Datasheet LT5400 (Analog Devices) - 6

LT5400
applicaTions inForMaTion Where to Connect the Exposed Pad
To protect the LT5400 against large ESD strikes, external The exposed pad is not DC connected to any resistor protection can be added using diodes to the circuit supply terminal. Its main purpose is to reduce the internal tem- rails or bidirectional Zeners to ground (Figure 1). perature rise when the application calls for large amounts of dissipated power in the resistors. The exposed pad can V+ V– be tied to any voltage (such as ground) as long as the BAV99 LT5400 LT5400 absolute maximum ratings are observed. EXTERNAL EXTERNAL There is capacitive coupling between the resistors and the CONNECTOR CONNECTOR UMZ36K exposed pad, as specified in the Electrical Characteristics table. To avoid interference, do not tie the exposed pad to 5400 F01
Figure 1
noisy signals or noisy grounds. Connecting the exposed pad to a quiet AC ground is
Matching Specification
recommended as it acts as an AC shield and reduces the The LT5400 specifies matching in the most conservative amount of resistor-resistor capacitance. possible way. In each device, the ratio error of the largest
Thermal Considerations
of the four resistors to the smallest of the four resistors meets the specified matching level. Looser definitions Each resistor is rated for relatively high power dissipation, would compare each resistor value to the average of the as listed in the Absolute Maximum Ratings section of resistor values, which would typically result in specifica- this data sheet. To calculate the internal temperature rise tions that appear twice as good as they are per the LT5400’s inside the package, add together the power dissipated in more conservative definition. The following two examples all of the resistors, and multiply by the thermal resistance illustrate this point. coefficient of the package (θJA or θJC as applicable). In an inverting gain-of-1 amplifier, if the largest resistor For example, if each resistor dissipates 250mW, for a is allowed to deviate only 0.01% from the smallest resis- total of 1W, the total temperature rise inside the package tor, then the worst-case gain can be –1.00005/0.99995 = equals 40°C. All 4 resistors will be at the same temperature, –1.0001, which is a 0.01% error from the ideal –1.0000. regardless of which resistor dissipates more power. The That is the LT5400 definition. In a looser definition, if each junction temperature must be kept within the Absolute resistor would be allowed to deviate by 0.01% from the Maximum Rating. At elevated ambient temperatures, this average, then the worst-case gain could be –1.0001/0.9999 places a limit on the maximum power dissipation. = –1.0002, which is a 0.02% error from the ideal –1.0000. In addition to limiting the maximum power dissipation, In a divide-by-2 resistor divider network, if the largest the maximum voltage across any two pins must also be resistor is allowed to deviate only 0.01% from the smallest kept less than the absolute maximum rating. resistor, then the worst-case ratio can be 1.00005/(1.00005 + 0.99995) = 0.500025, which is a 0.005% error from the
ESD
ideal 0.50000. That is the LT5400 definition. In a looser The LT5400 can withstand up to ±1kV of electrostatic definition, if each resistor would be allowed to deviate by discharge (ESD, human body). To achieve the highest 0.01% from the average, then the worst-case ratio could precision matching, the LT5400 is designed without explicit be 1.0001/(1.0001 + 0.9999) = 0.50005, which is a 0.01% ESD internal protection diodes. ESD beyond this voltage error from the ideal 0.50000. can damage or degrade the device including causing pin-to-pin shorts. 5400fc 6 For more information www.linear.com/LT5400 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Available Options Order Information Electrical Characteristics Typical Performance Characteristics Applications Information Typical Applications Package Description Revision History Typical Application Related Parts