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 V_CC=30V±10%, 8.445V_REF 9.555, ΔV_REF200 mV, 100 kΩR_LOAD200 kΩ, and 0°CT_A80°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, V_REF=(1.05)(9.1)=9.555V, but the temperature-induced drift is ΔV_REF=(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 ΔV_REF=181.5 mV, which meets specifications, but the minimum reference voltage, V_REF=(0.95)(8.2)+0.5=8.29V, is less than the specified limit of 8.445V.

This analysis is incomplete because additional zener characteristics, such as zener impedance and bias current, affect ΔV_REF. 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 I_Z=7.5 mA, where I_Z is the zener-test current. The reference-voltage range is 8.62V_REF9.53. A quick calculation of the temperature-coefficient error yields a maximum voltage change of ΔV_REF1=110 mV. So far, so good, but you need to do further calculations to get the complete picture.

Calculate R_BIAS as R_BIAS=(V_CC–V_REF)/I_Z=(30–9)/7.5=2885Ω; select R_BIAS=2800Ω±2%. (The temperature-compensated zener includes the diode in Figure 1.) Because of power-supply and resistor tolerances, the change in I_Z 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 I_Z, which is insignificant. This change in the temperature-compensated-zener current corresponds to a zener-impedance change of approximately ±5Ω (Reference 1), but ΔV_REF changes only about ±37.5 mV. You should also consider the zener-voltage change: Because of the I_Z shift, the zener-operating point changes when V_REF 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 V_CC had been as low as 12V, a zener diode would fail to meet specifications. The load-resistance variation does not affect design calculations, because R_LOAD is large. A smaller, 2-kΩ load resistance with a variation of 1 kΩ causes a great change in I_Z, necessitating a zener-diode buffer. If the zener diode is part of an IC, then you can trim R_BIAS 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.

Author Information

Ron Mancini is staff scientist at Texas Instruments. You can reach him at 1-352-569-9401, rmancini@ti.com.