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MCP1612-ADJIMS Datasheet(PDF) 10 Page - Microchip Technology |
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MCP1612-ADJIMS Datasheet(HTML) 10 Page - Microchip Technology |
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10 / 22 page  MCP1612 DS21921B-page 10 © 2005 Microchip Technology Inc. 4.0 DETAILED DESCRIPTION 4.1 Device Overview The MCP1612 is a 1A synchronous buck converter switching at 1.4 MHz to minimize external component size and cost. While utilizing a fixed-frequency Current mode architecture, the MCP1612 provides fast response to sudden load changes, as well as overcurrent protection in the event of a shorted load. The input voltage range is 2.7V to 5.5V, while the output voltage is adjustable by properly setting an external resistor divider and can range from 0.8V to VIN. Integrated soft-start, UVLO and overtemperature protection minimize external circuitry and component count. 4.2 Current Mode Control Scheme The MCP1612 incorporates a Peak Current mode control scheme. Peak Current mode is used to obtain high gain in the PWM control loop for very fast response to dynamic line and load conditions. With both the P-channel and N-channel MOSFETs turned off, the beginning of a cycle occurs on the negative edge of the internal 1.4 MHz oscillator, the P-channel MOSFET turns on and current ramps up into the buck inductor. The inductor current is sensed and tied to one input of a high-speed comparator. The other input of the high-speed comparator is the error amplifier output. This is the amplified difference between the internal 0.8V reference and the divided-down VOUT signal at the FB pin of the MCP1612. When the sensed inductor current ramps up to the point that is equal to the amplified error signal, the high-speed comparator output switches states and the P-channel MOSFET is turned off until the beginning of the next clock cycle and the N-channel is turned on. The width of the pulse (or duty cycle) is ideally determined by the VOUT/VIN ratio of the DC/DC converter. The actual duty cycle is slightly larger to account for the non-ideal losses of the integrated MOSFET switches and the losses in the external inductor. 4.3 Low-Dropout Operation The MCP1612 is capable of operating over a wide range of input voltages. The PWM architecture allows for the P-channel MOSFET to achieve 100% duty cycle operation for applications that have minimal input volt- age headroom. During 100% Duty Cycle mode, the output voltage (VOUT) is equal to the Output Current (IOUT) x Resistance (P-channel RDSON + RINDUCTOR). 4.4 Current Limit Cycle-by-cycle current limit is used to protect the MCP1612 from being damaged when an external short circuit is applied. The typical peak current limit is 2.3A. If the sensed inductor current reaches the 2.3A limit, the P-channel MOSFET is turned off, even if the output voltage is not in regulation. 4.5 Soft-Start During normal power-up, as VIN rises above the UVLO protection setting (or, in the case of a logic-low to logic- high transition on the shutdown pin), the rise time of the MCP1612 output voltage is controlled by the soft-start feature. This is accomplished by allowing the output of the error amplifier to slowly rise. This feature prevents the output voltage from overshooting the desired value and the sudden inrush of current, depleting the input capacitors and causing a large dip in input voltage. This large dip in the input voltage can trip the UVLO thresh- old, causing the converter to shut down prior to reach- ing steady-state operation. 4.6 Undervoltage Lockout (UVLO) The UVLO feature uses a comparator to sense the input voltage level (VIN). If the input voltage is lower than the voltage necessary to properly operate the MCP1612, the UVLO feature will hold the converter off. When VIN rises above the necessary input voltage, the UVLO is released and soft-start begins. For the MCP1612, the UVLO protection threshold is at a maximum of 2.7V. Hysteresis is built into the UVLO circuit to compensate for input impedance. For example, if there is any resistance between the input voltage source and the converter (once it starts), there will be a voltage drop at the converter input equal to IIN x RIN. The typical hysteresis for the MCP1612 is 200 mV. 4.7 Overtemperature Protection The MCP1612 has an integrated overtemperature protection circuit that monitors the device junction temperature and shuts the device off if the junction temperature exceeds the typical 160°C threshold. If the overtemperature threshold is reached, the soft-start is reset so that, when the junction temperature cools to approximately 151°C, the device will automatically restart and the output voltage will not overshoot. 4.8 Shutdown Input Operation The SHDN pin is used to turn the MCP1612 on and off. When the SHDN pin is tied low, the MCP1612 is off. When tied high, the MCP1612 will be enabled and begin operation as long as the input voltage is not below the UVLO threshold. |
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