4
IRU1150(PbF)
www.irf.com
APPLICATION INFORMATION
Introduction
The IRU1150 adjustable regulator is a five-terminal de-
vice designed specifically to provide extremely low drop-
out voltages comparable to the PNP type without the
disadvantage of the extra power dissipation due to the
base current associated with PNP regulators. This is
done by bringing out the control pin of the regulator that
provides the base current to the power NPN and con-
necting it to a voltage that is grater than the voltage present
at the VIN pin. This flexibility makes the IRU1150 ideal
for applications where dual inputs are available such as
a computer mother board with an ATX style power sup-
ply that provides 5V and 3.3V to the board. One such
application is the new graphic chip sets that require any-
where from 2.4V to 2.7V supply such as the Intel I740
chip set. The IRU1150 can easily be programmed with
the addition of two external resistors to any voltages
within the range of 1.25 to 5.5 V. Another major require-
ment of these graphic chips such as the Intel I740 is the
need to switch the load current from zero to several amps
in tens of nanoseconds at the processor pins, which
translates to an approximately 300 to 500ns of current
step at the regulator. In addition, the output voltage tol-
erances are also extremely tight and they include the
transient response as part of the specification.
The IRU1150 is specifically designed to meet the fast
current transient needs as well as providing an accurate
initial voltage, reducing the overall system cost with the
need for fewer number of output capacitors. Another fea-
ture of the device is its true remote sensing capability
which allows accurate voltage setting at the load rather
than at the device.
Output Voltage Setting
The IRU1150 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:
V
OUT
R1
R2
V
IN
V
CTRL
V
REF
I
ADJ
= 50uA
IRU1150
V
SENSE
Adj
V
OUT
V
CTRL
V
IN
R1
R2
V
IN
V
CTRL
R
L
IRU1150
V
SENSE
Adj
V
OUT
V
CTRL
Vin
Figure 3 - Typical application of the IRU1150
for programming the output voltage.
The IRU1150 keeps a constant 1.25V between the VSENSE
pin and the VADJ pin. By placing a resistor R1 across
these two pins and connecting the VSENSE and VOUT pin
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)×R2 + IADJ×R2. This voltage
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 121Ω resistor so
that it automatically satisfies this condition. Notice that
since the IADJ is typically in the range of 50µA it only
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 (VOUT)
and the sense (VSENSE), it is ideal for providing true re-
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.
Figure 4 - Schematic showing connection
for best load regulation.
VOUT = VREF× 1+
+IADJ×R2
R2
R1
( )
Where:
VREF = 1.25V Typically
IADJ = 50µA Typically
R1 and R2 as shown in Figure 3:
