Data Sheet No. PD94118
IRU431L
LOW -VOLTAGE ADJUSTABLE PRECISION
SHUNT REGULATOR
DESCRIPTION
The IRU431L is a three-terminal adjustable shunt regu-
lator that can also be used as a precision voltage refer-
ence. Its output voltage may be set to any value be-
tween V
REF
(1.24V) and 15V with two external resistors
as shown in the typical application circuit. Other appli-
cations of this device include being used as a merged
amplifier and reference in applications such as a linear
regulator or as the secondary side controller in low volt-
age power supply applications. The IRU431L only re-
quires 80mA maximum quiescent current before regu-
lating, making it ideal as an efficient voltage reference
for battery type applications. The IRU431L has
61%
initial accuracy.
FEATURES
SOT-23 Packages
Voltage Reference Initial Accuracy (1%)
Low Operating Cathode Current (80mA max)
Unconditionally Stable with only 1mF
Adjustable Output from 1.24V to 15V
0.25V Typical Output Impedance
Pin to Pin Compatible with TLV431
APPLICATIONS
Precision Voltage Reference
Linear Regulator Controller
Secondary Side Controller for the low voltage
power supply applications
TYPICAL APPLICATION
V
IN
R
B
R1
V
REF
V
OUT
K (Cathode)
IRU431L
Vo = V
REF
3
1 +
Ref
A (Anode)
C
O
(
R1
R2
)
R2
Figure 1 - Typical application of the IRU431L as a shunt regulator / voltage reference.
PACKAGE ORDER INFORMATION
T
A
(°C)
0 To 70
3-PIN SOT-23 (L3)
IRU431LCL3
5-PIN SOT-23 (L5)
IRU431LCL5
Rev. 2.0
08/05/03
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1
IRU431L
ABSOLUTE MAXIMUM RATINGS
Input Voltage (V
IN
) ....................................................
Continuous Cathode Current Range ..........................
Reference Current Range ..........................................
Storage Temperature Range ......................................
Operating Junction Temperature Range .....................
15V
-15mA to +15mA
-0.05mA to 15mA
-658C to 1508C
08C to 1508C
CAUTION:
Stresses above those listed in "Absolute Maximum Ratings" may cause permanent damage to the device.
PACKAGE INFORMATION
3-PIN SOT-23 (L3)
TOP VIEW
2
5-PIN SOT-23 (L5)
TOP VIEW
Cathode
NC
1
NC
2
5
Anode
Anode
3
1
Ref
Cathode
3
4
Ref
θ
JA
=336°C/W
θ
JA
=256°C/W
ELECTRICAL SPECIFICATIONS
Unless otherwise specified, these specifications apply over T
A
=0 to 708C, Co =1mF. Typical values refer to T
A
=258C.
Low duty cycle pulse testing is used which keeps junction and case temperatures equal to the ambient tempera-
ture.
PARAMETER
SYM
TEST CONDITION
I
K
=10mA, V
KA
=V
REF
, T
A
=258C
I
K
=10mA, V
KA
=V
REF
V
KA
=V
REF
, I
K
=10mA
I
K
=10mA,
DV
KA
=V
REF
to 6V
I
K
=10mA, R1=10KV, R2=open
I
K
=10mA, R1=10KV, R2=open
V
KA
=V
REF
V
KA
=6V, V
REF
=0V
V
KA
=10V, V
REF
=0V
V
KA
=15V, V
REF
=0V
V
KA
=V
REF
, f<1KHz,
I
K
=0.1 to 15mA, Note 1
MIN
1.228
1.221
TYP
1.240
1.240
6
-1
0.15
0.05
55
0.6
1.8
3.2
0.25
MAX UNITS
1.252
1.259
V
mV
-6
1
mV/V
mA
mA
mA
mA
Reference Voltage
V
REF
IRU431L
V
REF(DEV)
V
REF
Deviation over full
temperature range
Ratio of V
REF
change to
DV
REF
/DV
KA
cathode voltage change
Reference Pin Current
I
REF
Deviation over full
I
REF(DEV)
temperature range
Minimum Cathode Current
I
K(MIN)
Off State Cathode Current
Ioff
Dynamic Impedance
Z
KA0
80
0.75
5
10
0.4
V
Note 1:
The dynamic impedance when V
KA
=V
REF
is de-
fined as:
?Z
KA0
?
=
DV
KA
DI
K
When the device is operating with two external resistors
(See Figure 3), the total dynamic impedance of the cir-
cuit is given by:
?Z
KA
?
=
DV
R1
=
?Z
KA0
? 3
1 +
DI
R2
(
)
2
www.irf.com
Rev. 2.0
08/05/03
IRU431L
The deviation parameters, V
REF(DEV)
and I
REF(DEV)
are de-
fined as the differences between the maximum and the
minimum values obtained over the rated temperature
range. The average full range temperature coefficient of
the reference input voltage is defined as:
V
(
V
?
=
REF(DEV)
REF(258C)
?aV
REF
)
3
10
6
Where:
?aV
REF
?
unit is ppm/8C
DT
A
is the rated operating free air temperature of the
device.
aV
REF
can be positive or negative depending on whether
minimum V
REF
or maximum V
REF
respectively occurs at
the lower temperature.
DT
A
PIN DESCRIPTIONS
5-PIN
4
3-PIN
1
PIN SYMBOL
Ref
PIN DESCRIPTION
Resistors from the Ref pin to the Cathode pin and to ground form a divider
that sets the output voltage.
The output of the shunt regulator. A capacitor of 1mF minimum value must
be connected from this pin to Anode pin to insure unconditional stability.
Ground pin. This pin must be connected to the lowest potential in the
system and all other pins must be at higher potential with respect to this
pin.
These pins are not connected internally.
3
2
Cathode
5
3
Anode
1, 2
NA
NC
BLOCK DIAGRAM
Cathode
Ref
+
1.24V
Anode
Figure 2 - Simplified block diagram of the IRU431L.
Rev. 2.0
08/05/03
www.irf.com
3
IRU431L
APPLICATION INFORMATION
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:
Vo = V
KA
= V
REF
3
1 +
An example is given below on how to properly select the
biasing resistor.
Assuming:
V
MIN
= 4.5V
V
MAX
= 6V
V
KA
= 3.3V
I
L
= 10mA
The maximum value for the biasing resistor is calcu-
lated using the following equations:
R
B(MAX)
=
V
MIN
- V
KA
I
B(MAX)
+ I
L(MAX)
(
R1
R2
)
+ I
REF
3
R1
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
REF
current and into the R1
resistor producing a voltage equal to:
(
1.240
)
3
R1 + I
R2
REF
3
R1
I
B(MAX)
= I
K(MIN)
+ I
R
Where:
V
MIN
= Minimum supply voltage
I
L(MAX)
= Maximum load current
I
B(MAX)
= Maximum bias current
I
K(MIN)
= Maximum value for the minimum
cathode current spec
I
R
= Current through R1
Assuming R1 = 2KV as before,
V
KA
= V
OUT
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 0.5mA 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=1.21KV and R1=2KV the
error due to the I
ADJ
is only 1mV which is about 0.03% of
the nominal set point.
V
IN
R
B
I
K
R1
I
L
Co
R2
R
L
I
R
=
3.3 - 1.24
= 1.03mA
2
IRU431L
I
B(MAX)
= 0.08 + 1.03 = 1.11mA
R
B(MAX)
=
4.5 - 3.3
= 108V
1.11 +10
Select R
B
= 100V
Figure 3 - Typical application of the IRU431L
for programming the output voltage.
Biasing Resistor (R
B
) Selection
The biasing resistor R
B
is selected such that it does not
limit the input current under the minimum input supply
and maximum load and biasing current.
The maximum power dissipation of the resistor is calcu-
lated under the maximum supply voltage as follows:
P
R (MAX)
B
(V
MAX
- V
KA
)
=
R
B
2
Where:
V
MAX
= Maximum supply voltage
P
R (MAX)
= Maximum R
B
power dissipation
B
P
R (MAX)
B
(6 - 3.3)
=
= 73mW
100
2
4
www.irf.com
Rev. 2.0
08/05/03
IRU431L
Thermal Design
The IRU431L is offered in the surface mount SOT-23 (L)
packages. The SOT-23 package has the maximum power
dissipation capability of 150mW at T
A
=25°C with the de-
rating factor of -1.2mW /
°C.
The table below summarizes the maximum power dissi-
pation capability of each package versus ambient tem-
perature.
Ambient Temperature (T
A
)
8
C
25
40
50
60
70
150mW
132mW
120mW
108mW
96mW
Pkg
SOT-23
Stability
The IRU431L has many different domains of stability as
a function of the cathode current. As is typical of three-
terminal shunt regulators, the IRU431L has many do-
mains of stability. The actual domain in which any prac-
tical circuit operates is related to cathode current. In
general the device will be unconditionally stable for any
cathode current if a capacitor, 1mF or larger, is connected
between the cathode and the anode. If the cathode cur-
rent is always higher than 3mA under minimum line and
maximum load conditions, the capacitor value can be
reduced to 0.01mF and the system will be stable.
In our previous example, the maximum power dissipa-
tion of the device is calculated under no load and maxi-
mum input supply condition.
The maximum power is calculated using the following
equation:
P
MAX
= V
KA
3
(
V
MAX
- V
KA
R
B
)
Where:
P
MAX
= Maximum power dissipation of the IRU431L
For our example:
P
MAX
= 3.3
3
- 3.3
(
6100
)
= 89mW
As shown in the power dissipation table, both packages
can handle this power dissipation.
Rev. 2.0
08/05/03
www.irf.com
5
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