Rainbow Electronics MAX5547 - Manual (Page 8)

MAX5547
Detailed Description
The MAX5547 10-bit, dual-range, current-sink DAC
operates with serial data clock rates up to 10MHz. The
double-buffered DAC input consists of a 16-bit input
register and two 10-bit DAC registers, followed by a
current-steering array (see the Functional Diagram).
The MAX5547 sinks full-scale output currents of 1.2mA
or 3.6mA per DAC. Each DAC’s full-scale current can
be independently programmed.
Operating from a single +2.7V to +5.25V supply, the
MAX5547 typically consumes 1mA. The MAX5547
operates from an internal +2.5V reference or an exter-
nal reference in the +2.45V to +2.55V range.
The MAX5547 is ideal as the digital/analog interface for
laser-diode drivers with current-controlled inputs, such
as the MAX3736 (see the Typical Operating Circuit).
Set the current levels at the MAX3736’s MODSET and
BIASSET current-controlled inputs from the MAX5547’s
DAC outputs. The MAX3736’s MODSET and BIASSET
lines set the laser driver’s desired modulation and bias
currents.
Reference Architecture and Operation
The MAX5547 operates from an internal +2.5V refer-
ence or accepts an external reference voltage source
between +2.45V and +2.55V. The internal reference is
capable of sinking up to 50µA and sourcing up to
300µA. REF serves as the input for a low-impedance
reference source in external reference mode. Bypass
REF to GND with a ceramic capacitor in the 0.1µF to
10µF range, as close to the device as possible, in both
internal and external reference modes.
During startup, when power is first applied, the
MAX5547 defaults to external reference mode, and to
the 1.2mA full-scale current-range mode. Use software
commands to select internal reference mode and
3.6mA full-scale current-range mode (see Table 1).
DAC Data
The MAX5547‘s internal registers set the DAC full-scale
output currents (I
FS ) to 1.2mA or 3.6mA (see Table 1).
The 10-bit DAC data is decoded as straight binary,
with 1 LSB = I
FS / 1023, and converted into the corre-
sponding current as shown in Table 2.
Serial Interface
The MAX5547 operates through a 3-wire, 10MHz SPI-
compatible serial interface. CS, SCLK, and DIN control
the serial interface timing and data. Ensure the SPI bus
master, typically a microcontroller (µC), runs in master
mode so that it generates the serial clock signal. Select
an SCLK frequency of 10MHz or less and set the clock
polarity (CPOL) and phase (CPHA) in the µC control
registers to opposite values. The MAX5547 operates
with SCLK idling high or low. Therefore, set CPOL = 0
and CPHA = 1, or CPOL = 1 and CPHA = 0.
Dual, 10-Bit, Current-Sink Output DAC
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Pin Description
PIN NAME FUNCTION
1 V DD
Supply Voltage. Set V DD between +2.7V to +5.25V. Bypass V DD with a 0.1µF capacitor to GND, as close to
the device as possible.
2 CS Active-Low Chip-Select Input. Set CS low to enable the serial interface.
3 SCLK Serial-Clock Input
4 DIN Serial-Data Input. DIN is clocked into the serial interface on the falling edge of SCLK.
5 GND Ground
6 REF
External Reference Input/Internal Reference Output. When programmed for internal reference mode, REF is a
+2.5V output. When programmed for external reference mode, apply a voltage between +2.45V and +2.55V
(see Table 1). Connect a 1µF ceramic capacitor from REF to GND, as close to the device as possible.
7 OUTB DAC B Current Output. OUTB sinks up to 3.6mA.
8 OUTA DAC A Current Output. OUTA sinks up to 3.6mA.
— EP Exposed Pad. Connect to GND. Do not use as the ground connection.