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AD5821BCBZ-REEL1 Fiches technique(PDF) 11 Page - Analog Devices |
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AD5821BCBZ-REEL1 Fiches technique(HTML) 11 Page - Analog Devices |
11 / 16 page 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 voltage also appears across the RSENSE resistor and generates the sink current required to drive the voice coil. Resistor R and Resistor RSENSE are interleaved and matched. 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. RSENSE 3.3Ω R AD5821 D1 10-BIT CURRENT OUTPUT DAC SDA AGND XSHUTDOWN VDD DGND SCL ISINK DGND VDD I2C SERIAL INTERFACE REFERENCE POWER-ON RESET Figure 20. Block Diagram Showing Connection to Voice Coil SERIAL INTERFACE The AD5821 is controlled using the industry-standard I2C 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. I2C BUS OPERATION An I2C bus operates with one or more master devices that generate the serial clock (SCL) and read and write data on the serial data line (SDA) to and from slave devices such as the AD5821. All devices on an I2C bus have their SDA pin connected to the SDA line and their SCL pin connected to the SCL line of the master device. I2C devices can only pull the bus lines low; pulling high is achieved by pull-up resistors, RP. The value of RP depends on the data rate, bus capacitance, and the maximum load current that the I2C device can sink (3 mA for a standard device). SCL SDA I2C MASTER DEVICE AD5821 I2C SLAVE DEVICE I2C SLAVE DEVICE RP RP 1.8V Figure 21. Typical I2C Bus 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 device. Each slave device on an I2C bus must have a unique address. 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. I2C data is divided into blocks of eight bits, and the slave generates 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. |
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