LT1005M/LT1005C
Logic Controlled Regulator Logic Controlled Regulator
Obsolete:
For informational purposes only. Please contact Linear Technology for potential replacements.
Features:
Two Regulated Outputs
+5V, 1 Amp; +5V, 35mA
2% Output Voltage Tolerance
66 dB Ripple Rejection
0.5% Load Regulation
TTL and CMOS Compatible Logic Control
All devices pass 100% thermal burn-in test
application:
Power sequence control
Remote on/off power control
Selective system power supply during emergency power operation
Backup memory power supply
describe:
The LT1005 has two positive 5 volt regulators packaged on the same die. The primary regulator does an excellent job delivering load currents up to 1 amp, while the secondary regulator provides similar performance at light loads (35mA). The main voltage regulator has a shutdown control function for logic signals. When the enable pin is pulled to a logic low level, the main regulator is turned off and its output voltage approaches zero volts. During this command, the auxiliary output continues to provide 5 volts regardless of the state of the main regulator.
The main output features current and power limiting which, combined with thermal shutdown, makes it nearly impossible to burn out. The auxiliary output is not affected by the thermal shutdown mechanism or the state of the main output and can be used as a backup in case of mains overload or short circuit conditions.
The logic input (enable pin) of the LT1005 has a threshold of 1.6 volts and can be driven by a high source impedance. This allows it to be driven by most logic families including TTL and CMOS.
Application information:
General information:
The LT1005 is a dual output 5 volt regulator. The main output is capable of delivering up to 1 amp of load current and can be shut down by a logic signal. The auxiliary output provides at least 35mA and is not affected by logic signals. The output is trimmed to an initial ±2% tolerance and has good line and load regulation.
Logic control functions make the LT1005 ideal in many system applications, especially where it is necessary to power part of the system for a period of time and then shut down the system during standby operation. For example, the LT1005 can be used to activate various memory space locations only when needed, saving significant power and cooling costs. LT1005 can also be used to provide power for microcomputers (such as 8048 series). Auxiliary power can be used to keep the RAM active during power down operation. Additional power savings can be realized by using the LT1005 to power PROM, EPROM and E2PROM devices. During program loading or lookup table operations, ROM-type devices can be activated and place their contents in RAM, after which the ROM can be powered off. Or for a high-speed but low-power data-acquisition system, power can be applied to the fast memory and then the data transferred to the CMOS memory. The main regulator can then be turned off, and the CMOS memory can be powered from an auxiliary supply for lower power consumption. Other applications include multiple power supply sequencing, elimination of costly AC and DC switches, delayed start-up applications, switching 5V DC loads, and more.
Timing functions can also be performed directly on the enable pin, such as delaying power-up or power-down. Since a logic low on the enable pin turns off the main regulator, feedback from the output to enable can be used to create a hysteresis or latching function. The LT1005's low quiescent current makes it useful in battery-powered or battery-backed applications. The enable pin can be used as a "low battery" detector or to shut down power to major parts of the system, allowing the memory part to continue operating from the auxiliary output. At low output currents, the auxiliary output will regulate with input voltages typically as low as 6.1V, allowing for maximum battery life.
As with all regulators, good design practice is to bypass the input and output terminals. It is recommended to use 1uF solid tantalum capacitors on both the input and output. For the applications below, bypass capacitors are still recommended but not shown for simplicity. It is also recommended that for maximum noise immunity, the enable pin be tied high if not used. As shown in Figure 1, it can be connected directly to VIN, or to an auxiliary output. If the enable pin is left open, it will float to a high logic level of about 1.6V, while the main output regulator will remain at 5V.
The enable pin is fully protected against input voltages up to 20 volts, even with zero supply input voltage.
A basic shutdown control circuit uses a direct gate driver or an open-drain driver and a pull-up resistor connected to VAux, as shown in Figure 2.
Drive enable pin
The equivalent schematic diagram of the enable pin is shown in Figure 3. Basically, the enable pin has zero current above the threshold, and between 2 and 70uA below the threshold, flowing out of the pin. Standard logic, such as TTL and CMOS, can interface directly with the enable pin even if the logic output swing is higher than the regulator's input voltage (VIN). Even when the regulator is not powered, the enable pin can be driven with 15V CMOS without loading the CMOS output.
Timing functions such as delayed power-up or power-down can be implemented by driving the enable pin with an RC network. In delayed power-up applications, the current from the enable pin should not be used as a timing current because it is temperature sensitive and varies from device to device. Instead, use resistors connected to auxiliary outputs, inputs, or logic signals. The selected timing resistor should provide at least 500uA of current to "negate" the effects of the internal current.
Main Output Current/Voltage Characteristics
After the high-to-low transition of the enable pin, the main regulator output will begin to fall after a delay of about 1 microsecond. With no capacitive load, the output will drop to zero in about 0.5 microseconds (RL = 5-100). For cases with capacitive loads, the fall time is limited by the RC product of the load and output capacitance. For light loads (R > 100), the discharge time is controlled by the equivalent load resistance, which is 200 for the output voltage down to 1 volt. Below 1 volt, the output decays linearly with a slope equal to the load capacitance and pulls down about 0.5mA. In shutdown mode, with an input voltage (ViN) of 15V, the DC output voltage is approximately 0.1V. If ViN is 20V, the output will be about 0.2V during shutdown due to the current path inside the regulator (see Figure 4).
Users should be aware that the output in the low state can only sink about 0.5mA. If you force current into the output, the output voltage will rise to 0.8V at 1mA and over 1V at 10mA. With no output capacitor, the main output rise time is about 1.5 microseconds. Rise time is limited by the LT1005's short-circuit current and load capacitance through the output capacitor; rise time tr = © (5V)/(1.5A). A 1uF output capacitor will slow the output rise time to about 3 microseconds, and a 10uF output capacitor will slow the output rise time to 30 picoseconds.
Output current
For input voltages below 19V, the main output current limit is approximately 1.7A. An internal foldover, or "power limit" circuit, detects the difference in input-to-output voltage and reduces the current limit if the input-to-output voltage exceeds 14V. For example, for a 20V input, the short circuit current is reduced to 1.1A.
Another feature of the LT1005 is that the auxiliary supply is not included and not affected by thermal shutdown. Any fault condition of the main regulator will not affect the auxiliary output voltage. The application circuit below will demonstrate the versatility of the LT1005.