IRU1010 International Rectifier, IRU1010 Datasheet - Page 4

• Manufacturer Part Number: IRU1010
• Description: 1A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR
• Manufacturer: International Rectifier

IRU1010
Output Voltage Setting
The IRU1010 can be programmed to any voltages in the
range of 1.25V to 5.5V with the addition of R1 and R2
external resistors according to the following formula:
The IRU1010 keeps a constant 1.25V between the out-
put pin and the adjust pin. By placing a resistor R1 across
these two pins a constant current flows through R1, add-
ing to the I adj current and into the R2 resistor producing
a voltage equal to the (1.25/R1)*R2 + Iadj*R2 which will
be 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 IRU1010 is 10mA, R1 is
typically selected to be 121Ω resistor so that it auto-
matically satisfies the minimum current requirement.
Notice that since Iadj is typically in the range of 50µA it
only adds a small error to the output voltage and should
only be considered when a very precise output voltage
setting is required. For example, in a typical 3.3V appli-
cation where R1=121Ω and R2=200Ω the error due to
Iadj is only 0.3% of the nominal set point.
Load Regulation
Since the IRU1010 is only a three-terminal device, it is
not possible to provide true remote sensing of the output
voltage at the load. Figure 4 shows that the best load
regulation is achieved when the bottom side of R2 is
connected to the load and the top side of R1 resistor is
connected directly to the case or the Vout pin of the
4
Vin
V
Where:
V
I
R
ADJ
Figure 3 - Typical application of the IRU1010
OUT
REF
1
and R
= 50µA Typically
= 1.25V Typically
= V
for programming the output voltage
REF
2
as shown in figure 3:
Vin
× o o o o o 1 +
IRU1010
Adj
R
R
2
1
p p p p p + I
IAdj = 50uA
Vout
ADJ
Vref
× R
2
R1
R2
1010app2-1.0
Vout
regulator and not to the load. In fact, if R1 is connected
to the load side, the effective resistance between the
regulator and the load is gained up by the factor of (1+R2/
R1), or the effective resistance will be, Rp(eff)=Rp*(1+R2/
R1). It is important to note that for high current applica-
tions, this can represent a significant percentage of the
overall load regulation and one must keep the path from
the regulator to the load as short as possible to mini-
mize this effect.
Stability
The IRU1010 requires the use of an output capacitor as
part of the frequency compensation in order to make the
regulator stable. Typical designs for microprocessor ap-
plications use standard electrolytic capacitors with a
typical ESR in the range of 50 to 100mΩ and an output
capacitance of 500 to 1000µF. Fortunately as the ca-
pacitance increases, the ESR decreases resulting in a
fixed RC time constant. The IRU1010 takes advantage
of this phenomena in making the overall regulator loop
stable. For most applications a minimum of 100µF alu-
minum electrolytic capacitor such as Sanyo MVGX se-
ries, Panasonic FA series as well as the Nichicon PL
series insures both stability and good transient response.
Thermal Design
The IRU1010 incorporates an internal thermal shutdown
that protects the device when the junction temperature
exceeds the maximum allowable junction temperature.
Although this device can operate with junction tempera-
tures in the range of 150!C, 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 for
a SCSI terminator application shows the steps in sellect-
Vin
Figure 4 - Schematic showing connection
Vin
IRU1010
for best load regulation
Adj
Vout
PARASITIC LINE
RESISTANCE
Rp
R1
R2
1010app3-1.0
06/29/01
Rev. 1.4
R L