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Data Sheet No. PD94136
IRU1117
800mA LOW DROPOUT POSITIVE
ADJUSTABLE REGULATOR
FEATURES
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
and 8-Pin SOIC Surface-Mount Packages
APPLICATIONS
VGA & Sound Card Applications
Low Voltage High Speed Termination Applications
Standard 3.3V Chip Set and Logic Applications
TYPICAL APPLICATION
D1
5V
C1
10uF
DESCRIPTION
The IRU1117 is a low dropout, three-terminal adjustable
regulator with minimum of 800mA output current capability. 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 applications 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.
IN
V
3
IRU1117
OUT
V
2
R1
121
Gnd
1
R2
154
Figure 1 - Typical application of IRU1117 in a 5V to 2.85V SCSI termination regulator.
C2
10uF
2.85V / 800mA
PACKAGE ORDER INFORMATION
TJ (°C) 2-PIN PLASTIC 3-PIN PLASTIC 2-PIN PLASTIC 8-PIN PLASTIC
TO-252 (D-Pak) SOT-223 (Y) Ultra Thin-Pak (P) SOIC (S)
0 To 150 IRU1117CD IRU1117CY IRU1117CP IRU1117CS
Rev. 1.5
09/13/01
1
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IRU1117
ABSOLUTE MAXIMUM RATINGS
Input Voltage (VIN) .................................................... 7V
Power Dissipation ..................................................... Internally Limited
Storage Temperature Range ...................................... -65°C To 150°C
Operating Junction Temperature Range ..................... 0°C To 150°C
PACKAGE INFORMATION
2-PIN PLASTIC TO-252 (D-Pak) 3-PIN PLASTIC SO-223 (Y) 2-PIN ULTRA THIN-PAK (P) 8-PIN PLASTIC SOIC (S)
Tab i s
OUT
V
FRONT VIEW
3
V
IN
Tab is
1
Adj
TOP VIEW
3
IN
V
2
V
V
OUT
OUT
1
Adj
θJA=70°C/W for 0.5" Sq pad θJA=90°C/W for 0.4" Sq pad θJA=70°C/W for 0.5" Sq pad θJA=55°C/W for 1" Sq pad
ELECTRICAL SPECIFICATIONS
Unless otherwise specified, these specifications apply over CIN=1µF, C OUT=10µF, and TJ=0 to 150!C.
Typical values refer to TJ=25!C.
Tab is
OUT
V
FRONT VIEW
3
V
IN
1
Adj
V
NC
NC
Adj
IN
TOP VIEW
1
2
3
4
8
V
OUT
7
V
OUT
6
V
OUT
5
V
OUT
PARAMETER SYM TEST CONDITION MIN TYP MAX UNITS
Reference Voltage VREF Io=10mA, TJ=25!C, (VIN-Vo)=1.5V 1.238 1.250 1.262 V
Io=10mA, (VIN-Vo)=1.5V 1.225 1.250 1.275
Line Regulation Io=10mA, 1.3V<(VIN-Vo)<7V 0.2 %
Load Regulation (Note 1) VIN=3.3V, VADJ=0, 10mA<Io<800mA 0.4 %
Dropout Voltage Io=1A 1.2 1.3 V
(Note 2) ∆Vo Io=800mA, Note 2 1.1 1.2
Current Limit VIN=3.3V, ∆Vo=100mV 1.1 A
Minimum Load Current VIN=3.3V, VADJ=0V 5 10 mA
(Note 3)
Thermal Regulation 30ms Pulse, VIN-Vo=3V, Io=800mA 0.01 0.02 %/W
Ripple Rejection f=120Hz, Co=25µF Tantalum
Io=0.5A, VIN-Vo=3V 60 70 dB
Adjust Pin Current IADJ Io=10mA, VIN-Vo=1.5V, TJ=25!C,
Io=10mA, VIN-Vo=1.5V 55 120 µA
Adjust Pin Current Change Io=10mA, VIN-Vo=1.5V, TJ=25!C 0.2 5 µA
Temperature Stability VIN=3.3V, VADJ=0V, Io=10mA 0.5 %
Long Term Stability TJ=125!C, 1000Hrs 0.3 1 %
RMS Output Noise TJ=25°C, 10Hz<f<10KHz 0.003 %Vo
Note 1: Low duty cycle pulse testing with Kelvin connections is required in order to maintain accurate data.
Note 3: Minimum load current is defined as the minimum current required at the output in order for the out-
put voltage to maintain regulation. Typically, the resistor
Note 2: Dropout voltage is defined as the minimum differential voltage between VIN and VOUT required to maintain regulation at VOUT. It is measured when the output
voltage drops 1% below its nominal value.
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.
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PIN DESCRIPTIONS
PIN # PIN SYMBOL PIN DESCRIPTION
1
Adj
A resistor divider from this pin to the VOUT pin and ground sets the output voltage.
IRU1117
2
3
VOUT
VIN
The output of the regulator. A minimum of 10µF 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 VOUT
in order for the device to regulate properly.
BLOCK DIAGRAM
V
3
IN
CURRENT
LIMIT
2 V
OUT
+
1.25V
+
THERMAL
SHUTDOWN
Figure 2 - Simplified block diagram of the IRU1117.
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 voltages within the range of 1.25 to 5.5V. This regulator
only needs 1.3V differential to maintain output regulation. Unlike the first generation of the three-terminal regulators such as LM117 that required 3V differential between the input and the regulated output.
The IRU1117 is specifically designed to meet the fast
current transient needs as well as providing an accurate
1 Adj
initial voltage, reducing the overall system cost with the
need for fewer output capacitors. This is a key requirement for today’s low voltage IC applications that typically 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 current step at the regulator. In addition, the output voltage
tolerances are sometimes tight and they include the transient response as part of the specification.
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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 = VREF ×
oo
o1 +
oo
pp
p + IADJ × R2
pp
R1
R2
Where:
VREF = 1.25V Typically
IADJ = 50µA Typically
R1 and R2 as shown in figure 3:
V
I
ADJ
OUT
V
= 50uA
REF
R2
R1
V
OUT
V
V
IN
IN
IRU1117
Adj
Figure 3 - Typical application of the IRU1117
for programming the output voltage.
The IRU1117 keeps a constant 1.25V between the output pin and the adjust pin. By placing a resistor R1 across
these two pins a constant current flows through R1, adding to the I
ADJ
current and into the R2 resistor producing
a voltage equal to the (1.25/R1)×R2 + IADJ×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 121Ω resistor so
that it automatically satisfies the minimum current requirement. Notice that since IADJ is typically in the range
of 50µA it only adds a small error to the output voltage
and should only be considered when a very precise output voltage setting is required. For example, in a typical
3.3V application where R1=121Ω and R2=200Ω the error 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
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)=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 regulator to the load as short as possible to minimize this effect.
PARASITIC LINE
RESISTANCE
P
V
V
V
IN
IN
OUT
R
IRU1117
Adj
R1
R2
R
L
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 applications use standard electrolytic capacitors with a
typical ESR in the range of 50 to 100mΩ and an output
capacitance of 500 to 1000µF. Fortunately as the capacitance 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 100µF aluminum electrolytic capacitor such as Sanyo MVGX series, 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 temperatures in the range of 150!C, it is recommended that the
heat sink be selected such that during maximum continuous load operation the junction temperature is kept
below this number. The example below for a SCSI terminator application shows the steps in selecting the proper
regulator in a surface-mount package. (See IRU1015 for
non-surface-mount packages)
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IRU1117
Assuming the following specifications:
VIN = 5V
VF = 0.5V
VOUT = 2.85V
IOUT(MAX) = 0.8A
TA = 35!C
Where:
VF is the forward voltage drop of the D1 diode as
shown in Figure 5.
+5V is significantly more than required to maintain dropout voltage across the IRU1117 when regulating VOUT 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 temperature below 135!C is given as:
1) Calculate the maximum power dissipation using:
PD = IOUT × (VIN - VF - VOUT)
PD = 0.8 × (5 - 0.5 - 2.85) = 1.32W
2) Calculate the maximum θ
JA allowed for our ex-
ample:
PD
135 - 35
1.32
θJA(MAX) = = = 75.6!C/W
TJ - TA
3) Select a package from the data sheet with lower θJA
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.
To set the output DC voltage, we need to select R1 and
R2:
4) Assuming R1 = 121Ω, 1%:
R2 =
VOUT
oo
o - 1
oo
VREF
pp
p× R
pp
2.85
oo
1 =
o - 1
oo
1.25
pp
p× 121 = 154.8Ω
pp
Select R2 = 154Ω, 1%
5) Calculate the power dissapation for the schottky diode (D1):
PD = VF × IOUT
PD = 0.5 × 0.8 = 0.4W
and select a suitable component.
D1
V
5V
C1
10uF
V
IN
IRU1117
Adj
OUT
R1
121
1%
R2
154
1%
2.85V
C2
22uF
Figure 5 - Final Schematic for half of the
GTL+ termination regulator.
Layout Consideration
The output capacitors must be located as close to the
VOUT terminal of the device as possible. It is recommended to use a section of a layer of the PC board as a
plane to connect the VOUT pin to the output capacitors to
prevent any high frequency oscillation that may result
due to excessive trace inductance.
Rev. 1.5
09/13/01
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IRU1117
Notes
6
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
Rev. 1.5
09/13/01
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