US1075
2-33
Rev. 1.0
9/14/99
TYPICAL APPLICATION
TYPICAL APPLICATION
DESCRIPTION
DESCRIPTION
The US1075 product is a low dropout three terminal ad-
justable regulator with minimum of 7.5A output current
capability. This product is specifically designed to pro-
vide well regulated supply for low voltage IC applications
such as Pentium
TM
P54C
TM
,P55C
TM
as well as GTL+
termination for Pentium Pro
TM
and Klamath
TM
pro-
cessor applications . The US1075 is also well suited for
other processors such as Cyrix
TM
,AMD and Power
PC
TM
applications. The US 1075 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 voltage as low as 4.5V minimum.
FEATURES
FEATURES
1V Dropout at Full Load Current
Fast Transient Response
1% Voltage Reference Initial Accuracy
Output Current Limiting
Built-in Thermal Shutdown
APPLICATIONS
APPLICATIONS
Low Voltage Processor Applications such as :
P54C
TM
,P55C
TM
,Cyrix M2
TM
,
POWER PC
TM
,AMD
GTL+ Termination
PENTIUM PRO
TM
, KLAMATH
TM
Low Voltage Memory Termination Applications
Standard 3.3V Chip-Set and Logic Applications
Notes: Pentium P54C,P55C ,Klamath,Pentium Pro,VRE,are trade marks of Intel Corp.Cyrix M2 is trade mark of Cyrix Corp.
Power PC is trade mark of IBM Corp.
Typical Application of US1075 in a 5V to 3.3V regulator
PACKAGE ORDER INFORMATION
PACKAGE ORDER INFORMATION
Tj (C) 3 PIN PLASTIC 3 PIN PLASTIC 3 PIN PLASTIC
TO220 (T) TO263 (M) POWER FLEX (P)
0 TO 150 US1075CT US1075CM US1075CP
7.5A LOW DROPOUT POSITIVE
ADJUSTABLE REGULATOR
5V
3.3V
R1
121
R2
200
C1
1500uF
C2
2x 1500uF
US1075
1075app1-1.0
1
2
3
Adj
Vin
Vout
US1075
2-34
Rev. 1.0
9/14/99
Note 1 : Low duty cycle pulse testing with Kelvin con-
nections are required in order to maintain accurate data.
Note 2 : Drop-out voltage is defined as the minimum
differential voltage between Vin and Vout required to main-
tain regulation at Vout. 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.
ABSOLUTE MAXIMUM RATINGS
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
PACKAGE INFORMATION
3 PIN PLASTIC TO220 ( T ) 3 PIN PLASTIC TO263 ( M ) 3 PIN PLASTIC POWER FLEX ( P )
JT
=2.7
C/W
JA
=60
C/W
JA
=35
C/W for 1" Square pad
JA
=35
C/W for 1" Square pad
ELECTRICAL SPECIFICATIONS
ELECTRICAL SPECIFICATIONS
Unless otherwise specified ,these specifications apply over ,Cin=1uF,Cout=10uF,and Tj=0 to 150
C.Typical
values refer to Tj=25
C.
PARAMETER
SYM
TEST CONDITION
MIN
TYP
MAX
UNITS
Reference Voltage V
REF
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<7.5A
0.4
%
Dropout Voltage
Io=7.5A
1.0
1.2
V
(note 2)
V
O
Io=4A
0.92
1.1
Current Limit
Vin=3.3V,dVo=100mV
7.6
9
A
Minimum Load Current
Vin=3.3V,Vadj=0V
5
10
mA
(note 3)
Thermal Regulation
30 mS PULSE,Vin-Vo=3V,Io=7.5A
0.02
%/W
Ripple Rejection
f=120HZ ,Co=25uF Tan
Io=7.5A,Vin-Vo=3V
60 70 dB
Adjust Pin Current
I
ADJ
Io=10mA,Vin-Vo=1.5V,Tj=25
Io=10mA,Vin-Vo=1.5V
55
120
uA
Adjust Pin Current Change
Io=10mA,Vin-Vo=1.5V,Tj=25
0.2
5
uA
Temperature Stability
Vin=3.3V,Vadj=0V,Io=10mA
0.5
%
Long Term Stability
Tj=125
C,1000 Hrs
0.3
%
RMS Output Noise
Tj=25
C 10hz<f<10khz
0.003
%Vo
1
2
3
FRONT VIEW
Adj
Tab is
Vout
Vin
Vout
1
2
3
FRONT VIEW
Adj
Vout
Vin
Tab is
Vout
FRONT VIEW
1
3
2
Adj
Vout
Vin
Tab is
Vout
US1075
2-35
Rev. 1.0
9/14/99
APPLICATION INFORMATION
APPLICATION INFORMATION
Introduction
The US1075 adjustable Low Dropout (LDO) regulator is
a 3 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 5.5 V.This regulator
unlike the first generation of the 3T regulators such as
LM117 that required 3V differential between 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 sup-
ply and are often generated from the 5V supply. Another
major requirement of these microprocessors such as
the Intel P54C
TM
is the need to switch the load current
from zero to several amps in tens of nanoseconds at
the processor pins ,which translates to an approximately
300 to 500 nS current step at the regulator . 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
TM
specification calls
for a total of
100mV including initial tolerance,load regu-
lation and 0 to 4.6A load step.
The US1075 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.
PIN DESCRIPTIONS
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.
2
Vout
The output of the regulator. A minimum of 10uF capacitor must be connected
from this pin to ground to insure stability.
3
Vin
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 Vout in order for the device to regulate properly.
BLOCK DIAGRAM
BLOCK DIAGRAM
Figure 1 - Simplified block diagram of the US1050
Vin 3
1075blk1-1.0
+
1 Adj
+
2 Vout
THERMAL
SHUTDOWN
CURRENT
LIMIT
1.25V
US1075
2-36
Rev. 1.0
9/14/99
Output Voltage Setting
The US1075 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:
Figure 2 - Typical application of the US1075
for programming the output voltage.
The US1075 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)*R2 +
I
adj*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 US1075 is 10 mA , R1 is
typically selected to be 121
resistor so that it auto-
matically satisfies the minimum current requirement.
Notice that since Iadj is typically in the range of 50uA 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 appli-
cation where R1=121
and R2=200
the error due to
Iadj is only 0.3% of the nominal set point.
Load Regulation
Since the US1075 is only a 3 terminal device , it is not
possible to provide true remote sensing of the output
voltage at the load.Figure 3 shows that the best load
Figure 3 - Schematic showing connection for best load
regulation
Stability
The US1075 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 m
and an output
capacitance of 500 to 1000uF. Fortunately as the ca-
pacitance increases, the ESR decreases resulting in a
fixed RC time constant. The US1075 takes advantage of
this phenomena in making the overall regulator loop
stable.For most applications a minimum of 100uF 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.
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 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 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.
V
V
R
R
I
R
Where :
V Typically
I
lly
R
in figure
OUT
REF
ADJ
REF
ADJ
=
+
+
1
125
50
2
2
1
2
1
2
V
= .
=
uA Typica
& R as shown
Vout
1075app2-1.0
R1
R2
Vin
Vref
IAdj = 50uA
Adj
Vout
Vin
US1075
1075app3-1.0
R1
R2
Vin
RL
Rp
PARASITIC LINE
RESISTANCE
US1075
Adj
Vout
Vin
US1075
2-37
Rev. 1.0
9/14/99
V
V
V
V
I
A
T
C
IN
O
OUT
A
MAX
=
=
=
=
5
3 5
4 6
35
.
.
(
)
(
)
P
I
V
V
P
.
.
. W
D
O UT
IN
O UT
D
=
-
=
-
=
4 6 5 3 5
6 9
JC
C W
=
2 7
.
/
(
)
(
)
T
T P
.
.
.
S
J
D
= -
+
=
-
+
=
JC
CS
S
T
C
135 6 9 2 7 0 05
116
T
S
A
T
T
C
=
-
=
- =
116 35 81
Thermal Design
The US1075 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
C ,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 .
Assuming the following specifications :
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 :
2) Select a package from the Regulator data sheet
and record its junction to case (or Tab) thermal
resistance.
Selecting TO220 package gives us :
3) Assuming that the heat sink is Black Anodized, cal-
culate the maximum Heat sink temperature allowed :
Assume ,
cs=0.05
C/W (Heat sink to Case thermal
resistance for Black Anodized)
4) With the maximum heat sink temperature calcu-
lated in the previous step, the Heat Sink to Air thermal
resistance (
sa) is calculated by first calculating the
temperature rise above the ambient as follows :
T
=Temperature Rise Above Ambient
5) Next , a heat sink with lower
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 temperature rise
than the one calculated in previous step. The following
heat sinks from AAVID and Thermaloy meet this crite-
ria.
Air Flow (LFM)
0 100 200 300 400
Thermalloy 6021PB 6021PB 6073PB 6109PB 7141D
AAVID 534202B 534202B 507302 575002 576802B
SA
T
D
SA
P
C W
=
=
=
81
6 9
117
.
.
/
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