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MIC24085AYML Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC24085AYML Datasheet(HTML) 9 Page - Micrel Semiconductor |
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9 / 16 page  Micrel, Inc. MIC24085 October 22, 2013 9 Revision 1.1 Functional Description Micrel’s MIC24085 is an 18V, 3A, 1MHz buck regulator with integrated MOSFETSs for low-cost high- performance applications. The MIC24085 provides basic features including enable, power good, internal soft-start, current-limit, and overtemperature protections. VCC Supply Voltage and UVLO The MIC24085 is held in the off state until VCC reaches the UVLO threshold, which has a 650mV hysteresis. A ceramic capacitor from VCC to PGND is recommended for clean operation. Enable and Soft-Start When the MIC24085 is enabled, the reference voltage is controlled to ramp-up from 0V to 0.9V in approximately 4ms. During the soft-start, the current limit is enabled to protect the device in case of a short circuit present at the output. Peak Current-Mode PWM To achieve fast transient response, the peak current- mode PWM control is employed in the MIC24085. As illustrated in the Functional Diagram, the high-side MOSFET current (ISW) is sensed and summed with a slope-compensating ramp. Then it is compared with the error amplifier output to generate a PWM signal. The loop compensation is designed internally in the MIC24085 to ensure loop stability, even with a low-ESR capacitor at the output, such as a ceramic output capacitor. PFM Mode at Light Load The MIC24085 runs in asynchronous buck mode at light load, where the bottom MOSFET is turned off when the inductor current drops to zero. This scheme prevents the output energy from being pumped back to the input, and improves the over-all efficiency. Current Limit and Frequency Foldback In the MIC24085 ISW is also used to determine over- current conditions. As the peak inductor current is limited, the output voltage of the buck converter drops as the load current increases. When the output voltage reaches 60% of regulated voltage (or feedback voltage reaches 60% of 0.9V), the switching frequency is forced to fold back, from 1MHz to 250kHz. This current-limit scheme reduces the average inductor current (even with the same peak inductor current), and improves the power consumption at the overload conditions. The current limit is always active when the regulator is enabled. Once the overcurrent condition is removed, the device runs to a soft-start and returns to a normal operation. Power Good (PG) Power good (PG) is an open drain output pin. A 10kΩ resistor to VOUT is required for pull-up. The PG pin turns low if the output voltage is below 93% of the reference voltage (0.9V). After VOUT fault and recovery, PG is held low for a typical duration of 11ms. See the Electrical Characteristics table for additional information. Channel 1: VOUT, Channel 2: PG, Hold Time: 11ms Output Voltage Protection The MIC24085 turns both high-side and low-side MOSFETs off to shut down the buck regulator output voltage when the output voltage exceeds 103% of the regulated voltage. After the output voltage drops back below the threshold, the MIC24085 runs to a soft-start. Thermal Shutdown The MIC24085 turns both high-side and low-side MOSFETs off to shut down the buck regulator output voltage when the internal temperature is over the setting threshold at around 170°C. After the temperature drops back below the threshold, the MIC24085 runs to a soft- start. |
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