MC642
DETAILED OPERATING DESCRIPTION
PWM
CAUTION: Shutdown mode is unconditional. i.e., the
fan will not activate regardless of the voltage on VIN. (Note:
The fan should not be shut down until all heat–producing
activity in the system is at a negligible level.)
The PWM (Pulse Width Modulation) 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 30Hz is recommended (CF
= 1µF). The PWM is also the timebase for the startup and
fault timer (see below). The PWM voltage control range is
1.25V to 2.65V (typical) for 0% to 100% output duty cycle.
SENSE Input
The SENSE input, pin 5, is connected to a low–value
current sensing resistor in the ground return leg of the fan
circuit. Duringnormal fanoperationcommutation occurs as
each pole of the fan is energized. This commutation causes
brief interruptions in the fan current, which is seen as pulses
across the sense resistor. When the device is not in
Shutdown Mode and pulses are not appearing at the SENSE
input, a fault condition exists.
V
Output
OUT
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 are shown in the
The short, rapid changes in fan current (high dI/dt) cause
following pages.
This output has asymmetric
correspondingdV/dt pulses across the sense resistor, RSENSE
.
complementary drive and is optimized for driving NPN
transistors or N–channel MOSFET’s. 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 package) will
suffice. (See Output Drive Transistor Selection paragraph in
Applications Information section.)
The waveform on RSENSE is differentiated and converted to
a logic–level pulse–train by CSENSE and the internal signal
processing circuitry (See Figure 1). The presence and
frequency of this pulse–train is a direct indication of fan
operation. See the Applications Information section for
more details.
FAULT Output
Start–Up Timer
The MC642 detects faults in two ways:
To ensure reliable fan startup, the StartUp 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
whencoming out of shutdown mode. IfthePWMfrequency
is 30Hz (CF = 1µF), the resulting start–up time will be about
onesecond. IfaFaultisdetected(seebelow), theDiagnostic
Timer is triggered once, followed by the Startup–Up Timer.
If the fault persists, the device is shut down. See FAULT
Output below.
(1) 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 32 PWM cycles (1 Sec if CF = 1µF), the
Diagnostic Timer is activated and VOUT is driven
continuously for three PWM cycles (100msec if CF = 1µF).
If a pulse is not detected within this window, the
Startup–Timer is triggered. 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.
(Diodes D1, D2 and resistor R5 (See Figure 1) are provided
to ensure that fan restarting is the result of a fan fault, and not
an over–temperature fault. A CMOS logic OR gate may be
substituted for these components if available).
Shutdown Control (Optional)
When VMIN (pin 3) is pulled below VSHDN, the MC642 will
go into Shutdown mode. This can be accomplished by
driving VMIN with an open drain logic signal or using an
external transistor as shown in Figure 1. All functions are
suspended until the voltage on VMIN becomes higher than
VREL (0.85V @ VDD = 5.0V). Pulling VMIN below VSHDN will
WhenFAULTisactivatedduetothiscondition, thedevice
is latched in Shutdown mode and will remain off
indefinitely. Important: At this point, action must be
taken to restart the fan by momentarily pulling VMIN
below VSHDN, or by cycling system power. In either case
the fan cannot be permitted to remain disabled due to a
fault condition, as severe system damage could result. If
the fan cannot be restarted, the system should be shut
down. The MC642 may be configured to continuously
attempt fan restarts if so desired.
always result in complete device shutdown and reset. The
FAULT output is unconditionally inactive in Shutdown
mode.
Asmallamountofhysteresis, typicallyonepercentofVDD
(50mV at VDD = 5.0V), is designed into the VSHDN/VREL
threshold. The levels specified for VSHDN and VREL in the
Electrical Characteristics section include this hysteresis
plus adequate margin to account for normal variations in the
absolute value of the threshold and hysteresis.
http://onsemi.com
4
5秒后页面跳转
快速解读数据手册：HT7533引脚图说明、参数分析、技术特点
一文带你了解TMS320F28335引脚图说明、封装形式、参数详情
89C51单片机技术资料解析：参数详情、引脚说明
深入解析CS8416引脚说明、参数、技术特点
微信公众号
抖音公众号
浙公网安备 33010502006866号 浙ICP备10014259号-119
营业执照ICP证
技术参数
数据手册
如果您发现信息不准确，
