iru431al International Rectifier Corp., iru431al Datasheet - Page 4

• Manufacturer Part Number: iru431al
• Description: Low -voltage Adjustable Precision Shunt Regulator -
• Manufacturer: International Rectifier Corp.

IRU431L / IRU431AL
APPLICATION INFORMATION
4
Output Voltage Setting
The IRU431L can be programmed to any voltages in the
range of 1.24 to 15V with the addition of R1 and R2
external resistors according to the following formula:
The IRU431L keeps a constant voltage of 1.240V be-
tween the Ref pin and ground pin. By placing a resistor
R2 across these two pins a constant current flows
through R2, adding to the I
resistor producing a voltage equal to:
which will be added to the 1.240V to set the output volt-
age as shown in the above equation. Since the input
bias current of the Ref pin is 0.5 A max, it adds a very
small error to the output voltage and for most applica-
tions can be ignored. For example, in a typical 5V to
3.3V application where R2=1.21K
error due to the I
the nominal set point.
Biasing Resistor (R
The biasing resistor R
limit the input current under the minimum input supply
and maximum load and biasing current.
V
IRU431L
IN
Vo = V
o
o
IRU431L for programming the output voltage.
1.240
R2
R
Figure 3 - Typical application of the
p
KA
p
B
= V
R1 + I
ADJ
I
REF
K
is only 1mV which is about 0.03% of
B
) Selection
B
REF
o o 1 +
is selected such that it does not
R1
R2
REF
R1
R1
R2
current and into the R1
p p + I
Co
I
L
REF
and R1=2K
R
L
V
R1
KA
= V
OUT
www.irf.com
the
An example is given below on how to properly select the
biasing resistor.
Assuming:
The maximum value for the biasing resistor is calcu-
lated using the following equations:
Assuming R1 = 2K
The maximum power dissipation of the resistor is
calculated under the maximum supply voltage as
follows:
R
I
I
I
R
Select R
P
Where:
V
P
P
V
V
V
I
Where:
V
I
I
I
I
B(MAX)
R
B(MAX)
L
L(MAX)
B(MAX)
K(MIN)
R
B(MAX)
B(MAX)
R
MAX
R
R
MIN
MAX
KA
MIN
= 10mA
=
= Current through R1
B
B
B
(MAX)
(MAX)
(MAX)
= 3.3V
3.3 - 1.24
= 4.5V
= Minimum supply voltage
= Maximum supply voltage
= 6V
= Maximum value for the minimum
= I
= Maximum load current
= 0.08 + 1.03 = 1.11mA
= Maximum bias current
=
=
cathode current spec
=
= Maximum R
=
K(MIN)
2
B
I
1.11 +10
4.5 - 3.3
(V
(6 - 3.3)
B(MAX)
= 100
V
100
MAX
MIN
+ I
R
= 1.03mA
+ I
- V
B
- V
R
as before,
2
L(MAX)
KA
KA
= 73mW
= 108
)
2
B
power dissipation
02/20/02
Rev. 1.8