LTC1695CS5 datasheet(15/20 Pages) LINER

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LTC1695

scale (VFS) until junction temperature decreases to

approximately 105

°C. This extended timer period is an

attempt to cool down the system and the LTC1695 by

running the fan at full speed. In most cases, such elevated

ambient temperatures require the fan to run at full speed

anyway. The remaining LTC1695’s functionality remains

unchanged.

Thermal Shutdown, Overcurrent

The LTC1695 shuts down the P-channel linear regulator if

die temperature exceeds 155

°C typically. The thermal

shutdown circuitry employs about 30

°C of hysteresis. As

previously mentioned, the LTC1695 sets bit 6 (THE) in the

SMBus data byte register HIGH during thermal shutdown

conditions. During a fault condition, the LTC1695’s SMBus

logic continues to operate so that the SMBus host can read

back the fault status data.

During an overload or short-circuit fault condition, the

LTC1695’s current-limit detector sets bit 7 (OCF) in the

SMBus data byte register HIGH and actively limits output

current to 390mA typically. This protects the LTC1695’s

P-channel pass transistor. Under dead short conditions

with VOUT = 0V, the LTC1695 also clamps the output

current. However, the increased power dissipation

(5V • 390mA = 1.95W) eventually forces the LTC1695 into

thermal shutdown. The LTC1695 will then thermally oscil-

late until the fault condition is removed.

During recovery from thermal shutdown (typically 125

°C),

the LTC1695 automatically activates the boost start timer,

programming the fan voltage to full scale for 250ms

(TBST_ST), before switching to the user programmed out-

put voltage value. This again eliminates fan start-up prob-

lems if the thermal shutdown fault occurred while the fan

was previously operating at an output voltage below the

fan’s starting voltage. In addition, as discussed, the boost

start timer will keep VOUT at VFS for an extended time

period beyond TBST_ST until the LTC1695’s junction tem-

perature drops below 105

°C.

The LTC1695’s protection features protect itself, the fan,

and more importantly alerts the SMBus host to any system

thermal management fault conditions.

For further information, refer to the Junction Temperature

Increase (above ambient temperature) vs ILOAD graph in

the Typical Performance Characteristics section. This

graph provides a fast and simple junction temperature

estimation with various VOUT (DAC code) and ILOAD

combinations for a typical application.

Boost Start Timer

In general, a 5V brushless DC fan starts at a voltage value

higher than the voltage at which it stalls. This behavior is

directly attributed to the force necessary to overcome the

back EMF of the fan. For example, one fan measured

started at 3.5V but operated until its terminal voltage fell

below 2.1V. Therefore, users must ensure start-up in the

fan before programming the fan voltage to a value lower

than the starting voltage. Monitoring the fan’s DC current

for a stalled condition does not work due to the fan’s

resistive nature. Fans can sink load current even though

they are not rotating. Other approaches include detecting

absence of the fan’s commutation ripple current and

tachometers. In general, these approaches are more com-

plex, require more circuitry, add cost and have to be

customized for the specific fan used.

The LTC1695 contains a programmable boost start timer

offering three flexible solutions to the user:

1.) Enable the boost start timer bit (D6 in the DAC com-

mand code). Each time a new output voltage is pro-

grammed, the timer forces VOUT to full scale (4.922V

nominal with VCC = 5V) for 250ms before assuming the

programmed output voltage value. This ensures fan start

up even if the programmed output voltage is below the

fan’s start threshold.

2.) Users may also choose to use a software timer routine

inside the host controller to power the DC fan, at full scale,

for a programmed time period before programming VOUT

to a lower desired DAC output voltage code.

3.) Users may choose a tachometer fan that feedbacks its

speed to the SMBus host. If fan stall conditions are

detected, the SMBus host re-programs the LTC1695.

Beyond a typical 125

°C LTC1695 junction temperature,

the boost start timer (if activated) maintains VOUT at full

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