TC647 Microchip, TC647 Datasheet - Page 5

• Manufacturer Part Number: TC647
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

3.0
3.1
The PWM circuit consists of a ramp generator and
threshold detector. The frequency of the PWM is deter-
mined by the value of the capacitor connected to the C
input. A frequency of 30 Hz is recommended
(C
Start-up Timer (see Section 3.3, “Start-Up Timer”). The
PWM voltage control range is 1.25V to 2.65V (typical)
for 0% to 100% output duty cycle.
3.2
The V
or MOSFET as the low side power switching element in
the system. Various examples of driver circuits will be
shown throughout this data sheet. This output has an
asymmetric complimentary drive and is optimized for
driving NPN transistors or N-channel MOSFETs. Since
the system relies on PWM rather than linear control,
the power dissipation in the power switch is kept to a
minimum. Generally, very small devices (TO-92 or SOT
packages) will suffice.
3.3
To ensure reliable fan start-up, the Start-up Timer turns
the V
the fan is started from the off state. This occurs at
power up and when coming out of shutdown mode. If
the PWM frequency is 30 Hz (C
start-up time will be approximately one second. If a
fault is detected, the Diagnostic Timer is triggered
once, followed by the Start-up Timer. If the fault
persists, the device is shut down (see Section 3.6,
“FAULT Output”).
3.4
If V
into shutdown mode. This can be accomplished by
driving V
external transistor, as shown in Figure 3-1. All functions
are suspended until the voltage on V
higher than V
below V
shutdown
unconditionally inactive in shutdown mode.
A small amount of hysteresis, typically one percent of
V
V
V
include this hysteresis plus adequate margin to
account for normal variations in the absolute value of
the threshold and hysteresis.
DD
REL
REL
2002 Microchip Technology Inc.
F
MIN
= 1 µF). The PWM is also the time base for the
(50 mV at V
OUT
OUT
threshold. The levels specified for V
(Pin 3) is pulled below V
in Section 1.0, “Electrical Characteristics”,
SHDN
MIN
Start-Up Timer
Shutdown Control (Optional)
DETAILED DESCRIPTION
PWM
V
output on for 32 cycles of the PWM whenever
pin is designed to drive a low cost transistor
OUT
and
with an open-drain logic signal or using an
REL
will always result in complete device
Output
DD
(0.85V @ V
reset.
= 5.0V), is designed into the V
The
DD
SHDN
F
= 5.0V). Pulling V
= 1 µF), the resulting
FAULT
, the TC647 will go
MIN
output
SHDN
becomes
SHDN
and
MIN
is
F
/
3.5
The SENSE input (Pin 5) is connected to a low value
current sensing resistor in the ground return leg of the
fan circuit. During normal fan operation, commutation
occurs as each pole of the fan is energized. This
causes brief interruptions in the fan current, seen as
pulses across the sense resistor. If the device is not in
shutdown mode, and pulses are not appearing at the
SENSE input, a fault exists.
The short, rapid change in fan current (high dI/dt)
causes a corresponding dV/dt across the sense
resistor, R
differentiated and converted to a logic-level, pulse-train
by C
The presence and frequency of this pulse-train is a
direct indication of fan operation. See Section 5.0,
“Typical Applications”, for more details.
3.6
Pulses appearing at SENSE due to the PWM turning
on are blanked with the remaining pulses being filtered
by a missing pulse detector. If consecutive pulses are
not detected for 32 PWM cycles ( 1 Sec if C
the Diagnostic Timer is activated and V
high continuously for three PWM cycles ( 100 msec if
C
the Start-up Timer is triggered (see Section 3.3, “Start-
Up Timer”). This should clear a transient fault condition.
If the missing pulse detector times out again, the PWM
is stopped and FAULT goes low. When FAULT is
activated due to this condition, the device is latched in
shutdown mode and will remain off indefinitely.
The TC647 may be configured to continuously attempt
fan restarts, if so desired.
CAUTION: Shutdown mode is unconditional. That is,
the fan will not be activated regardless of the voltage
at V
producing activity in the system is at a negligible
level.
F
Note:
= 1 µF). If a pulse is not detected within this window,
SENSE
IN
. The fan should not be shut down until all heat
SENSE Input
(FanSense
FAULT Output
At this point, action must be taken to restart
the fan by momentarily pulling V
V
case, the fan cannot remain disabled due
to a fault condition as severe system dam-
age could result. If the fan cannot be
restarted, the system should be shut down.
and the internal signal processing circuitry.
SENSE
SHDN
. The waveform on R
, or cycling system power. In either
™
Technology)
DS21447C-page 5
TC647
OUT
MIN
SENSE
F
is driven
= 1 µF),
below
is