AD5821

Rev. 0 | Page 11 of 16

THEORY OF OPERATION

The AD5821 is a fully integrated, 10-bit digital-to-analog

converter (DAC) with 120 mA output current sink capability.

It is intended for driving voice coil actuators in applications

such as lens autofocus, image stabilization, and optical zoom.

The circuit diagram is shown in Figure 20. A 10-bit current

output DAC coupled with Resistor R generates the voltage that

drives the noninverting input of the operational amplifier. This

sink current required to drive the voice coil.

Therefore, the temperature coefficient and any nonlinearities

over temperature are matched, and the output drift over tempera-

ture is minimized. Diode D1 is an output protection diode.

3.3Ω

R

AD5821

D1

10-BIT

CURRENT

OUTPUT DAC

SDA

AGND

XSHUTDOWN

DGND

SCL

DGND

INTERFACE

REFERENCE

POWER-ON

RESET

Figure 20. Block Diagram Showing Connection to Voice Coil

SERIAL INTERFACE

2-wire serial protocol. Data can be written to or read from the

DAC at data rates of up to 400 kHz. After a read operation, the

contents of the input register are reset to all 0s.

generate the serial clock (SCL) and read and write data on the

serial data line (SDA) to and from slave devices such as the

to the SDA line and their SCL pin connected to the SCL line of

depends on the data rate, bus capacitance, and the maximum load

SCL

SDA

DEVICE

AD5821

DEVICE

DEVICE

1.8V

When the bus is idle, SCL and SDA are both high. The master

device initiates a serial bus operation by generating a start

condition, which is defined as a high-to-low transition on the

SDA low while SCL is high. The slave device connected to the

bus responds to the start condition and shifts in the next eight

data bits under control of the serial clock. These eight data bits

consist of a 7-bit address, plus a read/write (R/W) bit that is 0 if

data is to be written to a device, and 1 if data is to be read from a

The address of the AD5821 is 0001100; however, 0001101,

0001110, and 0001111 address the part because the last two bits

are unused/don’t cares (see Figure 22 and Figure 23). Because the

address plus the R/W bit always equals eight bits of data, the write

address of the AD5821 is 00011000 (0x18) and the read address

is 00011001 (0x19) (see Figure 22 and Figure 23).

At the end of the address data, after the R/W bit, the slave

device that recognizes its own address responds by generating

an acknowledge (ACK) condition. This is defined as the slave

device pulling SDA low while SCL is low before the ninth clock

pulse and keeping it low during the ninth clock pulse. Upon

receiving ACK, the master device can clock data into the AD5821

in a write operation, or it can clock it out in a read operation.

Data must change either during the low period of the clock

(because SDA transitions during the high period define a start

condition, as described previously), or during a stop condition,

as described in the Data Format section.

an ACK at the end of each block. Because the AD5821 requires

10 bits of data, two data-words must be written to it when a

write operation occurs, or read from it when a read operation

occurs. At the end of a read or write operation, the AD5821

acknowledges the second data byte. The master generates a stop

condition, defined as a low-to-high transition on SDA while SCL

is high, to end the transaction.

DATA FORMAT

Data is written to the AD5821 high byte first, MSB first, and is

shifted into the 16-bit input register. After all data is shifted in,

data from the input register is transferred to the DAC register.

Because the DAC requires only 10 bits of data, not all bits of the

input register data are used. The MSB is reserved for an active-

high, software-controlled, power-down function. Bit 14 is unused;

Bit 13 to Bit 4 correspond to the DAC data bits, Bit 9 to Bit 0.

Bit 3 to Bit 0 are unused.

During a read operation, data is read in the same bit order.