IRU1117
1
Rev. 1.8
02/04/03
www.irf.com
TYPICAL APPLICATION
DESCRIPTION
The IRU1117 is a low dropout, three-terminal adjustable
regulator with minimum of 800mA output current capa-
bility. This product is specifically designed to provide
well regulated supply for low voltage IC applications such
as high speed bus termination and low current 3.3V logic
supply. The IRU1117 is also well suited for other applica-
tions such as VGA and sound cards. The IRU1117 is
guaranteed to have <1.2V dropout at full load current
making it ideal to provide well regulated outputs of 2.5V
to 3.6V with 4.75V to 7V input supply.
800mA LOW DROPOUT POSITIVE
ADJUSTABLE REGULATOR
Figure 1 - Typical application of IRU1117 in a 5V to 2.85V SCSI termination regulator.
T
J
(C) 2-PIN PLASTIC 2-PIN PLASTIC 8-PIN PLASTIC 3-PIN PLASTIC
TO-252 (D-Pak) Ultra Thin-Pak
TM
(P) SOIC (S) SOT-223 (Y)
0 To 150 IRU1117CD IRU1117CP IRU1117CS IRU1117CY
Data Sheet No. PD94136
PACKAGE ORDER INFORMATION
VGA & Sound Card Applications
Low Voltage High Speed Termination Applications
Standard 3.3V Chip Set and Logic Applications
FEATURES
APPLICATIONS
Guaranteed < 1.2V Dropout at 800mA Load
Current
Fast Transient Response
1% Voltage Reference Initial Accuracy
Built-In Thermal Shutdown
Available in SOT-223, D-Pak, Ultra Thin-Pak
TM
and 8-Pin SOIC Surface-Mount Packages
5V
2.85V / 800mA
C1
10uF
C2
10uF
IRU1117
3
1
2
V
I N
V
O U T
Adj
D1
R1
121
R2
154
IRU1117
2
Rev. 1.8
02/04/03
www.irf.com
ABSOLUTE MAXIMUM RATINGS
Input Voltage (V
IN
) .................................................... 7V
Power Dissipation ..................................................... Internally Limited
Storage Temperature Range ...................................... -65C To 150C
Operating Junction Temperature Range ..................... 0C To 150C
PACKAGE INFORMATION
2-PIN PLASTIC TO-252 (D-Pak) 2-PIN ULTRA THIN-PAK
TM
(P) 8-PIN PLASTIC SOIC (S) 3-PIN PLASTIC SOT-223 (Y)
ELECTRICAL SPECIFICATIONS
Unless otherwise specified, these specifications apply over C
IN
=1
m
F, C
OUT
=10
m
F, and T
J
=0 to 150
8
C.
Typical values refer to T
J
=25
8
C.
Note 1: Low duty cycle pulse testing with Kelvin con-
nections is required in order to maintain accurate data.
Note 2: Dropout voltage is defined as the minimum dif-
ferential voltage between V
IN
and V
OUT
required to main-
tain regulation at V
OUT
. It is measured when the output
voltage drops 1% below its nominal value.
Note 3: Minimum load current is defined as the mini-
mum current required at the output in order for the out-
put voltage to maintain regulation. Typically, the resistor
dividers are selected such that it automatically main-
tains this current. Typically, the values of the resistors
used to build the voltage divider are selected to ensure
that minimum load current is maintained.
Adj
1
3
FRONT VIEW
Tab is
V
OUT
V
IN
Adj
V
OUT
V
IN
Tab is
V
OUT
3
1
2
TOP VIEW
Adj
FRONT VIEW
1
3
Tab is
V
OUT
V
IN
Adj
NC
NC
V
IN
V
OUT
4
3
2
1
5
6
7
8
TOP VIEW
V
OUT
V
OUT
V
OUT
Reference Voltage
Line Regulation
Load Regulation (Note 1)
Dropout Voltage (Note 2)
Current Limit
Minimum Load Current (Note 3)
Thermal Regulation
Ripple Rejection
Adjust Pin Current
Adjust Pin Current Change
Temperature Stability
Long Term Stability
RMS Output Noise
PARAMETER
SYM TEST CONDITION MIN TYP MAX UNITS
Io=10mA, T
J
=25
8
C, (V
IN
-Vo)=1.5V
Io=10mA, (V
IN
-Vo)=1.5V
Io=10mA, 1.3V<(V
IN
-Vo)<7V
V
IN
=3.3V, V
ADJ
=0, 10mA<Io<800mA
Io=1A
Io=800mA, Note 2
V
IN
=3.3V,
D
Vo=100mV
V
IN
=3.3V, V
ADJ
=0V
30ms Pulse, V
IN
-Vo=3V, Io=800mA
f=120Hz, Co=25
m
F Tantalum,
Io=0.5A, V
IN
-Vo=3V
Io=10mA, V
IN
-Vo=1.5V, T
J
=25
8
C,
Io=10mA, V
IN
-Vo=1.5V
Io=10mA, V
IN
-Vo=1.5V, T
J
=25
8
C
V
IN
=3.3V, V
ADJ
=0V, Io=10mA
T
J
=125
8
C, 1000Hrs
T
J
=25
C, 10Hz<f<10KHz
1.238
1.225
1.1
60
1.250
1.250
1.2
1.1
5
0.01
70
55
0.2
0.5
0.3
0.003
1.262
1.275
0.2
0.4
1.3
1.2
10
0.02
120
5
1
V
%
%
V
A
mA
%/W
dB
m
A
m
A
%
%
%V
O
V
REF
D
V
O
I
ADJ
JA
=70
C/W for 0.5" Sq pad
JA
=70
C/W for 0.5" Sq pad
JA
=55
C/W for 1" Sq pad
JA
=90
C/W for 0.4" Sq pad
IRU1117
3
Rev. 1.8
02/04/03
www.irf.com
APPLICATION INFORMATION
Introduction
The IRU1117 adjustable Low Dropout (LDO) regulator is
a three-terminal device which can easily be programmed
with the addition of two external resistors to any volt-
ages within the range of 1.25 to 5.5V. This regulator
only needs 1.3V differential to maintain output regula-
tion. Unlike the first generation of the three-terminal regu-
lators such as LM117 that required 3V differential be-
tween the input and the regulated output.
The IRU1117 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 output capacitors. This is a key require-
ment for today's low voltage IC applications that typi-
cally need 3.3V supply and are often generated from the
5V supply. Other applications such as high speed
memory termination need to switch the load current from
zero to full load in tens of nanoseconds at their pins,
which translates to an approximately 300 to 500ns cur-
rent step at the regulator. In addition, the output voltage
tolerances are sometimes tight and they include the tran-
sient response as part of the specification.
BLOCK DIAGRAM
Figure 2 - Simplified block diagram of the IRU1117.
PIN DESCRIPTIONS
PIN # PIN SYMBOL PIN DESCRIPTION
A resistor divider from this pin to the V
OUT
pin and ground sets the output voltage.
The output of the regulator. A minimum of 10
m
F capacitor must be connected from this pin
to ground to insure stability.
The input pin of the regulator. Typically a large storage capacitor is connected from this
pin to ground to insure that the input voltage does not sag below the minimum dropout
voltage during the load transient response. This pin must always be 1.3V higher than V
OUT
in order for the device to regulate properly.
1
2
3
Adj
V
OUT
V
IN
V
IN
3
1 Adj
2 V
OUT
THERMAL
SHUTDOWN
CURRENT
LIMIT
1.25V
+
+
IRU1117
4
Rev. 1.8
02/04/03
www.irf.com
Output Voltage Setting
The IRU1117
ca
n 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:
Where:
VREF = 1.25V Typically
I
ADJ
= 50
m
A Typically
R1 and R2 as shown in Figure 3:
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 I
ADJ
current and into the R2 resistor producing
a voltage equal to the (1.25/R1)
3
R2 + I
ADJ
3
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 IRU1117 is
10mA, R1 is typically selected to be 121
V
resistor so
that it automatically satisfies the minimum current re-
quirement. Notice that since I
ADJ
is typically in the range
of 50
m
A 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=121
V
and R2=200
V
the er-
ror due to I
ADJ
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 V
OUT
pin of the
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 multiplied by the factor of (1+R2/
R1), or the effective resistance will be R
P(eff)
=R
P
3
(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.
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 100m
V
and an output
capacitance of 500 to 1000
m
F. 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 100
m
F 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 150
8
C, 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)
V
O U T
R 1
R 2
V
I N
V
REF
I
ADJ
= 50uA
IRU1117
Adj
V
OUT
V
IN
R1
R2
V
IN
R
L
R
P
PARASITIC LINE
RESISTANCE
IRU1117
Adj
V
OUT
V
IN
V
OUT
= V
REF
3
1+
+I
ADJ
3
R2
R2
R1
(
)
IRU1117
5
Rev. 1.8
02/04/03
www.irf.com
To set the output DC voltage, we need to select R1 and
R2:
4) Assuming R1 = 121
V
, 1%:
Select R2 = 154
V
, 1%
5) Calculate the power dissapation for the schottky di-
ode (D1):
and select a suitable component.
Figure 5 - Final Schematic for half of the
GTL+ termination regulator.
Layout Consideration
The output capacitors must be located as close to the
V
OUT
terminal of the device as possible. It is recom-
mended to use a section of a layer of the PC board as a
plane to connect the V
OUT
pin to the output capacitors to
prevent any high frequency oscillation that may result
due to excessive trace inductance.
V
IN
= 5V
V
F
= 0.5V
V
OUT
= 2.85V
I
OUT(MAX)
= 0.8A
T
A
= 35
8
C
P
D
= I
OUT
3
(V
IN
- V
F
- V
OUT
)
P
D
= 0.8
3
(5 - 0.5 - 2.85) = 1.32W
u
JA(MAX)
= = = 75.6
8
C/W
T
J
- T
A
P
D
135 - 35
1.32
P
D =
V
F
3
I
OUT
P
D =
0.5
3
0.8 = 0.4W
Assuming the following specifications:
Where:
V
F
is the forward voltage drop of the D1 diode as
shown in Figure 5.
+5V is significantly more than required to maintain drop-
out voltage across the IRU1117 when regulating V
OUT
to
2.85V. The "Extra" voltage means additional power is
dissapated in the IRU1117. The diode dissapates some
of this additional power, allowing the IRU1117 to run cooler.
The steps for selecting the right package with proper
board area for heat sinking to keep the junction tem-
perature below 135
8
C is given as:
1) Calculate the maximum power dissipation using:
2) Calculate the maximum
u
JA
allowed for our ex-
ample:
3) Select a package from the data sheet with lower
u
JA
than the one calculated in the previous step.
Selecting TO-252 (D-Pak) with at least 0.5" square
of 0.062" FR4 board using 1oz. copper has 70
C/W
which is lower than the calculated number.
R2 =
3
R1 =
3
121 = 154.8
V
V
OUT
V
REF
-1
(
)
2.85
1.25
-1
(
)
IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: (310) 252-7105
TAC Fax: (310) 252-7903
Visit us at www.irf.com for sales contact information
Data and specifications subject to change without notice. 02/01
2.85V
R1
121
1%
R2
154
1%
5V
C2
22uF
C1
10uF
IRU1117
Adj
V
OUT
V
IN
D1
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