1
LT1587-1.5
Fixed 1.5V, 3A
Low Dropout
Fast Response
GTL+ Regulator
The LT
1587-1.5 is a low dropout 3-terminal regulator
with a fixed 1.5V output voltage and 3A output current
capability. The design is optimized for low voltage applica-
tions where transient response and minimum input voltage
are critical. Similar to the LT1085, it has lower dropout and
faster transient response. These improvements make it
ideal for low voltage microprocessor applications, espe-
cially as the regulator in an Intel Pentium Pro processor
GTL+ supply.
Current limit is trimmed to ensure specified output current
and controlled short-circuit current. On-chip thermal lim-
iting provides protection against any combination of over-
load that creates excessive junction temperatures.
The LT1587-1.5 is available in both the through-hole and
surface mount versions of the industry standard 3-pin
TO-220 power package.
DESCRIPTIO
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U
s
Fast Transient Response
s
Guaranteed Dropout Voltage at Multiple Currents
s
Load Regulation: 0.05% Typ
s
Trimmed Current Limit
s
On-Chip Thermal Limiting
FEATURES
APPLICATIO
N
S
U
s
Intel Pentium
Pro Processor GTL+ Supply
s
Low Voltage Logic Supplies
s
Battery-Powered Circuitry
, LTC and LT are registered trademarks of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation
TYPICAL APPLICATIO
N
U
Intel Pentium Pro Processor GTL+ Supply
C2, C3
100
F
10V
AVX TPS
C1
100
F
10V
AVX TPS
+
C4 TO
C6
1
F
C8, C9
100
F
10V
AVX TPS
C10 TO C12
1
F
C7
100
F
10V
AVX TPS
R1
75
R5
100
R2
150
V
REF
R4
150
V
REF
R7
100
R8
100
R6
100
R3
75
Q1
Q3
+
+
V
OUT
V
IN
GND
LT1587-1.5
3.3V
3.3V
V
TT
= 1.5V
3A
V
TT
= 1.5V
3A
V
IN
V
OUT
GND
LT1587-1.5
142 TOTAL SIGNAL LINES
C13
0.1
F
C14
0.1
F
Q4
Q2
+
LT1587 TA01
RX
TX
RX
TX
RX
TX
RX
TX
NOTE: LTC RECOMMENDS SEPARATE
V
TT
GENERATORS AT EACH BUS END
TO ENSURE CURRENT SHARING
BETWEEN THE REGULATORS AND TO
MINIMIZE V
TT
DISTRIBUTION DROPS
2
LT1587-1.5
V
IN
............................................................................. 7V
Operating Junction Temperature Range
Control Section.................................... 0
C to 125
C
Power Transistor ................................. 0
C to 150
C
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
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Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec) .................. 300
C
100% Thermal Limit Functional Test
PRECO
N
DITIO
N
I G
U
U
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PACKAGE/ORDER I
N
FOR
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ATIO
N
W
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ORDER PART
NUMBER
T PACKAGE
3-LEAD PLASTIC TO-220
FRONT VIEW
3
2
1
V
IN
V
OUT
GND
ORDER PART
NUMBER
V
IN
V
OUT
GND
M PACKAGE
3-LEAD PLASTIC DD
FRONT VIEW
3
2
1
JA
= 30
C/W*
LT1587CM-1.5
* With package soldered to 0.5 square inch copper area over backside
ground plane or internal power plane.
JA
can vary from 20
C/W to
> 40
C/ W with other mounting techniques.
Consult factory for Industrial and Military grade parts.
The
q
denotes specifications which apply over the specified operating
temperature range.
Note 1: See thermal regulation specifications for changes in output voltage
due to heating effects. Load and line regulation are measured at a constant
junction temperature by low duty cycle pulse testing.
Note 2: Line and load regulation are guaranteed up to the maximum
power dissipation (17W for the LT1587-1.5 in T package). Power
dissipation is determined by input/output differential and the output
current. Guaranteed maximum output power will not be available over
the full input/output voltage range.
Note 3: I
FULL LOAD
is defined as the maximum value of output load current
as a function of input-to-output voltage. I
FULL LOAD
is equal to 3A. The
LT1587-1.5 has constant current limit with changes in input-to-output
voltage.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Output Voltage
V
IN
= 5V, T
J
= 25
C, I
OUT
= 0mA
1.485 ( 1%)
1.5
1.515 (+ 1%)
V
3V
V
IN
7V, 0mA
I
OUT
3A
q
1.470 ( 2%)
1.5
1.530 (+ 2%)
V
Line Regulation (Notes 1, 2)
3V
V
IN
7V, I
OUT
= 0mA
0.005
0.2
%
Load Regulation
V
IN
= 5V, T
J
= 25
C, 0mA
I
OUT
I
FULL LOAD
0.05
0.3
%
(Notes 1, 2, 3)
q
0.05
0.5
%
Dropout Voltage
V
OUT
= 1%, I
OUT
= 3A
q
1.150
1.300
V
Current Limit (Note 3)
(V
IN
V
OUT
) = 5.5V
q
3.100
3.750
A
Quiescent Current
V
IN
= 5V
q
7
13
mA
Ripple Rejection
f = 120Hz, C
OUT
= 25
F Tant., V
IN
= 4.5V, I
OUT
= 3A
q
60
72
dB
Thermal Regulation
T
A
= 25
C, 30ms pulse
0.004
0.02
%/W
Temperature Stability
q
0.5
%
Long-Term Stability
T
A
= 125
C, 1000 Hrs.
0.03
1.0
%
RMS Output Noise
T
A
= 25
C, 10Hz
f
10kHz
0.003
%
(% of V
OUT
)
Thermal Resistance
T Package: Control Circuitry/Power Transistor
0.7/3.0
C/W
Junction to Case
M Package: Control Circuitry/Power Transistor
0.7/3.0
C/W
ELECTRICAL CHARACTERISTICS
JA
= 50
C/W
LT1587CT-1.5
3
LT1587-1.5
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Short-Circuit Current
vs Temperature
OUTPUT CURRENT (A)
0
0.5
DROPOUT VOLTAGE (V)
0.7
0.8
0.9
1.0
1.1
1.2
0.5
1.0
1.5
2.0
LT1587 TPC01
2.5
1.3
1.4
1.5
0.6
3.0
T = 5
C
T = 125
C
T = 25
C
GUARANTEED
TEST POINTS
TEMPERATURE (
C)
75
SHORT-CIRCUIT CURRENT (A)
4.0
4.5
125
LT1587 TPC02
3.5
3.0
25
25 50
175
5.0
75
50
0
150
100
TEMPERATURE (
C)
0.20
OUTPUT VOLTAGE DEVIATION (%)
0.10
0
0.10
0.15
0.05
0.05
25
25
75
125
LT1587 TPC03
175
50
75
0
50
100
150
I = 3A
Dropout Voltage vs Output Current
Load Regulation vs Temperature
Output Voltage vs Temperature
TEMPERATURE (
C)
OUTPUT VOLTAGE (V)
1.53
1.52
1.51
1.50
1.49
1.48
1.47
25
25
75
125
LT1587 TPC04
175
50
75
0
50
100
150
Ripple Rejection vs Frequency
FREQUENCY (Hz)
20
RIPPLE REJECTION (dB)
40
50
70
90
10
1k
10k
100k
LT1587 TPC06
0
100
60
30
10
80
LT1587-1.5: (V
IN
V
OUT
)
3V
0.5V
V
RIPPLE
2V
I
OUT
= I
FULL LOAD
Quiescent Current vs Temperature
TEMPERATURE (
C)
75
QUIESCENT CURRENT (mA)
9
11
13
125
LT1587 TPC05
7
5
8
10
12
6
4
3
25
25
75
50
150
0
50
100
175
Maximum Power Dissipation*
CASE TEMPERATURE (C)
50
POWER (W)
20
25
30
80
100
LT1587 TPC07
15
10
60 70
90
110 120 130 140 150
5
0
LT1587-1.5
*AS LIMITED BY MAXIMUM JUNCTION TEMPERATURE
4
LT1587-1.5
SI PLIFIED SCHE ATIC
W
W
APPLICATIO
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FOR
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THERMAL
LIMIT
GND
V
IN
V
OUT
LT1587 BD
+
General
The LT1587-1.5 3-terminal regulator is easy to use and
has all the protection features expected in a high perfor-
mance linear regulator. The device is short-circuit pro-
tected, safe-area protected and provides thermal shut-
down to turn off the regulator if the junction temperature
exceeds about 150
C.
The IC is pin compatible with the LT1083/LT1084/LT1085
family of linear regulators but offers lower dropout voltage
and faster transient response. The trade-off for this im-
proved performance is a 7V maximum supply voltage.
Similar to the LT1083/LT1084/LT1085 family, the LT1587-
1.5 regulator requires an output capacitor for stability.
However, the improved frequency compensation permits
the use of capacitors with much lower ESR while still
maintaining stability. This is critical in addressing the needs
of modern low voltage, high speed microprocessors.
Current generation microprocessors and their associated
circuitry cycle load current from almost zero to several
amps in tens of nanoseconds. Output voltage tolerances
are tighter and include transient response as part of the
specification. The LT1587-1.5 is specifically designed to
meet the fast current load step requirements of these
applications and saves total cost by needing less output
capacitance in order to maintain regulation.
Stability
The circuit design in the LT1587-1.5 requires the use of an
output capacitor as part of the frequency compensation.
For all operating conditions, the addition of a 22
F solid
tantalum or a 100
F aluminum electrolytic on the output
ensures stability. Normally, the LT1587-1.5 can use smaller
value capacitors. Many different types of capacitors are
available and have widely varying characteristics. These
capacitors differ in capacitor tolerance (sometimes rang-
ing up to
100%), equivalent series resistance, equivalent
series inductance and capacitance temperature coefficient.
The LT1587-1.5 frequency compensation optimizes fre-
quency response with low ESR capacitors. In general, use
capacitors with an ESR of less than 1
.
Normally, capacitor values on the order of several hundred
microfarads are used on the output of the regulators to
ensure good transient response with heavy load current
changes. Output capacitance can increase without limit
and larger values of output capacitance further improve the
stability and transient response of the LT1587-1.5.
5
LT1587-1.5
APPLICATIO
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Large load current changes are exactly the situation pre-
sented by modern microprocessors and their peripheral
circuitry. The load current step contains higher order
frequency components that the output decoupling network
must handle until the regulator throttles to the load current
level. Capacitors are not ideal elements and contain para-
sitic resistance and inductance. These parasitic elements
dominate the change in output voltage at the beginning of
a transient load step change. The ESR of the output
capacitors produces an instantaneous step in output volt-
age [
V =
I(ESR)]. The ESL of the output capacitors
produces a droop proportional to the rate of change of
output current [V = L(
I/
t)]. The output capacitance
produces a change in output voltage proportional to the
time until the regulator can respond [
V =
t(
I/C)]. These
transient effects are illustrated in Figure 1.
Figure 1
ESR
EFFECTS
LT1587 F01
ESL
EFFECTS
CAPACITANCE
EFFECTS
POINT AT WHICH REGULATOR
TAKES CONTROL
SLOPE,
=
V
t
I
C
The use of capacitors with low ESR, low ESL and good high
frequency characteristics is critical in meeting the output
voltage tolerances of these high speed microprocessor
applications. These requirements dictate a combination of
high quality surface mount tantalum capacitors and ce-
ramic capacitors. The location of the decoupling network is
critical to transient response performance. Place the
decoupling network as close as possible to the micropro-
cessor control circuitry because a trace run from the
decoupling capacitors to the actual circuitry is inductive. In
addition, use large power and ground plane areas to
minimize distribution drops.
A possible stability problem that occurs in monolithic linear
regulators is current limit oscillations. The LT1587-1.5
essentially has a flat current limit over the range of input
supply voltage. The lower current limit rating and 7V
maximum supply voltage rating for this device permits this
characteristic. Current limit oscillations are typically non-
existent unless the input and output decoupling capacitors
for the regulators are mounted several inches from the
terminals.
Protection Diodes
In normal operation, the LT1587-1.5 does not require any
protection diodes. Older 3-terminal regulators require
protection diodes between the output pin and the input pin
to prevent die overstress.
A protection diode between the input and output pins is
usually not needed. An internal diode between the input
and output pins on the LT1587-1.5 can handle microsec-
ond surge currents of 50A to 100A. Even with large value
output capacitors it is difficult to obtain those values of
surge currents in normal operation. Only with large values
of output capacitance, such as 1000
F to 5000
F, and with
the input pin instantaneously shorted to ground can dam-
age occur. A crowbar circuit at the input of the LT1587-1.5
can generate those levels of current, and a diode from
output to input is then recommended. This is shown in
Figure 2. Usually, normal power supply cycling or system
"hot plugging and unplugging" will not generate current
large enough to do any damage.
Figure 2
Ripple Rejection
The typical curve for ripple rejection reflects values for the
LT1587-1.5 as a function of frequency. In applications that
require improved ripple rejection, use the adjustable
LT1587. A bypass capacitor from the adjust pin to ground
reduces the output ripple by the ratio of V
OUT
/1.25V.
Load Regulation
It is not possible to provide true remote load sensing
because the LT1587-1.5 is a 3-terminal device. Load
LT1587-1.5
D1
1N4002
(OPTIONAL)
GND
V
IN
V
OUT
IN
OUT
C1
10
F
+
C2
10
F
+
LT1587 F02
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