term stability are equal to or
better than good quality discrete zeners.
Contents
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1
Description
2
1
IC Reference Zener ......................................................................................................... 2
2
Basic Biasing ................................................................................................................ 2
3
Bridge Compensation for Line Changes ................................................................................. 3
4
10 V Buffered Output Reference .......................................................................................... 3
5
Bipolar Output Reference .................................................................................................. 3
6
High Stability 10 V Regulator .............................................................................................. 4
7
External Reference For IC ................................................................................................. 5
8
Power Shunt Regulator
List of Figures
....................................................................................................
5
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Description
1
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Description
The LM129 uses a new subsurface breakdown IC zener combined with a buffer circuit to lower dynamic
impedance. The new subsurface zener has low noise and excellent long term stability since the
breakdown is in the bulk of the silicon. Circuitry around the zener supplies internal biasing currents and
buffers external current changes from the zener. The overall breakdown is about 6.9 V with devices
selected for temperature coefficients.
The zener is relatively straightforward. A buried zener D1 breaks down biasing the base of transistor Q1.
Transistor Q1 drives two buffers Q2 and Q3. External current changes through the circuit are fully
absorbed by the buffer transistors rather than by D1. Current through D1 is held constant at 250 μA by a
2k resistor across the emitter base of Q1 while the emitter-base voltage of Q1 nominally temperature
compensates the reference voltage.
The other components, Q4, Q5 and Q6, set the operating current of Q1. Frequency compensation is
accomplished with two junction capacitors.
All that is needed for biasing in most applications is a resistor as shown in Figure 2. Biasing current can
be anywhere from 0.6 mA to 15 mA with little change in performance. Optimally, however, the biasing
current should be as low as possible for the best regulation. The dynamic impedance of the LM129 is
about 1 Ω and is independent of current. Therefore, the regulation of the LM129 against voltage changes
is 1/Rs.
Lower currents or higher Rs give better regulation. For example, with a 15 V supply and 1 mA operating
current, the reference change for a 10% change in the 15 V supply is 180 μV. If the LM129 is run at 5 mA,
the change is 900 μV or 5 times worse. By comparison, a standard IN821 zener will change about 17 mV.
All discrete zeners have about the same regulation since their dynamic impedance is inversely
proportional to operating current.
If the zener does not have to be grounded, a bridge compensating circuit can be used to get virtually
perfect regulation, as shown in Figure 3. A small compensating voltage is generated across R1, which
matches the dynamic impedance of the LM129. Since the dynamic impedance of the LM129 is linear with
current, this circuit will work even with large changes in the unregulated input voltage.
Figure 1. IC Reference Zener
Figure 2. Basic Biasing
2
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Description
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Other output voltages are easily obtained with the simple op-amp circuit shown in Figure 4. A simple noninverting amplifier is used to boost and buffer the zener to 10 V. The reference is run directly from the
input power rather than the output of the op-amp. When the zener is powered from the op-amp, special
starting circuitry is sometimes necessary to insure the output comes up in the right polarity. For outputs
lower than the breakdown of the LM129 a divider can be connected across the zener to drive the op-amp.
An AC square wave or bipolarity output reference can easily be made with an op-amp and FET switch as
shown in Figure 5. When Q1 is “ON”, the LM108 functions as a normal inverting op-amp with a gain of −1
and an output of −6.9 V. With Q1 “OFF” the op-amp acts as a giving 6.9 V at the output. Some nonsymmetry will occur from loading change on the LM129 in the different states and mismatch of R1 and R2.
Trimming either R1 or R2 can make the output exactly symmetrical around ground.
Figure 3. Bridge Compensation for Line Changes
Figure 4. 10 V Buffered Output Reference
Figure 5. Bipolar Output Reference
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Description
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Figure 6. High Stability 10 V Regulator
By combining the LM129 with an LM117 three-terminal regulator a high stability power regulator can be
made. This is shown in Figure 6. Resistor R1 biases the LM129 at about 1 mA from the 1.25 V reference
in the LM117. The voltage of the LM129 is added to the 1.25 V of the LM117 to make a total reference
voltage of 8.1 V. The output voltage is then set at 10 V by R2 and R3. Since the internal reference of the
LM117 contributes only about 20% of the total reference voltage, regulation and drift are essentially those
of the external zener. The regulator has 0.2% load and line regulation and if a low drift zener such as the
LM129A is used overall temperature coefficient is less than 0.002%/°C.
The new zener can be used as the reference for conventional IC voltage regulators for enhanced
performance. Noise is lower, time stability is better, and temperature coefficient can be better depending
on the device selected. Further, the output voltage is independent of power changes in the regulator.
Figure 7 shows an LM723 using an external LM129 reference. The internal 7 V reference is not used and
a single resistor biases the LM129 as the reference. The 5k resistor chosen provides sufficient operating
current for the zener over the 10 V to 40 V input voltage range of the LM723. Since the dynamic
impedance of the LM129 is so low, the reference regulation against line changes is only 0.02%/V. This is
small compared to the regulation of 0.1%/V for the LM723; however, the resistor can be replaced by a 1
mA to 5 mA FET used as a constant current source for improved regulation. When the FET is used
reference regulation is easily 0.001%/V. Output voltage is set in the standard manner except that for low
output voltages sufficient current must be run through the zener to power the voltage divider supplying the
reference to the LM723.
An overload protected power shunt regulator is shown in Figure 8. The output voltage is about 7.8 V − the
7 V breakdown of the LM129 plus the 0.8 V emitter-base voltage of the LM395. The LM395 is an IC, 1.5 A
power transistor with complete overload protection on the chip. Included on the chip are current limiting
and thermal limiting, making the device virtually blowout-proof. Further, the base current is only 5 μA,
making it easy to drive as a shunt regulator. As the input voltage rises, more drive is applied to the base of
the LM395, turning it on harder and dropping more voltage across the series resistance. Should the input
voltage rise too high, the LM395 will current limit or thermal limit, protecting itself.
The new IC zener can replace existing zeners in just about any application with improved performance
and simpler external circuitry. As with any zener reference, devices are selected for temperature
coefficient and operating temperature range. Since the devices are made by a standard integrated circuit
process, cost is low and good reproducibility is obtained in volume production.
Finally, since the device is actually an IC, it is packaged in a rugged TO-46 metal can package or a 3-lead
plastic transistor package.
4
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Figure 7. External Reference For IC
Figure 8. Power Shunt Regulator
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