Datasheet

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LT1185

1185ff

Low Dropout Regulator

■

Low Resistance Pass Transistor: 0.25

Ω

■

Dropout Voltage: 0.75V at 3A

■

1% Reference Voltage

■

Accurate Programmable Current Limit

■

Shutdown Capability

■

Internal Reference Available

■

Full Remote Sense

■

Low Quiescent Current: 2.5mA

■

Good High Frequency Ripple Rejection

■

Available in 5-Lead TO-220 and DD Packages

The LT1185 uses a saturation-limited NPN transistor as

the pass element. This device gives the linear dropout

characteristics of a 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

shutdown.

5V, 3A Regulator with 3.5A Current Limit

Dropout Voltage

–

2.37k

2.67k

2µF

TANT

REF GND

OUT

LT1185

2µF

TANT

LIM

4.3k

–

6V TO 16V

OUT

5V AT 3A

LT1185 • TA01

*CURRENT LIMIT = 15k/R

LIM

= 3.5A

LOAD CURRENT (A)

– V

OUT

(V)

0.8

1.0

1.2

LT1185 • TA02

0.6

0.4

0.2

1.6

1.4

= 125°C

= –55°C

= 25°C

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.

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

Summary of content (16 pages)

PAGE 1
LT1185 Low Dropout Regulator FEATURES Low Resistance Pass Transistor: 0.25Ω Dropout Voltage: 0.75V at 3A ±1% Reference Voltage Accurate Programmable Current Limit Shutdown Capability Internal Reference Available Full Remote Sense Low Quiescent Current: 2.5mA Good High Frequency Ripple Rejection Available in 5-Lead TO-220 and DD Packages ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LT1185 uses a saturation-limited NPN transistor as the pass element.
PAGE 2
LT1185 W W W AXI U U ABSOLUTE RATI GS (Note 1) Input Voltage .......................................................... 35V Input-Output Differential ......................................... 30V FB Voltage ................................................................ 7V REF Voltage .............................................................. 7V Output Voltage ........................................................ 30V Output Reverse Voltage ............................................
PAGE 3
LT1185 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the operating temperature range, otherwise specifications are at TA = 25°C. Adjustable version, VIN = 7.4V, VOUT = 5V, IOUT = 1mA, RLIM = 4.02k, unless otherwise noted. PARAMETER CONDITIONS TYP MAX Load Regulation (Note 7) IOUT = 5mA to 3A VIN – VOUT = 1.5V to 10V, VOUT = 5V 0.05 0.3 % Line Regulation (Note 7) VIN – VOUT = 1V to 20V, VOUT = 5V 0.002 0.
PAGE 4
LT1185 U W TYPICAL PERFOR A CE CHARACTERISTICS 12 5 3 TYPICAL 2 GUARANTEED LIMIT 1 2.39 0 *DOES NOT INCLUDE REF CURRENT OR OUTPUT DIVIDER CURRENT 8 6 4 0 30 2.37 2.36 2.34 2.33 –50 –25 0 25 50 75 100 125 150 JUNCTION TEMPERATURE (°C) 0 25 5 15 20 10 INPUT-OUTPUT DIFFERENTIAL (V) 2.38 2.35 VOUT = 5V 2 TEST POINTS 0 2.40 VOLTAGE (V) GUARANTEED LIMIT 2.
PAGE 5
LT1185 U W U U APPLICATIO S I FOR ATIO Block Diagram 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.
PAGE 6
LT1185 U W U U APPLICATIO S I FOR ATIO Amplifier A2 is used to generate an internal current through Q4 when an external resistor is connected from the REF pin to ground. This current is equal to 2.37V divided by RLIM. It generates a current limit sense voltage across R1. The regulator will current limit via A4 when the voltage across R2 is equal to the voltage across R1. These two resistors essentially form a current “amplifier” with a gain of 350/0.055 = 6,360.
PAGE 7
LT1185 U W U U APPLICATIO S I FOR ATIO Large output capacitors (electrolytic or solid tantalum) will not cause the LT1185 to oscillate, but they will cause a damped “ringing” at light load currents where the ESR of the capacitor is several orders of magnitude lower than the load resistance. This ringing only occurs as a result of transient load or line conditions and normally causes no problems because of its low amplitude (≤ 25mV). Heat Sinking The LT1185 will normally be used with a heat sink.
PAGE 8
LT1185 U W U U APPLICATIO S I FOR ATIO factor is that local ambient temperature may be somewhat higher because of the point source of heat. The consequences of excess junction temperature include poor reliability, especially for plastic packages, and the possibility of thermal shutdown or degraded electrical characteristics. The final design should be checked in situ with a thermocouple attached to the regulator case under worstcase conditions of high ambient, high input voltage and full load.
PAGE 9
LT1185 U W U U APPLICATIO S I FOR ATIO for remote sense applications, they may need to be considered. Ground lead resistance of 0.4Ω would cause an output voltage error of up to (3A/40)(0.4Ω) = 30mV, or 0.6% at VOUT = 5V. Note that if the sense leads are connected as shown in Figure 2, with ra ≈ 0Ω, this error is a fixed number of millivolts, and does not increase as a function of DC output voltage. Shutdown Techniques The LT1185 can be shut down by open-circuiting the REF pin.
PAGE 10
LT1185 U W U 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 established within 50ms of a power change.
PAGE 11
LT1185 U TYPICAL APPLICATIO S Foldback Current Limiting + + R3 15k VIN 1.6 R1 2.37k 1.4 Q1 2N3906 2µF TANT + GND 2µF TANT VOUT (NORMALIZED) + R4 5.36k VOUT REF FB – VIN 1.0 0.8 0.6 0.4 R2 2.61k LT1185 IFULL LOAD = 15k + 10.8k R4 R3 1.2 0.2 – VOUT ISHORT-CIRCUIT = 15k R3 0 IOUT LT1185 • TA03b LT1185 • TA03a Auxiliary + 12V Low Dropout Regulator for Switching Supply 12V REGULATED AUXILIARY * R1 2.37k RLIM + REF GND + FB VIN R2 9.
PAGE 12
LT1185 U TYPICAL APPLICATIO S Low Input Voltage Monitor Tracks Dropout Characteristics + + + R3 360k C2 2.2µF TANT VIN R1 2.37k 4k REF R5* 0.01Ω – + GND R4** 1k FB VIN C1 2.2µF VOUT TANT R2 2.6k LT1185 ( ) TRIP POINT FOR VIN = VOUT 1 + R4 • R7 + IOUT R5 • R7 R6 R3 • R6 FOR VALUES SHOWN, TRIP POINT FOR VIN IS: VOUT + 0.37V AT IOUT = 0 AND VOUT = 1.18V AT IOUT = 3A † DO NOT SUBSTITUTE.
PAGE 13
Q49 Q3 Q1 Q50 R2 3k R56 600Ω R4 520Ω 10k Q47 Q39 Q6 R54 4k Q5 C1 10pF R5 600Ω Q4 Q2 R55 30k R3 3k R1 5.5k Q48 R50 160Ω R49 700Ω Q8 R46 8k Q40 Q36 Q43 R53 10k Q11 Q52 Q51 R9 2.7k R6 750Ω Q37 Q7 500Ω Q46 R47 4k Q44 C2 Q35 R52 10k Q9 R11 220Ω Q12 R8 6.5k R7 500Ω Q41 Q42 Q33 R45 1.
PAGE 14
LT1185 U PACKAGE DESCRIPTIO K Package 4-Lead TO-3 Metal Can (Reference LTC DWG # 05-08-1311) 1.177 – 1.197 (29.90 – 30.40) .320 – .350 (8.13 – 8.89) .760 – .775 (19.30 – 19.69) .470 TP P.C.D. .060 – .135 (1.524 – 3.429) .655 – .675 (16.64 – 19.05) .151 – .161 (3.84 – 4.09) DIA 2 PLC .420 – .480 (10.67 – 12.19) .167 – .177 (4.24 – 4.49) R .038 – .043 (0.965 – 1.09) 72° 18° .490 – .510 (12.45 – 12.
PAGE 15
LT1185 U PACKAGE DESCRIPTIO Q Package 5-Lead Plastic DD Pak (Reference LTC DWG # 05-08-1461) .256 (6.502) .060 (1.524) TYP .060 (1.524) .390 – .415 (9.906 – 10.541) .165 – .180 (4.191 – 4.572) .045 – .055 (1.143 – 1.397) 15° TYP .060 (1.524) .183 (4.648) +.008 .004 –.004 +0.203 0.102 –0.102 .059 (1.499) TYP .330 – .370 (8.382 – 9.398) ( ) .095 – .115 (2.413 – 2.921) .075 (1.905) .300 (7.620) +.012 .143 –.020 +0.305 3.632 –0.
PAGE 16
LT1185 U TYPICAL APPLICATIO S Logic Controlled 3A Low-Side Switch with Fault Protection 5V RLIM 4k REF FB GND 1N4001 ADD FOR INDUCTIVE LOADS LOAD VOUT LT1185 VIN LT1185 • TA08 Improved High Frequency Ripple Rejection + + + C2 2.2µF TANT R1 2.37k RLIM VIN REF C1 4.7µF TANT GND FB – VIN R2 LT1185 VOUT C3 0.05µF – VOUT LT1185 • TA09 NOTE: C3 IMPOVES HIGH FREQUENCY RIPPLE REJECTION BY 6dB AT VOUT = 5V, AND BY 14dB AT VOUT = 12V. C1 IS INCREASED TO 4.