DS4422/DS4424
Two-/Four-Channel, I2C, 7-Bit Sink/Source
Current DAC
6
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Detailed Description
The DS4422/DS4424 contain two or four I2C adjustable
current sources that are each capable of sinking and
sourcing current. Each output (OUT0, OUT1, OUT2, and
OUT3) has 127 sink and 127 source settings that can be
controlled by the I2C interface. The full-scale ranges
and corresponding step sizes of the outputs are deter-
mined by external resistors, connected to pins FS0, FS1,
FS2, and FS3, that can adjust the output current over a
4:1 range. Pins OUT2, OUT3, FS2, and FS3 are only
available on the DS4424.
The formula to determine RFS (connected to the FSx
pins) to attain the desired full-scale current range is:
Equation 1:
Where IFS is the desired full-scale current value, VRFS is
the RFS voltage (see the DC Electrical Characteristics
table), and RFS is the external resistor value.
To calculate the output current value (IOUT) based on the
corresponding DAC value (see Table 1 for corresponding
memory addresses), use equation 2.
Equation 2:
On power-up the DS4422/DS4424 output zero current.
This is done to prevent them from sinking or sourcing an
incorrect amount of current before the system host con-
troller has had a chance to modify the device’s setting.
As a source for biasing instrumentation or other circuits,
the DS4422/DS4424 provide a simple and inexpensive
current source with an I2C interface for control. The
adjustable full-scale range allows the application to get
the most out of its 7-bit sink or source resolution.
When used in adjustable power-supply applications
(see
Typical Operating Circuit), the DS4422/DS4424 do
not affect the initial power-up voltage of the supply
because they default to providing zero output current on
power-up. As the devices source or sink current into the
feedback-voltage node, they change the amount of out-
put voltage required by the regulator to reach its steady-
state operating point. Using the external resistor, RFS, to
set the output current range, the DS4422/DS4424 pro-
vide some flexibility for adjusting the impedances of the
feedback network or the range over which the power
supply can be controlled or margined.
I2C Slave Address
The DS4422/DS4424 respond to one of four I2C slave
addresses determined by the two address inputs, A0
and A1. The address inputs should be connected to
either VCC or ground. Table 1 lists the slave addresses
determined by the address input combinations.
Memory Organization
To control the DS4422/DS4424’s current sources, write
to the memory addresses listed in Table 2.
The format of each output control register is given by:
Where:
I
D
1
I
OUT
FS
=×
AC Value dec
()
27
R
V
16
I
FS
RFS
FS
=
×
× 127
Table 1. Slave Addresses
A1
A0
SLAVE ADDRESS
(HEX)
GND
20h
GND
VCC
60h
VCC
GND
A0h
VCC
E0h
Table 2. Memory Addresses
MEMORY ADDRESS
(HEX)
CURRENT SOURCE
F8h
OUT0
F9h
OUT1
FAh*
OUT2*
FBh*
OUT3*
*Only for DS4424.
MSB
LSB
SD6
D5
D4
D3
D2
D1
D0
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
DX
Data
7-Bit Data Controlling DAC
Output. Setting 0000000b
outputs zero current regardless
of the state of the sign bit.
0000000b