Datasheet

Functional Block Diagram
10122221
*LM4051-ADJ only
**LM4051-1.2 only
Applications Information
The LM4051 is a precision micro-power curvature-corrected
bandgap shunt voltage reference. For space critical applica-
tions, the LM4051 is available in the sub-miniature SOT-23
surface-mount package. The LM4051 has been designed for
stable operation without the need of an external capacitor
connected between the “+” pin and the “−” pin. If, however, a
bypass capacitor is used, the LM4051 remains stable. De-
sign effort is further reduced with the choice of either a fixed
1.2V or an adjustable reverse breakdown voltage. The mini-
mum operating current is 60 µA for the LM4051-1.2 and the
LM4051-ADJ. Both versions have a maximum operating
current of 12 mA.
LM4051s using the SOT-23 package have pin 3 connected
as the (-) output through the package’s die attach interface.
Therefore, the LM4051-1.2’s pin 3 must be left floating or
connected to pin 2 and the LM4051-ADJ’s pin 3 is the (-)
output.
The typical thermal hysteresis specification is defined as the
change in +25˚C voltage measured after thermal cycling.
The device is thermal cycled to temperature -40˚C and then
measured at 25˚C. Next the device is thermal cycled to
temperature +125˚C and again measured at 25˚C. The re-
sulting V
OUT
delta shift between the 25˚C measurements is
thermal hysteresis. Thermal hysteresis is common in preci-
sion references and is induced by thermal-mechanical pack-
age stress. Changes in environmental storage temperature,
operating temperature and board mounting temperature are
all factors that can contribute to thermal hysteresis.
In a conventional shunt regulator application (Figure 1), an
external series resistor (R
S
) is connected between the sup-
ply voltage and the LM4051. R
S
determines the current that
flows through the load (I
L
) and the LM4051 (I
Q
). Since load
current and supply voltage may vary, R
S
should be small
enough to supply at least the minimum acceptable I
Q
to the
LM4051 even when the supply voltage is at its minimum and
the load current is at its maximum value. When the supply
voltage is at its maximum and I
L
is at its minimum, R
S
should
be large enough so that the current flowing through the
LM4051 is less than 12 mA.
R
S
should be selected based on the supply voltage, (V
S
), the
desired load and operating current, (I
L
and I
Q
), and the
LM4051’s reverse breakdown voltage, V
R
.
The LM4051-ADJ’s output voltage can be adjusted to any
value in the range of 1.24V through 10V. It is a function of the
internal reference voltage (V
REF
) and the ratio of the external
feedback resistors as shown in Figure 2 . The output voltage
is found using the equation
V
O
=V
REF
[(R2/R1) + 1] (1)
(2)
where V
O
is the output voltage. The actual value of the
internal V
REF
is a function of V
O
. The “corrected” V
REF
is
determined by
V
REF
=V
O
(∆V
REF
/∆V
O
)+V
Y
(3)
where
V
Y
= 1.22V
∆V
REF
/∆V
O
is found in the Electrical Characteristics and is
typically −1.55 mV/V. You can get a more accurate indication
of the output voltage by replacing the value of V
REF
in
equation (1) with the value found using equation (3).
LM4051
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