IRU1260
4
Rev. 2.1
09/19/02
www.irf.com
Introduction
The IRU1260 is a dual adjustable Low Dropout (LDO)
regulator which can easily be programmed with the ad-
dition of two external resistors to any voltages within
the range of 1.20 to 5.5V. This voltage regulator is de-
signed specifically for applications that require two sepa-
rate regulators such as the Intel Pentium II
TM
proces-
sors requiring 1.5V and 2.5V supplies, eliminating the
need for a second regulator which results in lower over-
all system cost. When VCTRL pin is connected to a sup-
ply which is at least 1V higher than VIN, the dropout
voltage improves by 30% which makes it ideal for appli-
cations requiring less than the standard 1.3V dropout
given in the LDO products such as IRU10XX series. The
IRU1260 also provides an accurate 1.20V voltage refer-
ence common to both regulators for programming each
output voltage. Other features of the device include: fast
response to sudden load current changes, such as GTL+
termination application for Pentium II
TM
family of micro-
processors. The IRU1260 also includes thermal shut-
down protection to protect the device if an overload con-
dition occurs.
Output Voltage Setting
The IRU1260 can be programmed to any voltages in the
range of 1.20V to 5.5V with the addition of R1 and R2
external resistors according to the following formula:
Figure 3 - Typical application of the IRU1260
for programming the output voltage.
(Only one output is shown here)
V
OUT
R2
R1
V
IN
V
CTRL
VREF
IB
IRU1260
Gnd
VOUT
VCTRL
VIN
VFB
R2
R1
V
IN
V
CTRL
RL
RP
PARASITIC LINE
RESISTANCE
IRU1260
Gnd
VOUT
V CTRL
V IN
VFB
Where:
VREF = 1.20V Typically
IB = 0.02mA Typical
R1 and R2 as shown in Figure 3:
VOUT = VREF3 +R23IB
R2
R1
( )
1+
The IRU1260 keeps a constant 1.2V between the VFB
pin and ground pin. By placing a resistor R1 across these
two pins a constant current flows through R1, adding to
the IFB current and into the R2 resistor producing a volt-
age equal to the (1.2/R1)3R2 + IFB3R2 which will be
added to the 1.2V to set the output voltage as shown in
the above equation. Since the input bias current of the
amplifier (IFB) is only 0.02mA typically, it adds a very
small error to the output voltage and for most applica-
tions can be ignored. For example, in a typical 1.5V
GTL+application if R1=10.2KV and R2=2.55KV the er-
ror due to the IADJ is only 0.05mV which is less than
0.004% of the nominal set point. The effective input im-
pedance seen by the feedback pins (The parallel combi-
nation of R1 and R2) must always be higher than 1.8KV
in order for the regulator to start up properly.
Load Regulation
Since the IRU1260 does not provide a separate ground
pin for the reference voltage, 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 R1 resistor is connected di-
rectly to the ground pin of IRU1260 (preferably to the tab
of the device) and the top side of R2 resistor is con-
nected 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)=RP3(1+R2/R1). It is
important to note that for high current applications, this
can represent a significant percentage of the overall load
regulation and one must keep the path from the regula-
tor to the load as short as possible to minimize this
effect.
Figure 4 - Schematic showing connection
for best load regulation.
(Only one output is shown here)
APPLICATION INFORMATION
