Datasheet

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VOLTAGE REFERENCE INPUTS
V
OUT
 2  V
REF


V
REF
V
REF


D
65536
(2)
ANALOG AND DIGITAL SUPPLIES
EXTERNAL REFERENCE VOLTAGE
FEEDBACK PINS
HOST PROCESSOR INTERFACING
DAC8544 to MSP430 Microcontroller
DAC8544
SLAS420A – MAY 2004 – REVISED JUNE 2005
When pin PD is high, the device works normally with its typical power consumption of 950 µA at V
DD
= 5 V.
However, when PD pin is in low state, the device is in power-down mode, the supply current falls to 200 nA at
V
DD
= 5 V (50 nA at V
DD
= 3 V), and the output is open circuit (high impedance).
All analog circuitry is shut down when a power-down mode is activated. However, the contents of the DAC
register are unaffected when in power-down mode. This allows the DAC output voltage to return to the previous
level when power up resumes. The delay time required to exit power-down is typically 2.5 µs for V
DD
= 5 V, and 5
µs for V
DD
= 3 V. (See the typical characteristics section for additional information.)
Two voltage inputs provide the reference set points for the DAC architecture. These are V
REF+
and V
REF–
. For
typical rail-to-rail operation, V
REF+
should be equivalent to V
DD
and V
REF–
tied to GND. The output voltage is given
by:
The use of the V
REF–
input allows minor adjustments to be made to the offset of the DAC output by applying a
small voltage to the V
REF–
input. A low output impedance source is needed, so that the accuracy of the DAC over
its operating range is not affected.
The analog supply (V
DD
) powers the output buffer and DAC while the digital supply (IOV
DD
) powers the digital
interface. V
DD
can operate from 2.7 V to 5.5 V while IOV
DD
can independently function from 1.8 V to 5.5 V. IOV
DD
determines the interface logic level. See the device specification table for details.
To take advantage of the absolute accuracy of DAC8544, a high-performance reference voltage generator must
be used. DAC8544 has a typical absolute accuracy error of 2 millivolts, and a typical voltage drift of 3 ppm/°C.
This level of performance requires an accurate external reference voltage generator with good temperature drift
characteristics. Accuracy, drift, supply voltage, power consumption, and cost are important factors in choosing a
voltage reference. TI's REF02 is recommended. TI's REF3140 and REF3040 are small and low-cost alternatives.
For regular operation, the feedback pins (V
FBA
through V
FBD
) must be tied to their corresponding output pins
(V
OUTA
through V
OUTD
) at the load. For higher current applications sensitive to gain error, the feedback pin should
be routed to the target node, to sense the node voltage accurately (DAC8544 gain error is typically low, around 1
mV).
Figure 32 shows a typical parallel interface connection between the DAC8544 and a MSP430 microcontroller.
The setup for the interface shown uses ports 4 and 5 of the MSP430 to send or receive the 16-bit data while bits
0-7 of port 2 provides the control signals for the DAC. When data is to be transmitted to the DAC8544, the data
is made available to the DAC via P4 and P5, and P2.1 is taken low. The MSP430 then toggles P2.0 from
high-to-low and back to high, transferring the 16-bit data to the DAC. This data is loaded into the DAC register by
applying a rising edge to P2.4. The remaining five I/O signals of P2 shown in the figure control the reset,
power-down, and data format functions of the DAC. Depending on the specific requirements of a given
application, these pins may be tied to IOGND or IOV
DD
, enabling the desired mode of operation.
15