DN238 - SOT-23 SMBus Fan Speed Controller Extends Battery Life and Reduces Noise

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SOT-23 SMBus Fan Speed Controller Extends Battery Life and
Reduces Noise – Design Note 238
David Canny
Introduction
Battery run times for notebook computers and other
portable devices can be improved and acoustic noise
reduced by using Linear Technology’s LTC®1695 to
optimize the operation of these products’ internal
cooling fans. The LTC1695 comes in a SOT-23 package
and provides all the functions necessary for a system
controller or microcontroller to regulate the speed of
a typical 5V/≤1W fan via a 2-wire SMBus interface.
By varying the fan speed according to the system’s
instantaneous cooling requirements, the power consumption of the cooling fan is reduced and battery
run times are improved. Acoustic noise is practically
eliminated by operating the fan below maximum speed
when the thermal environment permits. Designers also
have the option of controlling the temperature in portable
devices by using feedback from a temperature sensor
to control the fan speed.
Figure 1 shows a typical application. Fan speed is easily programmed by sending a 6-bit digital code to the
LTC1695 via the SMBus. This code is converted into
an analog reference voltage that is used to regulate the
output voltage of the LTC1695’s internal linear regulator. The system controller can enable an optional boost
feature that eliminates fan start-up problems by output5V
+
3.3μF
1
2
3
SYSTEM
CONTROLLER
VCC
ting 5V to the fan for 250ms before lowering the output
voltage to its programmed value. Another important
feature is that the system controller can read overcurrent
and overtemperature fault conditions from information
stored in the LTC1695. The part’s SMBus address is
hard-wired internally as 1110 100 (MSB to LSB, A6 to
A0) and the data code bits D0 to D6 are latched at the
falling edge of the SMBus Data Acknowledge signal
(D6 is a Boost-Start Enable bit and D5 to D0 translate
to a linearly proportional output voltage, 00–3F hex =
0V–5V). The LTC1694, which also appears in Figure 1,
is a dual SMBus accelerator/pull-up device that may
be used in conjunction with the LTC1695. Table 1 lists
some 5V brushless DC fans suitable for typical LT1695
fan speed control applications.
Boost-Start Timer, Thermal Shutdown and
Overcurrent Clamp Features
A DC fan typically requires a starting voltage higher than
its minimum stall voltage. For example, one Micronel 5V
fan requires a 3.5V starting voltage, but once started, it
will run until its terminal voltage drops below 2.1V (its
stall voltage). Thus, the user must ensure that the fan
starts up properly before programming the fan voltL, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
VOUT
5
+
LTC1695
GND
SCL
SDA
4.7μF
4
SUNON
KED0502PFB1-8
5VDC FAN
0.6W
DN238 F01
5V
1
5
VCC SMB1
LTC1694
2
4
GND SMB2
Figure 1. SMBus Fan Speed Controller
08/00/238_conv
age to a value that is lower than the starting voltage.
Monitoring the fan’s DC current for stall conditions
does not help because some fans consume almost
the same amount of current at the same terminal voltage in both stalled and operating conditions. Another
approach is to detect the absence of fan commutation
ripple current. However, this is complex and requires
customization for the characteristics of each brand of
fan. The LTC1695 offers a simple and effective solution
through the use of a boost-start timer. By setting the
Boost-Start Enable bit high via the system controller,
the LTC1695 outputs 5V for 250ms to the fan before
lowering the voltage to its programmed value (see
Figure 2 for the start-up voltage profile).
During a system controller Read command, bits 6 and
7 in the data byte code are defined as the Thermal
Shutdown Status (THE) and the Overcurrent Fault (OCF),
respectively. The remaining bits of the data byte (0 to 5)
are set low during host read back. The LTC1695 shuts
down its PMOS pass transistor and sets the THE bit
high if die junction temperature exceeds 155°C. During
an overcurrent fault, the LTC1695’s overcurrent detector
sets the OCF bit high and actively clamps the output
current to 390mA. This protects the LTC1695’s PMOS
pass transistor. Under dead short conditions (VOUT =
0), although the LTC1695 clamps the output current,
the large amount of power dissipated on the chip will
force the LTC1695 into thermal shutdown. These dual
protection features protect both the IC and the fan,
but more importantly, alert the host to system thermal
management faults. During a fault condition, the SMBus
logic continues to operate so that the host can poll the
fault status data.
Conclusion
The LTC1695 improves battery run times and reduces
acoustic noise in portable equipment. In addition, it
provides important performance and protection features by controlling the operation of the equipment’s
cooling fan. It comes in a SOT-23 package and is easily
programmed via the SMBus interface. Table 1. Some 5V DC Fans’ Characteristics
MANUFACTURER
PART NUMBER
AIRFLOW
(CFM)
POWER
(W)
SIZE(L • W • H)mm3
SUNON
KDE0501PFB2-8
0.65
0.50
20 • 20 • 10
ATC
AD0205HB-G51
0.80
0.45
25 • 25 • 10
SUNON
KDE0502PFB2-8
1.70
0.60
25 • 25 • 10
SUNON
KDE0503PFB2-8
3.20
0.60
30 • 30 • 10
SUNON
KDE0535PFB2-8
4.80
0.70
35 • 35 • 10
Micronel
F41MM-005XK-9
6.10
0.70
40 • 40 • 12
LTC1695
OUTPUT VOLTAGE
2V/DIV
100ms/DIV
DN238 F02
Figure 2. Fan Start-Up Voltage Profile
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn238f_conv LT/TP 0800 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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© LINEAR TECHNOLOGY CORPORATION 2000
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