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MIC502YN Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC502YN Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 15 page Micrel, Inc. MIC502 November 2006 9 M9999-112206 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. (CF = 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 VT1 or VT2. A typical voltage range of 30% to 70% of VDD 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 VDD, 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 VT1 below its VIL threshold. This is typically accomplished by connecting the VT1 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 VIL. If V T1 then rises above VIH, 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× tPWM. Once the startup interval is complete, PWM operation will commence and the duty- cycle of the output will be determined by the higher of VT1 or VT2. 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 VT1 exceeding the normal operating range of 30% to 70% of VDD by > 7% of VDD. Note that VOT is guaranteed by design to always be higher than VPWM(max). Sleep Mode When VT1 and VT2 fall below VSLP, 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 VSLP. Connecting the VSLP pin to ground disables sleep mode. Once in sleep mode, all device functions cease (/OTF in- active, PWM output off) unless VT1 or VT2 rise above VWAKE. (VWAKE = VSLP + VHYST). VHYST is a fixed amount of hysteresis added to the sleep comparator which prevents erratic operation around the VSLP 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 VT1 or VT2 rises above VWAKE. System Operation Power Up • A complete reset occurs when power is applied. • OUT is off (low) and /OTF is inactive (high/floating). • If VT1 < VIL, the MIC502 remains in shutdown. • The startup interval begins. OUT will be on (high) for 64 clock cycles (64 × tPWM). • Following the startup interval, normal operation begins. Reset Startup Timer; Deassert /OTF; OUT Off (Low) . VT1 >VOT ? POWER ON VT1 <VIL ? NO NO OUT Held On (High) During Startup Interval. Startup Interval Finished ? Deassert OUT (OUT = Low) YES NO Assert /OTF While VT1 >VOT NORMAL OP ERATION YES YE S Figure 3. Power-Up Behavior |
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