AD8557

Rev. C | Page 19 of 24

If a parameter value is less than eight bits long, the MSBs of the

shift register are padded with 0s.

For example, to read the second stage gain, this code is used:

1000 0000 0001 11 00 10 0000 0000 0111 1111 1110

Because the second stage gain parameter value is only three bits

long, the DIGOUT pin has a value of 0 when this code is

entered, and remains 0 during four additional pulses at DIGIN.

The fifth, sixth, and seventh pulses at DIGIN return the 3-bit

value at DIGOUT, the seventh pulse returns the LSB. An eighth

pulse at DIGIN terminates the read mode.

Sense Current

A sense current is sent across each polysilicon fuse to determine

whether it has been blown. When the voltage across the fuse is

less than approximately 1.5 V, the fuse is considered not blown,

and Logic 0 is output from the OTP cell. When the voltage

across the fuse is greater than approximately 1.5 V, the fuse is

considered blown, and Logic 1 is output.

When the AD8557 is manufactured, all fuses have a low

resistance. When a sense current is sent through the fuse, a

voltage less than 0.1 V is developed across the fuse. This is

much lower than 1.5 V, so Logic 0 is output from the OTP cell.

When a fuse is electrically blown, it should have a very high

resistance. When the sense current is applied to the blown fuse,

the voltage across the fuse should be larger than 1.5 V, so

Logic 1 is output from the OTP cell.

It is theoretically possible, though very unlikely, for a fuse to

be incompletely blown during programming, assuming the

required conditions are met. In this situation, the fuse could

have a medium resistance, neither low nor high, and a voltage of

approximately 1.5 V could be developed across the fuse. Thus,

the OTP cell could output Logic 0 or Logic 1, depending on

temperature, supply voltage, and other variables.

To detect this undesirable situation, the sense current can be

lowered by a factor of 4 using a specific code. The voltage devel-

oped across the fuse would then change from 1.5 V to 0.38 V,

and the output of the OTP would be a Logic 0 instead of the

expected Logic 1 from a blown fuse. Correctly blown fuses would

still output a Logic 1. In this way, incorrectly blown fuses can be

detected. Another specific code would return the sense current

to the normal (larger) value. The sense current cannot be

permanently programmed to the low value. When the AD8557

is powered up, the sense current defaults to the high value.

The low sense current code is

1000 0000 0001 00 00 10 XXXX XXX1 0111 1111 1110

The normal (high) sense current code is

1000 0000 0001 00 00 10 XXXX XXX0 0111 1111 1110

Programming Procedure

For reliable fuse programming, it is imperative to follow the

programming procedure requirements, especially the proper

supply voltage during programming:

1. When programming the AD8557, the temperature of the

device must be between 10°C to 40°C.

2. Set VDD and VSS to the desired values in the application.

Use simulation mode to test and determine the desired

codes for the second stage gain, first stage gain, and output

offset. The nominal values for these parameters are shown

in Table 6, Table 7, Equation 2, and Equation 3; use the

codes corresponding to these values as a starting point.

However, because actual parameter values for given codes

vary from device to device, some fine tuning is necessary

for the best possible accuracy.

One way to choose these values is to set the output offset to

an approximate value, such as Code 128 for midsupply, to

allow the required gain to be determined. Then, set the

second stage gain so the minimum first stage gain (Code 0)

gives a lower gain than required, and the maximum first

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. Finally, 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.

Note that once a programming attempt has been made for

any fuse, there should be no further attempt to blow that

fuse. If a fuse does not program to the expected state,

discard the unit. The expected incidence rate of attempted

but unblown fuses is very small when following the proper

programming procedure and conditions.

3. Set VSS to 0 V and VDD to 5.75 V (±0.25 V). Power

supplies should be capable of supplying 250 mA at the

required voltage and properly bypassed as described in the

Programming Mode section. Use program mode to

permanently enter the desired codes for the first stage gain,

second stage gain, and output offset. Blow the master fuse

to allow the AD8557 to read data from the fuses and to

prevent further programming.

4. Set VDD and VSS to the desired values in the application.

Use read mode with low sense current followed by high

sense current to verify programmed codes.

5. Measure gain and offset to verify correct functionality.