IRU1117 Просмотр технического описания (PDF) - International Rectifier
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IRU1117 Datasheet PDF : 10 Pages
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IRU1117
Output Voltage Setting
The IRU1117 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:
( ) VOUT
=
VREF3
1+
R2
R1
+IADJ3R2
Where:
VREF = 1.25V Typically
IADJ = 50mA Typically
R1 and R2 as shown in Figure 3:
to the load side, the effective resistance between the
regulator and the load is multiplied 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 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.
VIN
VIN
VOUT
PARASITIC LINE
RESISTANCE
RP
IRU1117
VI N
V IN
V OUT
IRU1117
VO U T
Adj
R1
RL
Adj
R2
VREF R 1
IADJ = 50uA
R2
Figure 3 - Typical application of the IRU1117
for programming the output voltage.
The IRU1117 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 IADJ current and into the R2 resistor producing
a voltage equal to the (1.25/R1)3R2 + IADJ3R2 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 IRU1117 is
10mA, R1 is typically selected to be 121V resistor so
that it automatically satisfies the minimum current re-
quirement. Notice that since IADJ is typically in the range
of 50mA it only adds a small error to the output voltage
and should only be considered when a very precise out-
put voltage setting is required. For example, in a typical
3.3V application where R1=121V and R2=200V the er-
ror due to IADJ is only 0.3% of the nominal set point.
Load Regulation
Since the IRU1117 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
regulator and not to the load. In fact, if R1 is connected
Figure 4 - Schematic showing connection
for best load regulation.
Stability
The IRU1117 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 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 IRU1117 takes advantage
of this phenomenon in making the overall regulator loop
stable. For most applications a minimum of 100mF 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 IRU1117 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 1508C, it is recommended that the
heat sink be selected such that during maximum con-
tinuous load operation the junction temperature is kept
below this number. The example below for a SCSI termi-
nator application shows the steps in selecting the proper
regulator in a surface-mount package. (See IRU1015 for
non-surface-mount packages)
4
www.irf.com
Rev. 1.8
02/04/03
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