TC649 datasheet(5/28 Pages) MICROCHIP

5 / 28 page

 2002 Microchip Technology Inc.

DS21449C-page 5

TC649

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.

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 (

≅1Sec 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.

TC649 Datasheet(PDF) 5 Page - Microchip Technology

TC649 Datasheet(HTML) 5 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL