Current DAC

6

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Detailed Description

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

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.

pins) to attain the desired full-scale current range is:

Equation 1:

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

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-

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.

addresses determined by the two address inputs, A0

and A1. The address inputs should be connected to

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

GND

20h

GND

60h

GND

A0h

E0h

Table 2. Memory Addresses

MEMORY ADDRESS

(HEX)

CURRENT SOURCE

F8h

OUT0

F9h

OUT1

FAh*

OUT2*

FBh*

OUT3*

*Only for DS4424.

MSB

LSB

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

Data

7-Bit Data Controlling DAC

Output. Setting 0000000b

outputs zero current regardless

of the state of the sign bit.

0000000b