9

LTC1663

SYNC Address/Quick Command

In addition to the slave address, the LTC1663 has an

address that can be shared by other devices so that they

may be updated synchronously. The address is called to

the SYNC address and uses the quick command protocol.

The SYNC Address is 1111 110

Ack

Stop

Start

1111 110

SY/CLR

11

71

1663 TA05

1

SYNC Address

SY/CLR

1

Update output latches on rising edge of SCL during

Acknowledge of SYNC Address

0

Clear all internal latches on rising edge of SCL during

Acknowledge of SYNC Address

The SY/CLR bit set high only has meaning when the “SY”

bit of the command byte was previously set HIGH. On the

otherhand, the SY/CLR bit set LOW will always clear the

part, independent of the state of the “SY” bit in the

command byte.

Input Threshold

Anticipating the trend toward lower supply voltages, the

While some SMBus parts may violate this stringent SMBus

spondingly higher input supply voltage, the LTC1663

meets and maintains the constant SMBus input threshold

specification across the entire supply voltage range of

2.7V to 5.5V. The logic input threshold is designed to be

1V with 50mV of hysteresis.

Voltage Output

The output amplifier contained in the LTC1663 can source

or sink up to 5mA. The output stage swings to within a few

millivolts of either supply rail when unloaded and has an

equivalent output resistance of 85

Ω when driving a load to

the rails. The output amplifier is stable driving capacitive

loads up to 1000pF.

A small resistor placed in series with the output can be

used to achieve stability for any load capacitance greater

than 1000pF. For example, a 0.1

µF load can be driven by

the LTC1663 if a 110

Ωseriesresistanceisused.Thephase

margin of the resulting circuit is 45

° and increases mono-

tonically from this point if larger values of resistance, ca-

pacitance or both are substituted for the values given.

Rail-to-Rail Output Considerations

As in any rail-to-rail device, the output is limited to

voltages within the supply range.

If the DAC offset is negative, the output for the lowest

codes limits at 0V as shown in Figure 1b.

error (FSE) is positive, the output for the highest codes

can occur if the internal reference is used.

Offset and linearity are defined and tested over the region

of the DAC transfer function where no output limiting can

occur.

Internal Reference

In applications where a predictable output is required that

is independent of supply voltage, the LTC1663 has a user-

selectable internal reference. Selecting the internal refer-

ence will set the full-scale output voltage to 2.5V. This can

be useful in applications where the supply voltage is

poorly regulated.

Using the LT

Power Supply for the LTC1663

In applications where the advantages of using the internal

reference are required but the full-scale range needs to be

greater than 2.5V, an external series reference can be

used. The LT1460 is ideal for use as a power supply for the

LTC1663 and can provide 3V, 3.3V and 5V full-scale

output voltage ranges. The LT1460 provides accuracy,

noise immunity and extended supply range to the LTC1663

both parts are available in SOT-23 packages, the PC board

space for this application is extremely small. See Figure 2.