analogangle By Ron Mancini
Designing a zener-diode regulator
I
C references are popular with circuit designers be-
change: Because of the IZ shift, the
zener-operating point changes
cause they are accurate and exhibit low drift. Some of when VREF varies by ⫾50 mV. Also,
keep in mind that the maximum
my future columns will cover the three types of IC ref- wideband semiconductor noise
that the dc voltage contains is 20
erences: buried zener, bandgap, and XFET. You develop the ␮V.
The final voltage-reference
reference-design procedure with a zener diode; the zener’s change is 110+37.5+50=197.5 mV.
Some people say that this
simplicity illustrates the design 9.075V; a tolerance of 30V⫾10%
analysis is not the most
procedure, and its problems make 5%; a temperature
rigorous, and they are
you appreciate IC references. The coefficient of 2 mV/⬚C;
right, but it gives you an
circuit specifications are VCC= and a zener impedance
idea of what using a
RBIAS
30V⫾10%, 8.445울VREF울9.555, of 20⍀ at IZ=7.5 mA,
zener reference involves.
⌬VREF울200 mV, 100 k⍀울RLOAD울 where IZ is the zenerIf VCC had been as low as
VREF
200 k⍀, and 0⬚C울TA울80⬚C.
test current. The refer12V, a zener diode would
IN4148
Select a 1N757 9.1V zener for ence-voltage range is
fail to meet specificathe first try. Note that the maxi- 8.62울VREF울9.53.
A
tions. The load-resistmum temperature coefficient is 6 quick calculation of the
ance variation does not
mV/⬚C, and the zener-voltage tol- temperature-coefficient
affect design calculaerance is ⫾5%. The calculated ref- error yields a maxitions, because RLOAD is
large. A smaller, 2-k⍀
erence voltage equals the maxi- mum voltage change of
Figure 1
load resistance with a
mum specification, VREF=(1.05) ⌬VREF1=110 mV. So far,
(9.1)=9.555V, but the tempera- so good, but you need This zener-regulator
variation of 1 k⍀ causes
ture-induced drift is ⌬VREF= to do further calcula- circuit includes a tem- a great change in IZ,
(80⫺25)(6 mV)=0.330V, thus tions to get the com- perature-compensation necessitating a zenerdiode.
exceeding the maximum-drift plete picture.
diode buffer. If the zener
voltage specification.
diode is part of an IC,
Calculate RBIAS as
Connect a signal diode in series RBIAS=(VCC⫺VREF)/IZ=(30⫺9)/7.5= then you can trim RBIAS with a
with a 1N756 8.2V zener so that 2885⍀; select RBIAS=2800⍀⫾2%. laser, and adding a buffer is trivial.
the diode’s negative-temperature (The temperature-compensated This buried-zener-voltage-refercoefficient cancels part of the
zener’s positive-temperature coef- THE ZENER’S SIMPLICITY ILLUSTRATES THE DESIGN
ficient (Figure 1). The diode’s temperature coefficient ranges from PROCEDURE, AND ITS PROBLEMS MAKE YOU
⫺2.1 to ⫺2.3 mV/⬚C, and the 8.2V
APPRECIATE IC REFERENCES.
zener’s temperature coefficient is
5.4 mV/⬚C, so the combination’s zener includes the diode in Figure ence method seems like a better
maximum temperature coefficient 1.) Because of power-supply and way to build a voltage reference.왏
is 3.3 mV/⬚C. This scenario yields resistor tolerances, the change in IZ
⌬VREF=181.5 mV, which meets ranges from [(30)(0.9)⫺9.53⫺ Reference
specifications, but the minimum 0.11]/2.8(1.02)=6.07 mA to
1. Motorola zener-diode manual,
reference voltage, VREF=(0.95)(8.2)+ [(30)(1.1)⫺8.62⫺0.05]/2.8(0.98) Motorola Inc, 1980.
0.5=8.29V, is less than the specified =8.86 mA. The load-resistance
limit of 8.445V.
change causes about 90 ␮A of Talk to us
This analysis is incomplete change in IZ, which is insignificant. Post comments via TalkBack at the
because additional zener charac- This change in the temperature- online version of this article at
teristics, such as zener impedance compensated-zener current corre- www.edn.com.
and bias current, affect ⌬VREF. sponds to a zener-impedance
Now’s the time to give up on a change of approximately ⫾5⍀ Ron Mancini is staff scientist
pure zener diode and go to a tem- (Reference 1), but ⌬VREF changes at Texas Instruments. You can
perature-compensated zener. The only about ⫾37.5 mV. You should reach him at 1-352-569-9401,
1N935 has a zener voltage of also consider the zener-voltage rmancini@ti.com.
24 edn | August 5, 2004
www.edn.com