Datasheet LT1010 (Analog Devices) - 8

LT1010
APPLICATIONS INFORMATION
a ground plane is prudent, especially when operating at Normally, thermal overload protection will limit dissipa- high frequencies. tion and prevent damage. However, with more than 30V The buffer slew rate can be reduced by inadequate sup- across the conducting output transistor, thermal limiting ply bypass. With output current changes much above is not quick enough to ensure protection in current limit. 100mA/µs, using 10µF solid tantalum capacitors on both The thermal protection is effective with 40V across the supplies is good practice, although bypassing from the conducting output transistor as long as the load current positive to the negative supply may suffice. is otherwise limited to 150mA. When used in conjunction with an op amp and heavily
Drive Impedance
loaded (resistive or capacitive), the buffer can couple into When driving capacitive loads, the LT1010 likes to be driven supply leads common to the op amp causing stability from a low source impedance at high frequencies. Certain problems with the overall loop and extended settling time. low power op amps (e.g., the LM10) are marginal in this Adequate bypassing can usually be provided by 10µF solid respect. Some care may be required to avoid oscillations, tantalum capacitors. Alternately, smaller capacitors could especially at low temperatures. be used with decoupling resistors. Sometimes the op amp has much better high frequency rejection on one supply, Bypassing the buffer input with more than 200pF will solve so bypass requirements are less on this supply. the problem. Raising the operating current also works.
Power Dissipation Parallel Operation
In many applications the LT1010 will require heat sinking. Parallel operation provides reduced output impedance, Thermal resistance, junction to still air is 50°C/W for the more drive capability and increased frequency response TO-220 package and 100°C/W for the miniDIP package. under load. Any number of buffers can be directly paral- Circulating air, a heat sink or mounting the package to leled as long as the increased dissipation in individual a printed circuit board will reduce thermal resistance. units caused by mismatches of output resistance and offset voltage is taken into account. In DC circuits, buffer dissipation is easily computed. In AC circuits, signal waveshape and the nature of the load When the inputs and outputs of two buffers are connected determine dissipation. Peak dissipation can be several together, a current, ∆IOUT , flows between the outputs: times average with reactive loads. It is particularly im- V portant to determine dissipation when driving large load ∆I OS1 – VOS2 OUT = capacitance. R OUT1 + ROUT2 With AC loading, power is divided between the two output where VOS and ROUT are the offset voltage and output transistors. This reduces the effective thermal resistance, resistance of the respective buffers. junction to case is 3°C/W for the TO-220 package as long as the peak rating of neither output transistor is exceeded. Normally, the negative supply current of one unit will The typical curves indicate the peak dissipation capabilities increase and the other decrease, with the positive supply of one output transistor. current staying the same. The worst-case (VIN → V+) increase in standby dissipation can be assumed to be
Overload Protection
∆IOUTVT , where VT is the total supply voltage. The LT1010 has both instantaneous current limit and Offset voltage is specified worst case over a range of sup- thermal overload protection. Foldback current limiting has ply voltages, input voltage and temperature. It would be not been used, enabling the buffer to drive complex loads unrealistic to use these worst-case numbers above because without limiting. Because of this, it is capable of power paralleled units are operating under identical conditions. dissipation in excess of its continuous ratings. 1010fe 8 Document Outline Fteatures Applications Description Typical Application Absolute Maximum Ratings Preconditioning Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Applications Information Schematic Diagram Definition Of Terms Package Description Revision History Related Parts