MIC502_11 Datasheet, PDF (9/15 Pages) Micrel Semiconductor – Fan Management IC

Micrel, Inc.
Functional Description
Oscillator
A capacitor connected to CF determines the frequency
of the internal time base which drives the state-machine
logic and determines the PWM frequency. This operating
frequency will be typically 30Hz to 60Hz. (C_F = 0.1µF for
30Hz.)
Pulse-Width Modulator
A triangle-wave generator and threshold detector
comprise the internal pulse-width modulator (PWM). The
PWM’s output duty-cycle is determined by the higher of
V_T1 or V_T2. A typical voltage range of 30% to 70% of V_DD
applied to the V _T1 and V _T2 pins corresponds to 0% to
100% duty-cycle. Since at least one of the control
voltage inputs is generally from a thermistor-resistor
divider connected to V_DD, the PWM out-put duty cycle
will not be affected by changes in the supply voltage.
Driver Output
OUT is a complementary push-pull digital output with
asymmetric drive (approximately 10mA source, 1mA
sink, see “Electrical Characteristics”). It is optimized for
directly driving an NPN transistor switch in the fan’s
ground-return. See “Applications Information” for circuit
details.
Shutdown/Reset
Internal circuitry automatically performs a reset of the
MIC502 when power is applied. The MIC502 may be
shut down at anytime by forcing V_T1 below its V_IL
threshold. This is typically accomplished by connecting
the V_T1 pin to open-drain or open-collector logic and
results in an immediate and asynchronous shutdown of
the MIC502. The OUT and /OTF pins will float while V _T1
is below V_IL.
If V _T1 then rises above V_IH, a device reset occurs. Reset
is equivalent to a power-up condition: the state of /OTF
is cleared, a startup interval is triggered, and normal fan
operation begins.
Startup Interval
Any time the fan is started from the off state (power-on
or coming out of sleep mode or shutdown mode), the
PWM output is automatically forced high for a startup
interval of 64× t_PWM. Once the startup interval is
complete, PWM operation will commence and the duty-
cycle of the output will be determined by the higher of
V_T1 or V_T2.
Overtemperature Fault Output
/OTF is an active-low, open-collector logic output. An
over-temperature condition will cause /OTF to be
asserted. An overtemperature condition is determined by
V_T1 exceeding the normal operating range of 30% to
November 2006
MIC502
70% of V_DD by > 7% of V_DD. Note that V_OT is guaranteed
by design to always be higher than V_PWM(max).
Sleep Mode
When V_T1 and V_T2 fall below V_SLP, the system is deemed
capable of operating without fan cooling and the MIC502
enters sleep mode and discontinues fan operation. The
threshold where the MIC502 enters sleep mode is deter-
mined by V_SLP. Connecting the V_SLP pin to ground
disables sleep mode.
Once in sleep mode, all device functions cease (/OTF in-
active, PWM output off) unless V_T1 or V_T2 rise above
V_WAKE. (V_WAKE = V_SLP + V_HYST). V_HYST is a fixed amount of
hysteresis added to the sleep comparator which
prevents erratic operation around the V_SLP operating
point. The result is stable and predictable thermostatic
action: whenever possible the fan is shut down to reduce
energy consumption and acoustic noise, but will always
be activated if the system temperature rises.
If the device powers-up or exits its reset state, the fan
will not start unless V_T1 or V_T2 rises above V_WAKE.
System Operation
Power Up
• A complete reset occurs when power is applied.
• OUT is off (low) and /OTF is inactive (high/floating).
• If V_T1 < V_IL, the MIC502 remains in shutdown.
• The startup interval begins. OUT will be on (high) for
64 clock cycles (64 × t_PWM).
• Following the startup interval, normal operation
begins.
POWER ON
Reset Startup Timer;
Deassert /OTF;
OUT Off (Low).
VT1 < VIL
?
YES
NO
VT1 > VOT
?
YES
Assert /OTF While
VT1 > VOT
NO
NO
OUT Held On (High)
During Startup
Interval.
Startup Interval
Finished
?
YES
Deassert OUT
(OUT = Low)
NORMAL
OPERATION
Figure 3. Power-Up Behavior
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