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LTC1695 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1695 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LTC1695 11 APPLICATIONS INFORMATION OVERVIEW The LTC1695 is a 5V brushless DC fan speed controller. Fan speed is controlled by linear regulating the applied voltage to the fan. To program fan speed, a system controller or microprocessor first sends a 6-bit digital code to the LTC1695 via a 2-wire SMBus/I2C interface. The LTC1695’s DAC then converts this digital code into a voltage reference. Finally, the LTC1695’s op amp loop regulates the gate bias of the internal P-channel pass transistor to control the corresponding output voltage. The LTC1695 is designed for portable, power-conscious systems that utilize small 5V brushless DC fans. These fans are increasingly popular in providing efficient cooling solutions in a small footprint. Smaller fans allow a user to employ multiple fans at strategic physical locations to govern a system’s thermal airflow (“air duct” concept). These brushless DC fans also make use of the 5V supply used by the main digital/analog circuitry, removing the need for a 12V supply required by higher power fans. The LTC1695’s P-channel linear regulator control ap- proach offers the lowest solution component count, the smallest PCB board space consumed, wide fan speed control range and low acoustical/electrical generated noise. Thermal concerns over the use of a linear regulator topol- ogy are eliminated by the fan’s generally resistive behav- ior. As the LTC1695 DAC codes are changed to lower the output voltage, the voltage across the internal P-channel pass transistor increases. However, the fan’s load current decreases almost linearly, thereby controlling power dis- sipation in the regulator. For example, a Micronel 5V, 0.7W fan (40mm2 • 12mm) draws 80mA at 4V and 20mA at 2V. Thus the P-channel pass transistor’s power loss de- creases from 80mW to 60mW. The LTC1695 incorporates several features to simplify the overall solution including a boost start timer to ensure fan start-up, output current limiting and thermal shutdown. The boost start timer is enabled via the SMBus commands and programs VOUT to full scale for 250ms before regulat- ing at the user programmed output voltage. This elimi- nates potential fan start-up problems at lower output voltage DAC codes. The LTC1695’s thermal shutdown circuit trips if die tem- perature exceeds 155 °C. The P-channel pass transistor is shut off and bit D6 in the LTC1695’s SMBus data register is set high. If an overload or short-circuit condition occurs, the LTC1695’s current-limit circuitry limits output current to 390mA typically. In addition, bit D7 in the SMBus data register is set high. The readback capability of the LTC1695 allows the host controller to monitor the status of the D6 and D7 bits for fault conditions. SMBus Serial Interface The LTC1695 is an SMBus slave device that supports both SMBus send byte and receive byte protocol (Figure 2) with two interface signals, SCL and SDA. The SMBus host initiates communication with the LTC1695 through a start bit followed by a 7-bit address code and a write bit. Each SMBus slave device in the system com- pares the address code with its specific address. For send byte and receive byte protocol, the write bit is LOW and HIGH respectively. If selected, the LTC1695 acknowl- edges by pulling SDA low. If send byte protocol is used, the host issues an 8-bit command code. After receiving the entire command byte, the LTC1695 again acknowledges by pulling SDA low. At the falling edge of the acknowledge pulse, the LTC1695’s DAC latches in the new command byte from its shift register. If receive byte protocol is used, the LTC1695 acknowl- edges by pulling SDA low after the write bit. The LTC1695 then transmits the data byte. After the host receives the entire data byte, the cycle is terminated by a “NOT Ac- knowledge” bit and a stop bit. |
Similar Part No. - LTC1695_15 |
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Similar Description - LTC1695_15 |
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