Rainbow Electronics DS4412 - Manual (Page 6)

Detailed Description
The DS4412 contains two I 2 C adjustable-current
sources that are each capable of sinking and sourcing
current. Each output, OUT0 and OUT1, has 15 sink and
15 source settings that are programmed through the
I
2 C interface. The full-scale ranges and corresponding
step sizes of the outputs are determined by external
resistors, connected to pins FS0 and FS1, which can
adjust the output currents over a 4:1 range. The formula
to determine the positive and negative full-scale current
ranges for each of the four outputs is given by:
R FS = (V RFS / I FS ) x (15 / 1.974)
where V RFS is the R FS voltage (see DC Electrical
Characteristics ), and R FS is the external resistor value.
On power-up, the DS4412 outputs zero current. This is
done to prevent it from sinking or sourcing an incorrect
current before the system host controller has had a
chance to modify the device’s setting.
As a source for biasing instrumentation or other cir-
cuits, the DS4412 provides a simple and inexpensive
current source with an I
2 C interface for control. The
adjustable full-scale range allows the application to get
the most out of its 4-bit sink or source resolution.
When used in adjustable power-supply applications
(see
Typical Operating Circuit) , the DS4412 does not
affect the initial power-up supply voltage because it
defaults to providing zero output current on power-up.
As it sources or sinks current into the feedback voltage
node, it changes the amount of output voltage required
by the regulator to reach its steady state operating
point. Using the external resistor, R
FS , to set the output
current range, the DS4412 provides some flexibility for
adjusting the range over which the power supply can
be controlled or margined.
Memory Organization
The DS4412’s current sources are controlled by writing
to the memory addresses in Table 1.
The format of each output control register is given by:
Where:
Example: R
FS0
= 4.8k Ω and register 0xF8h is written to
a value of 0x8Ah. Calculate the output current.
I
FS
= (0.607V / 4.8k Ω) x (15 / 1.974) = 949.85µA
The MSB of the output register is 1, so the output is
sourcing the value corresponding to position Ah (10 dec-
imal). The magnitude of the output current is equal to:
949.85µA x (10 / 15) = 633.23µA
DS4412
Dual-Channel, I
2
C Adjustable
Sink/Source Current DAC
6 _______________________________________________________________________________________
Table 1. Memory Addresses
MEMORY ADDRESS
(HEXADECIMAL)
CURRENT SOURCE
0xF8 OUT0
0xF9 OUT1
MSB LSB
S X X X D 3 D 2 D 1 D 0
BIT NAME FUNCTION
POWER-ON
DEFAULT
S Sign Bit
Determines if DAC sources
or sinks current. For sink
S = 0, for source S = 1.
0b
X Reserved Reserved. XXX
D X Data
4-Bit Data Word Controlling
DAC Output. Setting 0000b
outputs zero current
regardless of the state of the
sign bit.
0000b