IRU1260CM Ver la hoja de datos (PDF) - International Rectifier
Número de pieza
componentes Descripción
Fabricante
IRU1260CM Datasheet PDF : 10 Pages
| |||
IRU1260
APPLICATION INFORMATION
Introduction
The IRU1260 is a dual adjustable Low Dropout (LDO)
regulator packaged in a 7-pin TO-220 which can easily
be programmed with the addition of two external resis-
tors to any voltages within the range of 1.20 to 5.5V.
This voltage regulator is designed specifically for appli-
cations that require two separate regulators such as the
Intel Pentium II processors requiring 1.5V and 2.5V
supplies, eliminating the need for a second regulator
which results in lower overall system cost. When Vctrl
pin is connected to a supply which is at least 1V higher
than Vin, the dropout voltage improves by 30% which
makes it ideal for applications requiring less than the
standard 1.3V dropout given in the LDO products such
as IRU10XX series. The IRU1260 also provides an accu-
rate 1.20V voltage reference 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 family of microprocessors. The IRU1260 also
includes thermal shutdown protection to protect the de-
vice if an overload condition 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:
VOUT = VREF × o1 + RR21p + R2 × IB
Where:
VREF = 1.20V Typically
IB = 0.02µA Typical
R1 and R2 as shown in Figure 3:
Vin
Vctrl
Vin
Vout
IRU1260
R2
Ib
Vctrl
Vfb
Gnd
Vref R1
Vout
The IRU1260 keeps a constant 1.20V 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)*R2 + IFB* R2 which will be
added to the 1.20V to set the output voltage as shown
in the above equation. Since the input bias current of the
amplifier (IFB) is only 0.02µA 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=10.2KΩ and R2=2.55KΩ 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 1.8KΩ
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)=Rp*(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.
Vin
Vin
Vout
IRU1260
Vctrl
Vctrl
R2
Vfb
Gnd
RL
R1
1260app2-1.0
Figure 3 - Typical application of the IRU1260 for
programming the output voltage.
(Only one output is shown here)
Rp
1260app3-1.1
PARASITIC LINE
RESISTANCE
Figure 4 - Schematic showing connection for best load
regulation. (Only one output is shown here)
4
Rev. 1.9
07/03/01
Share Link: 