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Texas Instruments A Micropower Voltage Reference Application notes
LM4250
A Micropower Voltage Reference
Literature Number: SNOA859
National Semiconductor
Linear Brief 34
June 1976
A low-drift voltage reference can be easily made by converting a zero temperature coefficient current to a voltage.
JFETs biased slightly below pinch-off exhibit a zero temperature coefficient drain current (ID) as shown in Figure 1. With
the above property and a micropower operational amplifier,
used to convert the drain current to a voltage, a low power
consumption voltage reference can be built as shown in
Figure 2. The consumption of LM4250 op amp is programmed through resistor RSET. Potentiometer P1 should be
adjusted for low output (VREF) temperature coefficient. Actually, it can be trimmed for positive, negative or zero temperature coefficient. The output voltage is trimmed through P2
and it is expressed by:
are a function of its supply current, which depends on RSET,
and V+. V+ can be provided by VREF through the addition of
a second FET, J2, shown in Figure 3. This way the parameters of the op amp will be independent of the unregulated
input. The reference voltage can be taken from the wiper of
the potentiometer P2 (VREF = V+) or from the source of J2
(VREF > V+). In the first case, the output impedance of the
circuit is quite high and buffering may be required according
to the application. The output impedance in the second case
is low, essentially the 1/gm of (J2) divided by the loop gain of
the circuit. In this case, a small temperature coefficient due
to the supply current of the LM4250 is going to be added and
be compensated for by an additional trimming of P1. VREF is
computed by:
A Micropower Voltage Reference
A Micropower Voltage
Reference
With the values shown in Figure 2, the temperature coefficient of the output is 0.002%/˚C and the overall standby
current less than 100 µA. The characteristics of the LM4250
00873401
FIGURE 1. FET Transfer Characteristics
LB-34
© 2002 National Semiconductor Corporation
AN008734
www.national.com
LB-34
00873402
P1 = Drift Adjust
P2 = VREF Adjust
FIGURE 2. Basic Voltage Reference
00873403
FIGURE 3. Improved Voltage Reference
www.national.com
2
A Micropower Voltage Reference
Notes
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LB-34
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