Rainbow Electronics MAX545 - Manual (Page 8)

MAX544/MAX545
+5V, Serial-Input, Voltage-Output, 14-Bit DACs
8 _______________________________________________________________________________________
Digital Interface
The MAX544/MAX545’s digital interface is a standard
3-wire connection compatible with SPI/QSPI/
MICROWIRE interfaces. The chip-select input (CS)
frames the serial data loading at the data-input pin
(DIN). Immediately following CS’s high-to-low transition,
the data is shifted synchronously and latched into the
input register on the rising edge of the serial clock input
(SCLK). After 16 bits (14 data bits, plus 2 sub-bits set to
zero) have been loaded into the serial input register, it
transfers its contents to the DAC latch on CS’s low-to-
high transition (Figure 3a). Note that if CS is not kept
low during the entire 16 SCLK cycles, data will be cor-
rupted. In this case, reload the DAC latch with a new
16-bit word.
Alternatively, for the MAX545, LDAC allows the DAC
latch to update asynchronously by pulling LDAC low
after CS goes high (Figure 3b). Hold LDAC high during
the data-loading sequence.
External Reference
The MAX544/MAX545 operate with external voltage ref-
erences from 2V to 3V. The reference voltage deter-
mines the DAC’s full-scale output voltage. Kelvin
connections are provided with the MAX545 for optimum
performance. The 2.5V MAX873A, with ±15mV initial
accuracy and a 7ppm/°C (max) temperature coeffi-
cient, is a good choice.
Power-On Reset
The MAX544/MAX545 have a power-on reset circuit to
set the DAC’s output to 0V in unipolar mode when V
DD
is first applied. This ensures that unwanted DAC output
voltages will not occur immediately following a system
power-up, such as after a loss of power. In bipolar
mode, the DAC output is set to -V
REF .
; ; ;;
CS
SCLK
DIN
MSB LSB
D13 D6 D5 D4 D3 D2 D1 D0 S1 S0
SUB-BITS
DAC
UPDATED
D12 D11 D10 D9 D8 D7
; ;;
;;;
CS
SCLK
DIN
LDAC
MSB LSB
D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 S1 S0
DAC
UPDATED
SUB-BITS
Figure 3a. MAX544/MAX545 3-Wire Interface Timing Diagram ( LDAC = DGND for MAX545)
Figure 3b. MAX545 4-Wire Interface Timing Diagram