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LTC1695

SMBus/I2C Fan Speed Controller in SOT-23

Linear Technology 

LTC1695

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.

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