TC649VUATR View Datasheet(PDF) - Microchip Technology
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TC649VUATR Datasheet PDF : 28 Pages
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3.0 DETAILED DESCRIPTION
3.1 PWM
The PWM circuit consists of a ramp generator and
threshold detector. The frequency of the PWM is
determined by the value of the capacitor connected to
the CF input. A frequency of 30 Hz is recommended for
most applications (CF = 1 µF). The PWM is also the
time base for the 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 VOUT Output
The VOUT pin is designed to drive a low cost transistor
or MOSFET as the low side power switching element in
the system. Various examples of driver circuits will be
shown throughout the datasheet. This output has
asymmetric complementary 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 Start-Up Timer
To ensure reliable fan start-up, the Start-up Timer turns
the VOUT output on for 32 cycles of the PWM whenever
the fan is started from the off state. This occurs at
power-up and when coming out of shutdown or auto-
shutdown mode. If the PWM frequency is 30 Hz (CF =
1 µF) the resulting start-up time will be approximately
one second. If a fan fault is detected (see Section 3.5,
FAULT Output), the Diagnostic Timer is triggered once,
followed by the Start-up Timer. If the fault persists, the
device is shut down (see Section 3.5, FAULT Output).
3.4 SENSE Input
(FanSense™ Technology)
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 or auto-shutdown mode, and pulses are not
appearing at the SENSE input, a fault exists.
The short, rapid change in fan current (high dl/dt)
causes a corresponding dV/dt across the sense
resistor, RSENSE. The waveform on RSENSE is
differentiated and converted to a logic-level pulse-train
by CSENSE and the internal signal processing circuitry.
The presence and frequency of this pulse-train is a
direct indication of fan operation. See Section 5.0,
“Typical Applications”, for more details.
TC649
3.5 FAULT Output
Pulses appearing at SENSE due to the PWM turning
on are blanked, and the remaining pulses are filtered
by a missing pulse detector. If consecutive pulses are
not detected for thirty-two PWM cycles (≅1 Sec if
CF = 1 µF), the Diagnostic Timer is activated, and
VOUT is driven high continuously for three PWM cycles
(≅100 msec if CF = 1 µF). If a pulse is not detected
within this window, the Start-up Timer is triggered (see
Section 3.3, “Start-up Timer”). This should clear a tran-
sient 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 TC649 is thus prevented from
attempting to drive a fan under catastrophic fault
conditions.
One of two things will restore operation: Cycling power
off and then on again; or pulling VIN below VSHDN and
releasing it to a level above VREL. When one of these
two conditions is satisfied, the normal start-up cycle is
triggered and operation will resume, provided the fault
has been cleared.
3.6 Auto-Shutdown Mode
If the voltage on VIN becomes less than the voltage on
VAS, the fan is automatically shut off (auto-shutdown
mode). The TC649 exits auto-shutdown mode when
the voltage on VIN becomes higher than the voltage on
VAS by VHAS (the auto-shutdown hysteresis voltage,
see Figure 3-1). The Start-up Timer is triggered and
normal operation is resumed upon exiting auto-shut-
down mode. The FAULT output is unconditionally
inactive in auto-shutdown mode.
2002 Microchip Technology Inc.
DS21449C-page 5
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