2001-2012 Microchip Technology Inc. DS21450D-page 7
TC650/TC651
4.0 DETAILED DESCRIPTION
The TC650/TC651 acquire and convert their junction
temperature (T
J
) information from an on-chip, solid-
state sensor with a typical accuracy of ±1°C. The tem-
perature data is digitally stored in an internal register.
The register is compared with pre-defined threshold val-
ues. The six threshold values are equally distributed
over a pre-defined range of temperatures (see Table 4-
1). The TC650/TC651 control the speed of a DC brush-
less fan using a fractional speed-control scheme. The
output stage requires only a 2N2222-type, small-signal
BJT for fans up to 300 mA. For larger current fans (up
to 1 amp), a logic-level N-channel MOSFET may be
used. In addition to controlling the speed of the fan, the
TC650/TC651 include an on-chip overtemperature
alarm (T
OVER
) that gives a low signal when the temper-
ature of the chip exceeds T
H
by 10°C (typical). This fea-
ture eliminates the need for a separate temperature
sensor for overtemperature monitoring. Figure 4-1
shows the block diagram of the device.
FIGURE 4-1: Functional Block Diagram.
4.1 PWM Output
The PWM pin is designed to drive a low-cost transistor
or MOSFET as the low-side, power-switching element
in the system. This output has an asymmetric
complementary drive and is optimized for driving NPN
transistors or N-channel MOSFETs. Since the system
relies on PWM rather than linear power control, the
dissipation in the power switch is kept to a minimum.
Generally, very small devices (TO-92 or SOT
packages) will suffice. The frequency of the PWM is
about 15 Hz. The PWM is also the time base for the
Start-up Timer (see Section 4.2 “Start-Up Timer”).
The PWM duty cycle has a range of 40% to 100% for
the TC650 and 50% to 100% for the TC651.
4.2 Start-Up Timer
To ensure reliable fan start-up, the Start-up Timer turns
PWM high for about 2 seconds whenever the fan is
started from the off state. This occurs at power-up and
when coming out of Shutdown mode.
4.3 Overtemperature Alert (T
OVER
)
This pin goes low when the T
H
set point is exceeded by
10°C (typical). This indicates that the fan is at
maximum drive and the potential exists for system
overheating; either heat dissipation in the system has
gone beyond the cooling system's design limits or
some fault exists (such as fan bearing failure or an air-
flow obstruction). This output may be treated as a
“System Overheat” warning and be used to either
trigger system shutdown or bring other fans in the
system to full speed. The fan will continue to run at full
speed while T
OVER
is asserted. Built-in hysteresis
prevents T
OVER
from “chattering” when the measured
temperature is at or near the T
H
+ 10°C trip point. As
temperature falls through the T
H
+ 10°C trip point, hys-
teresis maintains the T
OVER
output low until the
measured temperature is 5°C above the trip point
setting.
4.4 Shutdown (SHDN)
The fan can be unconditionally shut down by pulling
the SHDN
pin low. During shutdown, the PWM output
is low; ideal for notebook computers and other portable
applications where you need to change batteries and
must not have the fan running at that time. Thermal
monitoring and T
OVER
are still in operation during
shutdown. I
DD
shutdown current is around 50 µA.
4.5 Auto-shutdown Mode
The TC651 features auto-shutdown. When the
temperature is below the factory set point at minimum
speed (T
L
), PWM is low and the fan is automatically
shut off (Auto-shutdown mode). This feature is ideal for
notebook computers and other portable equipment that
need to conserve as much battery power as possible
and, thus, run a fan when it is only absolutely needed.
The TC651 will continue to be active in order to monitor
temperature for T
OVER
. The TC651 exits Auto-
shutdown mode when the temperature rises above the
factory set point (T
1
).
Temperature Sensor
Duty Cycle
Logic
Control
A/D Converter
Oscillator
Temperature
Set Point and
Trim Range
SHDN
T
OVER
PWM
V
DD
V+
