All internal circuitry is designed to operate down to 1V input-to-output differential and the dropout voltage is fully specified as a function of load current. Dropout is guaranteed at a maximum of 1. Current limit is also trimmed, minimizing the stress on both the regulator and power source circuitry under overload conditions. F output capacitor is required on these new devices. However, this is included in most regulator designs. All other trademarks are the property of their respective owners.

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All internal circuitry is designed to operate down to 1V input-to-output differential and the dropout voltage is fully specified as a function of load current. Dropout is guaranteed at a maximum of 1. Current limit is also trimmed, minimizing the stress on both the regulator and power source circuitry under overload conditions. F output capacitor is required on these new devices. However, this is included in most regulator designs. All other trademarks are the property of their respective owners.

Note 2: 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 5: Dropout voltage is specified over the full output current range of the device.

Test points and limits are shown on the Dropout Voltage curve. Note 7: 1. Note 8: Dropout is 1. These regulators are pin compatible with older threeterminal adjustable devices, offer lower dropout voltage and more precise reference tolerance. Further, the reference stability with temperature is improved over older types of regulators.

The only circuit difference between using the LT family and older regulators is that this new family requires an output capacitor for stability. Stability The circuit design used in the LT family requires the use of an output capacitor as part of the device frequency compensation.

For all operating conditions, the addition of ? F aluminium electrolytic or a 22? F solid tantalum on the output will ensure stability. Normally, capacitors much smaller than this can be used with the LT Many different types of capacitors with widely varying characteristics are available.

The ? F or 22? F values given will ensure stability. When the adjustment terminal is bypassed to improve the ripple rejection, the requirement for an output capacitor increases.

The value of 22? F tantalum or ? F aluminum covers all cases of bypassing the adjustment terminal. Without bypassing the adjustment terminal, smaller capacitors can be used with equally good results and the table below shows approximately what size capacitors are needed to ensure stability.

F Tantalum, 50? F Aluminum 22? F Tantalum, ? F are used in the output of many regulators to ensure good transient response with heavy load current changes.

Output capacitance can be increased without limit and larger values of output capacitor further improve stability and transient response of the LT regulators. Another possible stability problem that can occur in monolithic IC regulators is current limit oscillations. These can occur because, in current limit, the safe area protection exhibits a negative impedance. The safe area protection decreases the current limit as the input-to-output voltage increases.

That is the equivalent of having a negative resistance since increasing voltage causes current to decrease. Negative resistance during current limit is not unique to the LT series and has been present on all power IC regulators. The value of the negative resistance is a function of how fast the current limit is folded back as input-to-output voltage increases. This negative resistance can react with capacitors or inductors on the input to cause oscillation during current limiting. Depending on the value of series resistance, the overall circuitry may end up unstable.

Since this is a system problem, it is not necessarily easy to solve; however, it does not cause any problems with the IC regulator and can usually be ignored. Protection Diodes In normal operation, the LT family does not need any protection diodes. Older adjustable regulators required protection diodes between the adjustment pin and the output and from the output to the input to prevent overstressing the die. The internal current paths on the LT adjustment pin are limited by internal resistors.

Therefore, even with capacitors on the adjustment pin, no protection diode is needed to ensure device safety under short-circuit conditions. Diodes between input and output are usually not needed. The internal diode between the input and the output pins of the LT family can handle microsecond surge currents of 50A to A. Even with large output capacitances, it is very difficult to get those values of surge currents in normal operations.

Only with a high value of output capacitors, such as ? F to ? F and with the input pin instantaneously shorted to ground, can damage occur. A crowbar circuit at the input of the LT can generate those kinds of currents, and a diode from output to input is then recommended.

Normal power supply cycling or even plugging and unplugging in the system will not generate current large enough to do any damage. Of course, as with any IC regulator, exceeding the maximum input to output voltage differential causes the internal transistors to break down and none of the protection circuitry is functional. The safe area protection decreases the current limit as input-to-output voltage increases and keeps the power transistor inside a safe operating region for all values of input-to-output voltage.

The LT protection is designed to provide some output current at all values of input-to-output voltage up to the device breakdown. When power is first turned on, as the input voltage rises, the output follows the input, allowing the regulator to start up into very heavy loads. During the start-up, as the input voltage is rising, the input-to-output voltage differential remains small, allowing the regulator to supply large output currents.

With high input voltage, a problem can occur wherein removal of an output short will not allow the output voltage to recover. Older regulators, such as the series, also exhibited this phenomenon, so it is not unique to the LT The load line for such a load may intersect the output current curve at two points. If this happens, there are two stable output operating points for the regulator. With this double intersection, the power supply may need to be cycled down to zero and brought up again to make the output recover.

Ripple Rejection The typical curves for ripple rejection reflect values for a bypassed adjustment pin. This curve will be true for all values of output voltage.

For proper bypassing and ripple rejection approaching the values shown, the impedance of the adjust pin capacitor at the ripple frequency should be less than the value of R1, normally ?

The size of the required adjust pin capacitor is a function of the input ripple frequency. At Hz the adjust pin capacitor should be 25? At 10kHz only 0. F is needed. For circuits without an adjust pin bypass capacitor, the ripple rejection will be a function of output voltage. Ripple rejection will be degraded by 12dB from the value shown on the typical curve.

Output Voltage The LT develops a 1. By placing a resistor R1 between these two terminals, a constant current is caused to flow through R1 and down through R2 to set the overall output voltage. Normally this current is the specified minimum load current of 10mA. Because IADJ is very small and constant when compared with the current through R1, it represents a small error and can usually be ignored. Basic Adjustable Regulator Load Regulation Because the LT is a three-terminal device, it is not possible to provide true remote load sensing.

Load regulation will be limited by the resistance of the wire connecting the regulator to the load. The data sheet specification for load regulation is measured at the bottom of the package. Negative side sensing is a true Kelvin connection, with the bottom of the output divider returned to the negative side of the load. Although it may not be immediately obvious, best load regulation is obtained when the top of the resistor divider R1 is connected directly to the case not to the load.

This is illustrated in Figure 2. If R1 were connected to the load, the effective resistance between the regulator and the load would be:? RP is about 0. Thermal Considerations The LT series of regulators have internal power and thermal limiting circuitry designed to protect the device under overload conditions.

For continuous normal load conditions however, maximum junction temperature ratings must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. This includes junction-to-case, caseto-heat sink interface, and heat sink resistance itself. New thermal resistance specifications have been developed to more accurately reflect device temperature and ensure safe operating temperatures.

The data section for these new regulators provides a separate thermal resistance and maximum junction temperature for both the Control Section and the Power Transistor.

Previous regulators, with a single junction-to-case thermal resistance specification, used an average of the two values provided here and therefore could allow excessive junction temperatures under certain conditions of ambient temperature and heat sink resistance. To avoid this possibility, calculations should be made for both sections to ensure that both thermal limits are met.

Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. This is the lowest resistance path for heat flow. Proper mounting is required to ensure the best possible thermal flow from this area of the package to the heat sink.

Thermal compound at the case-to-heat sink interface is strongly recommended. If the case of the device must be electrically isolated, a thermally conductive spacer can be used, as long as its added contribution to thermal resistance is considered.


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