Datasheet
2017-2018 Microchip Technology Inc. DS20005757B-page 19
LM4040/LM4041
4.0 APPLICATION INFORMATION
The stable operation of the LM4040 and LM4041
references require an external capacitor greater than
10 nF connected between the (+) and (–) pins. Bypass
capacitors with values between 100 pF and 10 nF have
been found to cause the devices to exhibit instabilities.
4.1 Schottky Diode
LM4040-x.x and LM4041-1.2 in the SOT-23 package
have a parasitic Schottky diode between Pin 2 (–) and
Pin 3 (die attach interface connect). Pin 3 of the
SOT-23 package must float or be connected to Pin 2.
The LM4041-ADJ use Pin 3 as the (–) output.
4.2 Conventional Shunt Regulator
In a conventional shunt regulator application (see
Figure 5-1), an external series resistor (R
S
) is
connected between the supply voltage and the
LM4040-x.x or LM4041-1.2 reference. R
S
determines
the current that flows through the load (I
L
) and the
reference (I
Q
). Because load current and supply
voltage may vary, R
S
should be small enough to supply
at least the minimum acceptable I
Q
to the reference
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 LM4040-x.x is less than 15 mA, and the
current flowing through the LM4041-1.2 or
LM4041-ADJ is less than 12 mA.
R
S
is determined by the supply voltage (V
S
), the load
and operating current, (I
L
and I
Q
), and the reference’s
reverse breakdown voltage (V
R
):
EQUATION 4-1:
4.3 Adjustable Regulator
The LM4041-ADJ’s output voltage can be adjusted to
any value between 1.24V and 10V. It is a function of the
internal reference voltage (V
REF
) and the ratio of the
external feedback resistors as shown in Figure 5-2.
The output is found using the following equation:
EQUATION 4-2:
The actual value of the internal V
REF
is a function of V
O
.
The corrected V
REF
is determined by:
EQUATION 4-3:
V
REF
/V
O
is found in the Electrical Characteristics
section and is typically –1.3 mV/V and V
Y
is equal to
1.233V. Replace the value of V
REF
in Equation 4-2 with
the value V
REF
found using Equation 4-3.
Note that actual output voltage can deviate from that
predicted using the typical V
REF
/V
O
in Equation 4-3;
for C-grade parts, the worst case V
REF
/V
O
is –
2.5 mV/V and V
Y
= 1.248V.
The following example shows the difference in output
voltage resulting from the typical and worst case values
of V
REF
/V
O
.
Let V
O
= +9V. Using the typical values of V
REF
/V
O
,
V
REF
is 1.223V. Choosing a value of R1 = 10 k, R2 =
63.272 k. Using the worst case V
REF
/V
O
for the
C-grade and D-grade parts, the output voltage is
actually 8.965V and 8.946V respectively. This results in
possible errors as large as 0.39% for the C-grade parts
and 0.59% for the D-grade parts. Once again, resistor
values found using the typical value of V
REF
/V
O
will
work in most cases, requiring no further adjustment.
R
S
V
S
V
R
–
I
L
I
Q
+
-----------------------
=
V
O
V
REF
R2 R11+=
Where:
V
O
Desired Output Voltage
V
REF
V
O
V
REF
V
O
----------------
V
Y
+=
Where:
V
O
Desired Output Voltage