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 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 overranged. 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.

 

INFORMATIONEN ÜBER ANWENDUNGEN

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.

What About Overloads?

IC regulators with thermal shutdown, like the LT1185,allow heat sink designs which concentrate on worst-case “normal” conditions and ignore “fault” conditions. An output overload or short may force the regulator to exceed its maximum junction temperature rating, but thermal shutdown is designed to prevent regulator failure under these conditions. A word of caution however; thermal shutdown temperatures are typically 175°C in the control portion of the die and 180°C to 225°C in the power transistor section. Extended operation at these temperatures can cause permanent degradation of plastic encapsulation. Designs which may be subjected to extended periods of overload should either use the hermetic TO-3 package or increase heat sink size. Fold back current limiting can be implemented to minimize power levels under fault conditions.

 

External Current Limit

The LT1185 requires a resistor to set current limit. The value of this resistor is 15k divided by the desired current limit (in amps). The resistor for 2A current limit would be 15k/2A = 7.5k. Tolerance over temperature is ±10%, so current limit is normally set 15% above maximum load current. Fold back limiting can be employed if short-circuit current must be lower than full load current (see Typical Applications).

The LT1185 has internal current limiting which will override external current limit if power in the pass transistor is excessive. The internal limit is ≈3.6A with a fold back characteristic which is dependent on input-output voltage, not output voltage per se (see Typical Performace Characteristics).

 

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.