Data Sheet No. PD94128
IRU1075
7.5A LOW DROPOUT POSITIVE
ADJUSTABLE REGULATOR
DESCRIPTION
The IRU1075 is a low dropout three-terminal adjustable
regulator with minimum of 7.5A output current capabil-
ity. This product is specifically designed to provide well
regulated supply for low voltage IC applications such as
Pentium™ P54C™ , P55C™ as well as GTL+ termina-
tion for Pentium Pro™ and Klamath™ processor appli-
cations. The IRU1075 is also well suited for other pro-
cessors such as Cyrix™ , AMD and Power PC™ appli-
cations. The IRU1075 is guaranteed to have <1.2V drop-
out at full load current making it ideal to provide well
regulated outputs such as 3.3V with input supply volt-
age as low as 4.5V minimum.
FEATURES
1V Dropout at Full Load Current
Fast Transient Response
1% Voltage Reference Initial Accuracy
Output Current Limiting
Built-In Thermal Shutdown
APPLICATIONS
Low Voltage Processor Applications such as:
P54C™ ,P55C™ , Cyrix M2™,
POWER PC™ , AMD
GTL+ Termination
PENTIUM PRO™ , KLAMATH™
Low Voltage Memory Termination Applications
Standard 3.3V Chip Set and Logic Applications
TYPICAL APPLICATION
5V
C1
1500uF
V
IN
3
IRU1075
V
OUT
2
R1
121
3.3V
C2
2x 1500uF
Adj 1
R2
200
Typical application of IRU1075 in a 5V to 3.3V regulator.
Notes:
Pentium P54C, P55C, Klamath, Pentium Pro, VRE are trademarks of Intel Corp. Cyrix M2 is trademark of Cyrix Corp.
Power PC is trademark of IBM Corp.
PACKAGE ORDER INFORMATION
T
J
(°C)
0 To 150
3-PIN PLASTIC
TO-220 (T)
IRU1075CT
3-PIN PLASTIC
TO-263 (M)
IRU1075CM
3-PIN PLASTIC
Ultra Thin-Pak
TM
(P)
IRU1075CP
Rev. 1.2
08/01/02
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1
IRU1075
ABSOLUTE MAXIMUM RATINGS
Input Voltage (V
IN
) ....................................................
Power Dissipation .....................................................
Storage Temperature Range ......................................
Operating Junction Temperature Range .....................
7V
Internally Limited
-65°C To 150°C
0°C To 150°C
PACKAGE INFORMATION
3-PIN PLASTIC TO-220 (T)
FRONT VIEW
3-PIN PLASTIC TO-263 (M)
FRONT VIEW
3-PIN PLASTIC ULTRA THIN-PAK
TM
(P)
FRONT VIEW
3
Tab is
V
OUT
3
V
IN
Tab is
V
OUT
3
V
IN
Tab is
V
OUT
V
IN
2
V
OUT
2
V
OUT
2
V
OUT
1
Adj
1
Adj
1
Adj
θ
JT
=2.7°C/W
θ
JA
=60°C/W
θ
JA
=35°C/W for 1" Square pad
θ
JA
=35°C/W for 1" Square pad
ELECTRICAL SPECIFICATIONS
Unless otherwise specified, these specifications apply over C
IN
=1mF, C
OUT
=10mF, and T
J
=0 to 1508C.
Typical values refer to T
J
=258C.
PARAMETER
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
I
ADJ
SYM
V
REF
TEST CONDITION
Io=10mA, T
J
=258C, (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<7.5A
Io=7.5A
Io=4A
V
IN
=3.3V,
DVo=100mV
V
IN
=3.3V, V
ADJ
=0V
30ms Pulse, V
IN
-Vo=3V, Io=7.5A
f=120Hz, Co=25mF Tantalum,
Io=7.5A, V
IN
-Vo=3V
Io=10mA, V
IN
-Vo=1.5V, T
J
=258C,
Io=10mA, V
IN
-Vo=1.5V
Io=10mA, V
IN
-Vo=1.5V, T
J
=258C
V
IN
=3.3V, V
ADJ
=0V, Io=10mA
T
J
=1258C, 1000Hrs
T
J
=258C, 10Hz<f<10KHz
MIN
1.238
1.225
TYP
1.250
1.250
0.4
1.0
0.92
9
5
0.02
70
55
0.2
0.5
0.3
0.003
120
5
MAX UNITS
1.262
V
1.275
0.2
%
%
1.2
V
1.1
A
10
mA
%/W
dB
mA
mA
%
%
%V
O
DV
O
7.6
60
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.
2
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Rev. 1.2
08/01/02
IRU1075
PIN DESCRIPTIONS
PIN # PIN SYMBOL
1
Adj
2
V
OUT
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 10mF 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 drop out
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.
3
V
IN
BLOCK DIAGRAM
V
IN
3
2 V
OUT
+
1.25V
CURRENT
LIMIT
+
THERMAL
SHUTDOWN
1 Adj
Figure 2 - Simplified block diagram of the IRU1075.
APPLICATION INFORMATION
Introduction
The IRU1075 adjustable Low Dropout (LDO) regulator
is a three-terminal device which can easily be pro-
grammed with the addition of two external resistors to
any voltages within the range of 1.25 to 5.5 V. This regu-
lator 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, only needs
1.3V differential to maintain output regulation. This is a
key requirement for today’s microprocessors that need
typically 3.3V supply and are often generated from the
5V supply. Another major requirement of these micro-
processors such as the Intel P54C™ is the need to switch
the load current from zero to several amps in tens of
Rev. 1.2
08/01/02
nanoseconds at the processor pins, which translates to
an approximately 300 to 500ns current step at the regu-
lator. In addition, the output voltage tolerances are also
extremely tight and they include the transient response
as part of the specification. For example Intel VRE™
specification calls for a total of
±100mV
including initial
tolerance, load regulation and 0 to 4.6A load step.
The IRU1075 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.
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3
IRU1075
Output Voltage Setting
The IRU1075 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:
V
OUT
= V
REF
3
1+
regulator and the load is gained up 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.
PARASITIC LINE
RESISTANCE
(
R2
R1
)
+I
ADJ
3R2
Where:
V
REF
= 1.25V Typically
I
ADJ
= 50mA Typically
R1 and R2 as shown in Figure 3:
V
I N
V
O U T
R
P
V
IN
V
IN
V
OUT
IRU1075
V
IN
V
OUT
Adj
IRU1075
Adj
V
REF
R1
R
L
R1
R2
I
ADJ
= 50uA
R2
Figure 3 - Typical application of the IRU1075
for programming the output voltage.
The IRU1075 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)3R2 + I
ADJ
3R2
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 IRU1075 is
10mA, R1 is typically selected to be 121V resistor so
that it automatically satisfies the minimum current re-
quirement. Notice that since I
ADJ
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 I
ADJ
is only 0.3% of the nominal set point.
Load Regulation
Since the IRU1075 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
Figure 4 - Schematic showing connection
for best load regulation.
Stability
The IRU1075 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 100 mV 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 IRU1075 takes advantage
of this phenomena 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 IRU1075 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
selected heat sink be chosen such that during maxi-
mum continuous load operation the junction tempera-
ture is kept below this number. The example below
shows the steps in selecting the proper regulator heat
sink for the worst case current consumption using Intel
200MHz microprocessor as the load.
4
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Rev. 1.2
08/01/02
IRU1075
Assuming the following specifications:
V
IN
= 5V
V
OUT
= 3.5V
I
OUT(MAX)
= 4.6A
T
A
= 358C
The steps for selecting a proper heat sink to keep the
junction temperature below 135°C is given as:
1) Calculate the maximum power dissipation using:
P
D
= I
OUT
3(V
IN
- V
OUT
)
P
D
= 4.63(5 - 3.5) = 6.9W
2) Select a package from the regulator data sheet and
record its junction to case (or tab) thermal resistance.
Selecting TO-220 package gives us:
θ
JC
= 2.78C/W
Air Flow (LFM)
4) With the maximum heat sink temperature calculated
in the previous step, the heat-sink-to-air thermal re-
sistance (u
SA
) is calculated by first calculating the
temperature rise above the ambient as follows:
DT
= T
S
- T
A
= 116 - 35 = 818C
∆T
= Temperature Rise Above Ambient
u
SA
=
DT
81
=
= 11.78C/W
P
D
6.9
5) Next, a heat sink with lower
u
SA
than the one calcu-
lated in Step 4 must be selected. One way to do this
is to simply look at the graphs of the “Heat Sink Temp
Rise Above the Ambient” vs. the “Power Dissipation”
and select a heat sink that results in lower tempera-
ture rise than the one calculated in previous step.
The following heat sinks from AAVID and Thermalloy
meet this criteria.
3) Assuming that the heat sink is black anodized, cal-
culate the maximum heat sink temperature allowed:
Assume,
ucs
= 0.05°C/W (heat-sink-to-case ther-
mal resistance for black anodized)
T
S
= T
J
- P
D
3(u
JC
+
u
CS
)
T
S
= 135 - 6.93(2.7 + 0.05) = 1168C
0
Thermalloy
AAVID
6021PB
100
6021PB
200
300
400
7141D
6073PB 6109PB
534202B 534202B 507302
575002 576802B
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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. 1.2
08/01/02
www.irf.com
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