Datasheet AD8556 (Analog Devices) - 23

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Data Sheet AD8556 Read Mode
It is theoretically possible, though very unlikely, for a fuse The values stored by the polysilicon fuses can be sent to the to be incompletely blown during programming, assuming the FILT/DIGOUT pin to verify correct programming. Normally, required conditions are met. In this situation, the fuse could the FILT/DIGOUT pin is only connected to the second gain stage have a medium resistance, neither low nor high, and a voltage output via RF. During read mode, however, the FILT/DIGOUT pin of approximately 1.5 V could be developed across the fuse. is also connected to the output of a shift register to al ow the Therefore, the OTP cell could output Logic 0 or Logic 1, depending polysilicon fuse contents to be read. Because VOUT is a buffered on temperature, supply voltage, and other variables. version of FILT/DIGOUT, VOUT also outputs a digital signal To detect this undesirable situation, the sense current can be during read mode. lowered by a factor of 4 using a specific code. The voltage Read mode is entered by setting Field 1 to 11 and selecting the developed across the fuse would then change from 1.5 V to desired parameter in Field 2. Field 4 is ignored. The parameter 0.38 V, and the output of the OTP would be Logic 0 instead of value, stored in the polysilicon fuses, is loaded into an internal the expected Logic 1 from a blown fuse. Fuses blown correctly shift register, and the MSB of the shift register is connected to would still output Logic 1. In this way, fuses blown incorrectly the FILT/DIGOUT pin. Pulses at DIGIN shift out the shift can be detected. Another specific code would return the sense register contents to the FILT/DIGOUT pin, al owing the 8‒bit current to the normal (larger) value. The sense current cannot parameter value to be read after seven additional pulses; shifting be permanently programmed to the low value. When the AD8556 occurs on the falling edge of DIGIN. An eighth pulse at DIGIN is powered up, the sense current defaults to the high value. disconnects FILT/DIGOUT from the shift register and terminates The low sense current code is: the read mode. If a parameter value is less than eight bits long, 1000 0000 0001 00 00 10 XXXX XXX1 0111 1111 1110. the MSBs of the shift register are padded with 0s. The normal (high) sense current code is: For example, to read the second stage gain, this code is used: 1000 0000 0001 00 00 10 XXXX XXX0 0111 1111 1110. 1000 0000 0001 11 00 10 0000 0000 0111 1111 1110. Because
Programming Procedure
the second stage gain parameter value is only three bits long, the FILT/DIGOUT pin has a value of 0 when this code is For reliable fuse programming, it is imperative to follow the entered, and remains 0 during four additional pulses at DIGIN. programming procedure requirements, especial y the proper The fifth, sixth, and seventh pulses at DIGIN return the 3-bit supply voltage during programming. value at FILT/DIGOUT, the seventh pulse returns the LSB. An 1. When programming the AD8556, the temperature of the eighth pulse at DIGIN terminates the read mode. device must be between 10°C and 40°C.
Sense Current
2. Set VDD and VSS to the desired values in the application. Use simulation mode to test and determine the desired A sense current is sent across each polysilicon fuse to determine codes for the second stage gain, first stage gain, and output whether it has been blown. When the voltage across the fuse is offset. The nominal values for these parameters are shown less than approximately 1.5 V, the fuse is considered not blown, in Table 6, Table 7, Equation 2, and Equation 3; use the and Logic 0 is output from the OTP cell. When the voltage across codes corresponding to these values as a starting point. the fuse is greater than approximately 1.5 V, the fuse is considered However, because actual parameter values for given codes blown, and Logic 1 is output. vary from device to device, some fine tuning is necessary When the AD8556 is manufactured, all fuses have a low for the best possible accuracy. resistance. When a sense current is sent through the fuse, a voltage less than 0.1 V is developed across the fuse, which is One way to choose these values is to set the output offset much lower than 1.5 V; therefore, Logic 0 is output from the to an approximate value, such as Code 128 for midsupply, OTP cell. When a fuse is electrically blown, it should have a to al ow the required gain to be determined. Then set the very high resistance. When the sense current is applied to the second stage gain so the minimum first stage gain (Code 0) blown fuse, the voltage across the fuse should be larger than gives a lower gain than required, and the maximum first 1.5 V; therefore, Logic 1 is output from the OTP cell. stage gain (Code 127) gives a higher gain than required. After choosing the second stage gain, the first stage gain can be chosen to fine tune the total gain. Final y, the output offset can be adjusted to give the desired value. After determining the desired codes for second stage gain, first stage gain, and output offset, the device is ready for permanent programming. Rev. B | Page 23 of 27 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY GENERAL DESCRIPTION SPECIFICATIONS ELECTRICAL SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION GAIN VALUES OPEN WIRE FAULT DETECTION SHORTED WIRE FAULT DETECTION FLOATING VPOS, VNEG, OR VCLAMP FAULT DETECTION DEVICE PROGRAMMING Digital Interface Initial State Simulation Mode Programming Mode Parity Error Detection Read Mode Sense Current Programming Procedure Determining Optimal Gain and Offset Codes EMI/RFI PERFORMANCE OUTLINE DIMENSIONS ORDERING GUIDE