iru1150cs International Rectifier Corp., iru1150cs Datasheet - Page 5

• Manufacturer Part Number: iru1150cs
• Description: 4a Ultra Low Dropout Positive Adjustable Regulator -
• Manufacturer: International Rectifier Corp.

The IRU1150 keeps a constant 1.25V between the V
pin and the V
these two pins and connecting the V
together, a constant current flows through R1, adding to
the Iadj current and into the R2 resistor producing a volt-
age equal to the (1.25/R1)3R2 + I
is then added to the 1.25V to set the output voltage.
This is summarized in the above equation. Since the
minimum load current requirement of the IRU1150 is
10mA, R1 is typically selected to be a 121V resistor so
that it automatically satisfies this condition. Notice that
since the I
adds a small error to the output voltage and should be
considered when very precise output voltage setting is
required.
Load Regulation
Since the IRU1150 has separate pins for the output (V
and the sense (V
mote sensing of the output voltage at the load. This
means that the voltage drops due to parasitic resistance
such as PCB traces between the regulator and the load
are compensated for using remote sensing. Figure 4
shows a typical application of the IRU1150 with remote
sensing.
Stability
The IRU1150 requires the use of an output capacitor as
part of the frequency compensation in order to make the
regulator stable. Typical designs for the microprocessor
applications use standard electrolytic capacitors with
typical ESR in the range of 50 to 100mV and an output
capacitance of 500 to 1000mF. Fortunately as the ca-
pacitance increases, the ESR decreases resulting in a
fixed RC time constant. The IRU1150 takes advantage
of this phenomena in making the overall regulator loop
stable.
Rev. 1.6
03/18/02
V
C T R L
V
I N
Figure 4 - Schematic showing connection
ADJ
Vin
V
CTRL
ADJ
is typically in the range of 50mA it only
for best load regulation.
IRU1150
pin. By placing a resistor R1 across
SENSE
Adj
), it is ideal for providing true re-
V
SENSE
V
OUT
ADJ
3R2. This voltage
SENSE
R 1
R 2
and V
OUT
www.irf.com
SENSE
OUT
R
pin
L
)
For most applications a minimum of 100mF aluminum
electrolytic capacitor such as Sanyo, MVGX series,
Panasonic FA series as well as the Nichicon PL series
insures both stability and good transient response.
Thermal Design
The IRU1150 incorporates an internal thermal shutdown
that protects the device when the junction temperature
exceeds the allowable maximum junction temperature.
Although this device can operate with junction tempera-
tures in the range of 1508C, it is recommended that the
selected heat sink be chosen such that during maxi-
mum continuous load operation the junction tempera-
ture is kept below this number. The example below
shows the steps in selecting the proper surface mount
package.
Assuming, the following conditions:
Calculate the maximum power dissipation using the fol-
lowing equation:
Using table below select the proper package and the
amount of copper board needed.
Pkg
TO-263 1.4"X1.4"
TO-263 1.0"X1.0"
TO-263 0.7"X0.7"
TO-263 Pad Size
SO-8
Note: Above table is based on the maximum junction
temperature of 1358C.
As shown in the above table, any of the two packages
will do the job. For low cost applications the SO-8 pack-
age is recommended.
V
V
V
I
P
P
OUT
OUT
IN
CTRL
D
D
= 2 3 (3.3 - 2.7) +o o p p 3 (5 - 2.7) = 1.28W
= I
= 3.3V
= 2A (DC Avg)
Copper
1.0"X1.0"
= 2.7V
Area
= 5V
OUT
3 (V
IN
JA
- V
( C/W)
25
30
35
45
55
OUT
) +o o
60
2
I
OUT
60
(
Max Pd
T
4.4W
3.7W
3.1W
2.4W
2.0W
A
p p 3 (V
=25 C
IRU1150
)
CTRL
Max Pd
(
T
- V
A
3.6W
3.0W
2.6W
2.0W
1.63W
=45 C
OUT
)
)
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