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PDF LT1185C Data sheet ( Hoja de datos )
| Número de pieza | LT1185C | |
| Descripción | Low Dropout Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT1185C (archivo pdf) en la parte inferior de esta página. Total 16 Páginas | ||
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LT1185
Low Dropout Regulator
FEATURES
s Low Resistance Pass Transistor: 0.25Ω
s Dropout Voltage: 0.75V at 3A
s ±1% Reference Voltage
s Accurate Programmable Current Limit
s Shutdown Capability
s Internal Reference Available
s Standard 5-Lead Packages
s Full Remote Sense
s Low Quiescent Current: ≈ 2.5mA
s Good High Frequency Ripple Rejection
DESCRIPTIO
The LT®1185 is a 3A low dropout regulator with adjustable
current limit and remote sense capability. It can be used as
a positive output regulator with floating input or as a
standard negative regulator with grounded input. The
output voltage range is 2.5V to 25V, with ±1% accuracy on
the internal reference voltage.
The LT1185 uses a saturation-limited NPN transistor as
the pass element. This device gives the linear dropout
characteristics of an FET pass element with significantly
less die area. High efficiency is maintained by using special
anti-saturation circuitry that adjusts base drive to track
load current. The “on resistance” is typically 0.25Ω.
Accurate current limit is programmed with a single 1/8W
external resistor, with a range of zero to three amperes. A
second, fixed internal limit circuit prevents destructive
currents if the programming current is accidentally over-
ranged. Shutdown of the regulator output is guaranteed
when the program current is less than 1µA, allowing
external logic control of output voltage.
The LT1185 has all the protection features of previous LTC
regulators, including power limiting and thermal shut-
down. The 4-lead TO-3 package is specified for –55°C to
150°C operation and the 5-lead TO-220 is specified over
0°C to 125°C.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATI
5V, 3A Regulator with 3.5A Current Limit
+
+
VIN
6V TO 16V
C2
2µF
TANT
RLIM*
4.3k
REF
GND
FB
– VIN LT1185
VOUT
R1
2.37k
+
+
R2
2.67k
C1 VOUT
2µF 5V AT 3A
TANT
–
LT1185 • TA01
*CURRENT LIMIT = 15k/RLIM = 3.5A
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
Dropout Voltage
TJ = 25°C
TJ = 125°C
TJ = –55°C
12 3
LOAD CURRENT (A)
4
LT1185 • TA02
1
1 page  
UU W U
APPLICATIO S I FOR ATIO
Block Diagram
A simplified block diagram of the LT1185 is shown in
Figure 1. A 2.37V bandgap reference is used to bias the
input of the error amplifier A1, and the reference amplifier
A2. A1 feeds a triple NPN pass transistor stage which has
the two driver collectors tied to ground so that the main
pass transistor can completely saturate. This topology
normally has a problem with unlimited current in Q1 and
Q2 when the input voltage is less than the minimum
required to create a regulated output. The standard “fix”
for this problem is to insert a resistor in series with Q1 and
Q2 collectors, but this resistor must be low enough in
value to supply full base current for Q3 under worst-case
LT1185
conditions, resulting in very high supply current when the
input voltage is low. To avoid this situation, the LT1185
uses an auxiliary emitter on Q3 to create a drive limiting
feedback loop which automatically adjusts the drive to Q1
so that the base drive to Q3 is just enough to saturate Q3,
but no more. Under saturation conditions, the auxiliary
emitter is acting like a collector to shunt away the output
current of A1. When the input voltage is high enough to
keep Q3 out of saturation, the auxiliary emitter current
drops to zero even when Q3 is conducting full load current.
VREF
2.37V
–
A2
+
RLIM (EXTERNAL)
REF
–+
A1
Q4
D2 D4 D3
A5
–+
A4
+–
300mV
VIN
I1 D1 R1
2µA 350Ω
A3
+–
200mV
Figure 1. Block Diagram
GND
FB
Q1 VOUT
Q2
Q3
R2
0.055Ω
LT1185 • BD
5
5 Page 
UU W U
APPLICATIO S I FOR ATIO
Thermal Regulation
IC regulators have a regulation term not found in discrete
designs because the power transistor is thermally coupled
to the reference. This creates a shift in the output voltage
which is proportional to power dissipation in the regulator.
∆VOUT = P(K1 + K2 θJA)
= (IOUT)(VIN – VOUT)(K1 + K2 θJA)
K1 and K2 are constants. K1 is a fast time constant effect
caused by die temperature gradients which are estab-
lished within 50ms of a power change. K1 is specified on
the data sheet as thermal regulation, in percent per watt.
K2 is a long time constant term caused by the temperature
drift of the regulator reference voltage. It is also specified,
but in percent per degree centigrade. It must be multiplied
by overall thermal resistance, junction-to-ambient, θJA.
As an example, assume a 5V regulator with an input
voltage of 8V, load current of 2A, and a total thermal
resistance of 4°C/W, including junction-to-case, (use
control area specification), interface, and heat sink resis-
tance. K1 and K2, respectively, from the data sheet are
0.014%/W and 0.01%/°C.
∆VOUT = (2A)(8V –␣ 5V)(0.014 + 0.01 • 4)
= 0.32%
LT1185
This shift in output voltage could be in either direction
because K1 and K2 can be either positive or negative.
Thermal regulation is already included in the worst case
reference specification.
Output Voltage Reversal
Some IC regulators suffer from a latch-up state when their
output is forced to a reverse voltage of as little as one diode
drop. The latch-up state can be triggered without a fault
condition when the load is connected to an opposite
polarity supply instead of to ground. If the second supply
is turned on first, it will pull the output of the first supply
to a reverse voltage through the load. The first supply may
then latch off when turned on. This problem is particularly
annoying because the diode clamps which should always
be used to protect against polarity reversal do not usually
stop the latch-up problem.
The LT1185 is designed to allow output reverse polarity of
several volts without damage or latch-up, so that a simple
diode clamp can be used.
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT1185C.PDF ] | |
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