LT1581/LT1581-2.5
10A, Very Low
Dropout Regulators
FEATURES
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DESCRIPTION
The LT
®
1581 is a 10A low dropout regulator designed to
power the new generation of microprocessors. The drop-
out voltage of this device is 100mV at light loads rising to
just 430mV at 10A. To achieve this dropout a second low
current input voltage 1V greater than the output voltage is
required. The device can also be used as a single supply
device where dropout is comparable to an LT1584.
Several other new features have been added to the LT1581.
A remote SENSE pin is brought out. This feature virtually
eliminates output voltage variations due to load changes.
Typical load regulation for a load current step of 100mA to
10A, measured at the SENSE pin, is less than 1mV.
The LT1581 has fast transient response, equal to the
LT1584. On fixed voltage devices, the ADJUST pin is
brought out. A small capacitor on the ADJUST pin further
improves transient response.
This device is ideal for generating processor supplies of
2V to 3V on motherboards where both 5V and 3.3V
supplies are available.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation.
PowerPC is a trademark of IBM Corporation.
Low Dropout, 430mV at 10A Output Current
Fast Transient Response
Remote Sense
1mV Load Regulation
Fixed 2.5V Output and Adjustable Output
No Supply Sequencing Problems in
Dual Supply Mode
APPLICATIONS
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Microprocessor Supplies
Post Regulators for Switching Supplies
High Current Regulators
5V to 3.XXV for Pentium
®
Processors Operating
at 90MHz to 166MHz and Beyond
3.3V to 2.9V for Portable Pentium Processor
PowerPC
TM
Series Power Supplies
TYPICAL APPLICATION
2.5V Microprocessor Supply
0.8
POWER
OUTPUT
2.5V/10A
Dropout Voltage –
Minimum Power Voltage
INDICATES GUARANTEED TEST POINTS
MINIMUM POWER VOLTAGE (V)
(V
POWER
– V
OUT
) (V)
3.3V
10A
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3 4 5 6 7 8
OUTPUT CURRENT (A)
9
10
T
J
= 25°C
T
J
= 125°C
DATA SHEET LIMIT
+
330µF
OS-CON
+
LT1581-2.5
100µF TANT
AVX TPS
×
7
5V
0.2A
CONTROL
SENSE
ADJ
0.1µF
1581 TA01
+
10µF
TANT
GND
U
U
U
1581 G03
1
LT1581/LT1581-2.5
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
FRONT VIEW
7
6
5
4
3
2
1
TAB IS
OUTPUT
T7 PACKAGE
7-LEAD PLASTIC TO-220
NC
V
CONTROL
V
POWER
OUTPUT
SENSE
NC/GND*
ADJUST
V
POWER
Input Voltage ................................................ 6V
V
CONTROL
Input Voltage ........................................... 13V
Operating Junction Temperature Range
Control Section ...................................... 0°C to 125°C
Power Transistor ................................... 0°C to 150°C
Storage Temperature ............................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1581CT7
LT1581CT7-2.5
θ
JA
= 50°C/ W
PRECONDITIONING
100% Thermal Limit Functional Test
*PIN2 = NC FOR LT1581CT7, GND FOR LT1581CT7-2.5
Consult factory for Industrial and Military grade parts.
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. (Note 2)
PARAMETER
Output Voltage:
LT1581-2.5
CONDITIONS
V
CONTROL
= 5V, V
POWER
= 3.3V, I
LOAD
= 0mA
V
CONTROL
= 4V to 12V, V
POWER
= 3V to 5.5V, I
LOAD
= 0mA to 4A
V
CONTROL
= 4V to 12V, V
POWER
= 3.3V to 5.5V, I
LOAD
= 0mA to 10A
V
CONTROL
= 2.75V, V
POWER
= 2V, I
LOAD
= 10mA
V
CONTROL
= 2.7V to 12V, V
POWER
= 1.75V to 5.5V, I
LOAD
= 0mA to 4A
V
CONTROL
= 2.7V to 12V, V
POWER
= 2.05V to 5.5V, I
LOAD
= 0mA to 10A
V
CONTROL
= 3.65V to 12V, V
POWER
= 3V to 5.5V, I
LOAD
= 10mA
V
CONTROL
= 2.5V to 12V, V
POWER
= 1.75V to 5.5V, I
LOAD
= 10mA
V
CONTROL
= 5V, V
POWER
= 3.3V, I
LOAD
= 0mA to 10A
V
CONTROL
= 2.75V, V
POWER
= 2.1V, I
LOAD
= 10mA to 10A
V
CONTROL
= 5V, V
POWER
= 3.3V, V
ADJ
= 0V (Note 4)
V
CONTROL
= 5V, V
POWER
= 3.3V, I
LOAD
= 100mA
V
CONTROL
= 5V, V
POWER
= 3.3V, I
LOAD
= 4A
V
CONTROL
= 5V, V
POWER
= 3.3V, I
LOAD
= 7A
V
CONTROL
= 5V, V
POWER
= 3.3V, I
LOAD
= 10A
V
CONTROL
= 2.75V, V
POWER
= 2.05V, I
LOAD
= 100mA
V
CONTROL
= 2.75V, V
POWER
= 2.05V, I
LOAD
= 4A
V
CONTROL
= 2.75V, V
POWER
= 2.05V, I
LOAD
= 7A
V
CONTROL
= 2.75V, V
POWER
= 2.05V, I
LOAD
= 10A
V
CONTROL
= 5V, V
POWER
= 3.3V, I
LOAD
= 0mA
V
CONTROL
= 5V, V
POWER
= 3.3V,
∆V
OUT
= 100mV
V
CONTROL
= 2.75V, V
POWER
= 2.05V,
∆V
OUT
= 100mV
V
CONTROL
= V
POWER
= 5V Avg, V
RIPPLE
= 1V
P-P
, f
RIPPLE
= 120Hz,
I
OUT
= 4A, T
J
= 25°C
V
CONTROL
= V
POWER
= 3.75V Avg, V
RIPPLE
= 1V
P-P
, f
RIPPLE
= 120Hz,
V
ADJ
= 0V, I
OUT
= 4A, T
J
= 25°C
30ms Pulse
Control Circuitry/Power Transistor
MIN
2.485
2.475
2.475
1.243
1.237
1.237
TYP
2.500
2.500
2.500
1.250
1.250
1.250
1
1
1
1
3
5
20
40
70
5
20
40
70
6
60
10.1
10.1
55
60
11
11
80
80
0.004
0.65/2.50
0.020
MAX
2.515
2.525
2.525
1.257
1.263
1.263
3
3
10
5
10
10
50
100
170
10
50
100
170
10
120
UNITS
V
V
V
V
V
V
mV
mV
mV
mV
mA
mA
mA
mA
mA
mA
mA
mA
mA
mA
µA
A
A
dB
dB
%/W
°C/W
ELECTRICAL CHARACTERISTICS
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
Reference Voltage: LT1581
(V
ADJ
= 0V)
Line Regulation: LT1581-2.5
LT1581
Load Regulation: LT1581-2.5
LT1581 (V
ADJ
= 0V)
Minimum Load Current: LT1581
Control Pin Current: LT1581-2.5
(Note 5)
Control Pin Current: LT1581
(Note 5)
Ground Pin Current: LT1581-2.5
Current Limit:
LT1581-2.5
LT1581 (V
ADJ
= 0V)
Adjust Pin Current: LT1581 (V
ADJ
= 0V) V
CONTROL
= 2.75V, V
POWER
= 2.05V, I
LOAD
= 10mA
Ripple Rejection: LT1581-2.5
LT1581
Thermal Regulation
Thermal Resistance, Junction-to-Case
2
U
W
U
U
W W
UU
W
U
LT1581/LT1581-2.5
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. (Note 2)
PARAMETER
Dropout Voltage (Note 3)
Minimum V
CONTROL
: LT1581-2.5
(V
CONTROL
– V
OUT
)
CONDITIONS
V
POWER
= 3.3V, I
LOAD
= 100mA
V
POWER
= 3.3V, I
LOAD
= 1A
V
POWER
= 3.3V, I
LOAD
= 4A
V
POWER
= 3.3V, I
LOAD
= 7A
V
POWER
= 3.3V, I
LOAD
= 10A
V
POWER
= 2.05V, I
LOAD
= 100mA
V
POWER
= 2.05V, I
LOAD
= 1A
V
POWER
= 2.05V, I
LOAD
= 4A
V
POWER
= 2.05V, I
LOAD
= 7A
V
POWER
= 2.05V, I
LOAD
= 10A
V
CONTROL
= 5V, I
LOAD
= 100mA
V
CONTROL
= 5V, I
LOAD
= 1A
V
CONTROL
= 5V, I
LOAD
= 4A, T
J
= 25°C
V
CONTROL
= 5V, I
LOAD
= 4A
V
CONTROL
= 5V, I
LOAD
= 7A, T
J
= 25°C
V
CONTROL
= 5V, I
LOAD
= 7A
V
CONTROL
= 5V, I
LOAD
= 10A, T
J
= 25°C
V
CONTROL
= 5V, I
LOAD
= 10A
V
CONTROL
= 2.75V, I
LOAD
= 100mA
V
CONTROL
= 2.75V, I
LOAD
= 1A
V
CONTROL
= 2.75V, I
LOAD
= 4A, T
J
= 25°C
V
CONTROL
= 2.75V, I
LOAD
= 4A
V
CONTROL
= 2.75V, I
LOAD
= 7A, T
J
= 25°C
V
CONTROL
= 2.75V, I
LOAD
= 7A
V
CONTROL
= 2.75V, I
LOAD
= 10A, T
J
= 25°C
V
CONTROL
= 2.75V, I
LOAD
= 10A
q
q
q
q
q
q
q
q
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
TYP
1.02
1.04
1.06
1.10
1.12
1.02
1.04
1.06
1.10
1.12
0.10
0.13
0.22
0.31
MAX
1.25
1.27
1.30
1.33
1.35
1.25
1.27
1.30
1.33
1.35
0.20
0.25
0.33
0.37
0.45
0.55
0.63
0.70
0.20
0.25
0.33
0.37
0.45
0.55
0.63
0.70
UNITS
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
Minimum V
CONTROL
: LT1581
(V
CONTROL
– V
OUT
)
(V
ADJ
= 0V)
Minimum V
POWER
:
(V
POWER
– V
OUT
)
LT1581-2.5
q
0.43
q
q
q
q
Minimum V
POWER
:
(V
POWER
– V
OUT
)
(V
ADJ
= 0V)
LT1581
0.10
0.13
0.22
0.31
q
0.43
q
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
Unless otherwise specified, V
OUT
= V
SENSE
. For the LT1581
adjustable device, V
ADJ
= 0V.
Note 3:
For the LT1581, dropout is caused by either minimum control
voltage (V
CONTROL
) or minimum power voltage (V
POWER
). Both parameters
are specified with respect to the output voltage. The specifications represent
the minimum input-to-output voltage required to maintain 1% regulation.
Note 4:
For the LT1581 adjustable device the minimum load current is the
minimum current required to maintain regulation. Normally, the current in
the resistor divider used to set the output voltage is selected to meet the
minimum load current requirement.
Note 5:
The CONTROL pin current is the drive current required for the
output transistor. This current will track output current with roughly a
1:100 ratio. The minimum value is equal to the quiescent current of the
device.
3
LT1581/LT1581-2.5
TYPICAL PERFORMANCE CHARACTERISTICS
Control Pin Current vs
Output Current
200
180
CONTROL PIN CURRENT (mA)
INDICATES GUARANTEED TEST POINTS
160
140
120
100
80
60
40
20
0
0
1
2
8
3 4 5 6 7
OUTPUT CURRENT (A)
9
10
TYPICAL
DEVICE
DATA SHEET LIMIT
MINIMUM POWER VOLTAGE (V)
(V
POWER
– V
OUT
) (V)
MINIMUM CONTROL VOLTAGE
(V
CONTROL
– V
OUT
) (V)
LT1581 Reference Voltage vs
Temperature
1.262
1.259
2.512
2.509
REFERENCE VOLTAGE (V)
OUTPUT VOLTAGE (V)
1.256
1.253
1.250
1.247
1.244
1.241
1.238
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1581 G04
PIN FUNCTIONS
ADJUST (Pin 1):
This pin is the negative side of the
reference voltage for the device. Transient response can
be improved by adding a small bypass capacitor from the
ADJUST pin to ground. For fixed voltage devices the
ADJUST pin is also brought out to allow the user to add a
bypass capacitor.
GND (Pin 2, Fixed Voltage Devices Only):
For fixed
voltage devices this is the bottom of the resistor divider
that sets the output voltage.
SENSE (Pin 3):
This pin is the positive side of the reference
voltage for the device. With this pin it is possible to Kelvin
sense the output voltage at the load.
OUTPUT (Pin 4):
This is the power output of the device.
V
POWER
(Pin 5):
This is the collector to the power device
of the LT1581. The output load current is supplied through
this pin. For the device to regulate, the voltage at this pin
must be between 0.1V and 0.7V greater than the output
voltage (see Dropout specifications).
V
CONTROL
(Pin 6):
This pin is the supply pin for the control
circuitry of the device. The current flow into this pin will
be about 1% of the output current. For the device to
regulate, the voltage at this pin must be between 1.0V and
1.35V greater than the output voltage (see Dropout
specifications).
4
U W
1581 G01
Dropout Voltage —
Minimum Control Voltage
2
INDICATES GUARANTEED TEST POINTS
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3 4 5 6 7 8
OUTPUT CURRENT (A)
9
10
0
0.8
Dropout Voltage —
Minimum Power Voltage
INDICATES GUARANTEED TEST POINTS
DATA SHEET LIMIT
DATA SHEET LIMIT
1
T
J
= 125°C
T
J
= 25°C
T
J
= 125°C
T
J
= 25°C
0
1
2
3 4 5 6 7 8
OUTPUT CURRENT (A)
9
10
1581 G02
1581 G03
LT1581-2.5 Output Voltage vs
Temperature
V
OUT
50mV/DIV
Load Current Step Response
2.506
2.503
2.500
10A
2.497
2.494
2.491
2.488
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1581 G05
LOAD
400mA
50µs/DIV
1581 G06
U
U
U
LT1581/LT1581-2.5
BLOCK DIAGRA
APPLICATIONS INFORMATION
The LT1581 is a low dropout regulator designed to power
the new generation of microprocessors. Low dropout
regulators have become more common in desktop com-
puter systems as microprocessor manufacturers have
moved away from 5V only CPUs. A wide range of supply
requirements exists today with new voltages just over the
horizon. In many cases the input/output differential is very
small, effectively disqualifying many of the low dropout
regulators on the market today. The LT1581 is designed to
make use of multiple power supplies present in most
systems to reduce the dropout voltage. This 2-supply
approach maximizes efficiency.
The second supply, at least 1V greater than the output
voltage, is used to provide power for the control circuitry
and supply the drive current to the NPN output transistor.
This allows the NPN to be driven into saturation, thereby
reducing the dropout voltage by a V
BE
compared to
conventional designs. The current requirement for the
control voltage is relatively small, equal to approximately
1% of the output current or about 100mA for a 10A load.
The bulk of this current is drive current for the NPN output
transistor. This drive current becomes part of the output
current.
The control voltage must be at least 1V greater than the
output voltage to obtain optimum performance. The maxi-
mum voltage on the V
CONTROL
pin is 13V. The maximum
voltage at the V
POWER
pin is limited to 7V. GND pin current
for fixed voltage devices is 6mA (typ) and is constant as a
function of load. ADJUST pin current for adjustable de-
vices is 60µA at 25°C and varies proportional to absolute
temperature.
The LT1581 has improved frequency compensation which
permits the use of capacitors with very low ESR. This is
critical in addressing the needs of modern, low voltage,
high speed microprocessors. Current generation micro-
U
W
W
CONTROL
POWER
+
–
SENSE
OUTPUT
FOR FIXED
VOLTAGE
DEVICE
1581 BD
ADJ
GND
U
U
5
FPGA/CPLD
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