Designing a zener-diode regulator
By Ron Mancini -- EDN, August 5, 2004
IC references are popular with circuit designers because they are accurate and exhibit low drift. Some of my future columns will cover the three types of IC references: buried zener, bandgap, and XFET. You develop the reference-design procedure with a zener diode; the zener's simplicity illustrates the design procedure, and its problems make you appreciate IC references. The circuit specifications are VCC=30V±10%, 8.445
VREF 9.555, ΔVREF200 mV, 100 kΩRLOAD200 kΩ, and 0°CTA80°C.
Select a 1N757 9.1V zener for the first try. Note that the maximum temperature coefficient is 6 mV/°C, and the zener-voltage tolerance is ±5%. The calculated reference voltage equals the maximum specification, VREF=(1.05)(9.1)=9.555V, but the temperature-induced drift is ΔVREF=(80–25)(6 mV)=0.330V, thus exceeding the maximum-drift voltage specification.
Connect a signal diode in series with a 1N756 8.2V zener so that the diode's negative-temperature coefficient cancels part of the zener's positive-temperature coefficient (Figure 1). The diode's temperature coefficient ranges from –2.1 to –2.3 mV/°C, and the 8.2V zener's temperature coefficient is 5.4 mV/°C, so the combination's maximum temperature coefficient is 3.3 mV/°C. This scenario yields ΔVREF=181.5 mV, which meets specifications, but the minimum reference voltage, VREF=(0.95)(8.2)+0.5=8.29V, is less than the specified limit of 8.445V.
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This analysis is incomplete because additional zener characteristics, such as zener impedance and bias current, affect ΔVREF. Now's the time to give up on a pure zener diode and go to a temperature-compensated zener. The 1N935 has a zener voltage of 9.075V; a tolerance of 5%; a temperature coefficient of 2 mV/°C; and a zener impedance of 20Ω at IZ=7.5 mA, where IZ is the zener-test current. The reference-voltage range is 8.62VREF9.53. A quick calculation of the temperature-coefficient error yields a maximum voltage change of ΔVREF1=110 mV. So far, so good, but you need to do further calculations to get the complete picture.
Calculate RBIAS as RBIAS=(VCC–VREF)/IZ=(30–9)/7.5=2885Ω; select RBIAS=2800Ω±2%. (The temperature-compensated zener includes the diode in Figure 1.) Because of power-supply and resistor tolerances, the change in IZ ranges from [(30)(0.9)–9.53–0.11]/2.8(1.02)=6.07 mA to [(30)(1.1)–8.62–0.05]/2.8(0.98)=8.86 mA. The load-resistance change causes about 90 µA of change in IZ, which is insignificant. This change in the temperature-compensated-zener current corresponds to a zener-impedance change of approximately ±5Ω (Reference 1), but ΔVREF changes only about ±37.5 mV. You should also consider the zener-voltage change: Because of the IZ shift, the zener-operating point changes when VREF varies by ±50 mV. Also, keep in mind that the maximum wideband semiconductor noise that the dc voltage contains is 20 µV.
The final voltage-reference change is 110+37.5+50=197.5 mV. Some people say that this analysis is not the most rigorous, and they are right, but it gives you an idea of what using a zener reference involves. If VCC had been as low as 12V, a zener diode would fail to meet specifications. The load-resistance variation does not affect design calculations, because RLOAD is large. A smaller, 2-kΩ load resistance with a variation of 1 kΩ causes a great change in IZ, necessitating a zener-diode buffer. If the zener diode is part of an IC, then you can trim RBIAS with a laser, and adding a buffer is trivial. This buried-zener-voltage-reference method seems like a better way to build a voltage reference.
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| Author Information |
 Ron Mancini is staff scientist at Texas Instruments. You can reach him at 1-352-569-9401, rmancini@ti.com. |
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Why not choose an actual reference from the plethora of chips available? Then you get specified accuracy and tempco.
Nicholas Allen - 2005-12-1 06:10:00 PST -
Sometimes you use a zener just because it has a lower impedance than a capacitor, it's smaller, and it has no nasty time constants. IE you often don't care about drift or voltage precision.
Zener diodes like to be 6 or 7 volts. Look at the specs for different diode voltages and you will see that silicon zeners are best in that range. You may want to design your circuit so that it uses parts in that range.
Likewise, an old trick is to wire a bipolar transistor upside down and you have a b-e zener in series with a c-b forward diode, in one part. Common signal transistors will give you b-e zeners around 7 volts.
Today parts are cheap. Given the cost of PCBs etc, you probably don't want to play around with anything unpredictable, such as, JFET current sources. Which are otherwise, a good way to improve a zener cct.
Today, it's no trim-pots allowed! But that just means we gave that job to the silicon vendors and their laser trimmer.
Cheers,
Steve
Steve Ungstad - 2004-6-8 22:33:00 PDT
