US431L/431AL
The maximum value for the biasing resistor is calcu-
lated using the following equations :
APPLICATION INFORMATION
VMIN - VKA
Output Voltage Setting
RBMAX
=
IB MAX + ILMAX
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:
IBMAX = IKMIN + IR
Where :
VMIN = Minimum supply voltage
ILMAX = Maximum load current
IBMAX = Maximum bias current
IKMIN = Maximum value for the minimum
cathode current spec
R1
R2
æ
è
ö
Vo = VKA = Vref ´ 1+
+ Iref ´ R1
ç
÷
ø
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=1.21kW and R1=2kW
the error due to the Iadj is only 1mV which is about
0.03% of the nominal set point.
IR = Current through R1
Assuming R1=2kW as before,
3.3 - 1.24
IR =
= 1.03 mA
2
IBMAX = 0.08 +1.03 = 1.11 mA
4.5 - 3.3
RB MAX
=
= 108 W
1.11+10
Selecting RB = 100 W
The maximum power dissipation of the resistor is
calculated under the maximum supply voltage as
follows :
2
RB
VMAX - VKA
PR (MAX)
B
=
VIN
VKA = VO
RB
IK
IL
Co
R1
R2
Where :
RL
US431
VMAX = Maximum supply voltage
PRB(MAX) = Maximum RB power dissipation
431app2-1.0
2
6 - 3.3
PRB(MAX) =
= 73 mW
100
Thermal Design
Figure 2 - Typical application of the US431L for program-
ming the output voltage.
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/
°C.The SOT23-5 package has the maximum power dis-
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.
Biasing Resistor (RB) Selection
The biasing resistor RB is selected such that it does
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 :
VMIN = 4.5 V
VMAX = 6 V
VKA = 3.3 V
IL = 10 mA
Ambient Temperature (Ta) -°C
Pkg
25
40
50
60
70
TO 92 775mW 682mW 620mW 558mW 496mW
SOT23-5 150mW 132mW 120mW 108mW 96mW
Rev. 1.2
5/11/98
2-4
UNISEM [ UNISEM ]
