US431ALCS UNISEM [UNISEM], US431ALCS Datasheet - Page 4

• Manufacturer Part Number: US431ALCS
• Description: LOW -VOLTAGE ADJUSTABLE PRECISION SHUNT REGULATOR
• Manufacturer: UNISEM [UNISEM]

US431L/431AL
APPLICATION INFORMATION
APPLICATION INFORMATION
Output Voltage Setting
The US431L can be programmed to any voltages in the
range of 1.24 to 6V with the addition of R1 and R2 exter-
nal resistors according to the following formula:
The US431L 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 Iref current and into the R1
resistor producing a voltage equal to the (1.240/R2)*R1
+ Iref * R1 which will be added to the 1.240V to set the
output voltage as shown in the above equation. Since
the Input bias current of the Ref pin is 0.5 uA max , it
adds a very small error to the output voltage and for most
applications can be ignored . For example, in a typical
5V to 3.3V application where R2=1.21k and R1=2k
the error due to the Iadj is only 1mV which is about
0.03% of the nominal set point.
Figure 2 - Typical application of the US431L for program-
ming the output voltage.
Biasing Resistor (R
The biasing resistor R
not limit the input current under the minimum input
supply and maximum load and biasing current.
An example is given below on how to properly select
the biasing resistor.
Assuming :
V
V
V
I
2-4
L
MIN
MAX
KA
= 10 mA
Vo
= 3.3 V
431app2-1.0
= 4.5 V
V
= 6 V
IN
US431
V
KA
R
B
Vref
I
K
B
) Selection
B
1
is selected such that it does
R1
R2
R
R
2
1
Iref R
Co
I
L
R
1
L
V
KA
= V
O
The maximum value for the biasing resistor is calcu-
lated using the following equations :
Assuming R1=2k as before,
The maximum power dissipation of the resistor is
calculated under the maximum supply voltage as
follows :
Thermal Design
The US431L is offered in the plastic TO-92 (Z) or the
surface mount SOT23-5 (L) packages. The TO-92 pack-
age has the maximum power dissipation capability of
775mW at Ta=25 C with the derating factor of -6.2mW/
sipation capability of 150mW at Ta =25 C with the
derating factor of -1.2mW/ C.
Table below summarizes the maximum power dissipa-
tion capability of each package versus ambient tempera-
ture.
TO 92
SOT23-5 150mW 132mW 120mW 108mW 96mW
Pkg
C.The SOT23-5 package has the maximum power dis-
P
P
I =
I
R
Selecting
Where :
V
P
R
I
I
I
I
I = Current through R1
R
B
Where :
V
cathode current spec
B
L
B
K
R
R MAX
R MAX
MAX
B
R MAX
MAX
MAX
MAX
MAX
MIN
B
MIN
B
B
B
MAX
(
(
MAX
(
3.3 - 1.24
= 0.08 + 1.03 = 1.11 mA
= Maximum value for the minimum
= I
= Maximum load current
= Maximum bias current
= Maximum supply voltage
= Minimum supply voltage
)
)
)
775mW 682mW 620mW 558mW 496mW
1.11+ 10
I
4
V
B
K
2
=
.5 - 3.3
MIN
MIN
MAX
25
R = 100
Maximum R power dissipation
V
B
+ I
MAX
6
+ I
- V
Ambient Temperature (Ta) - C
- 3.3
100
R
R
L
- V
MAX
KA
103
B
.
40
108
KA
2
mA
B
2
73
mW
50
60
70
Rev. 1.2
5/11/98