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MCP1640T-IMC Datasheet(PDF) 11 Page - Microchip Technology |
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MCP1640T-IMC Datasheet(HTML) 11 Page - Microchip Technology |
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11 / 32 page  2010 Microchip Technology Inc. DS22234A-page 11 MCP1640/B/C/D 4.0 DETAILED DESCRIPTION 4.1 Device Option Overview The MCP1640/B/C/D family of devices is capable of low start-up voltage and delivers high efficiency over a wide load range for single cell, two cell, three cell alkaline, NiMH, NiCd and single cell Li-Ion battery inputs. A high level of integration lowers total system cost, eases implementation and reduces board area. The devices feature low start-up voltage, adjustable output voltage, PWM/PFM mode operation, low IQ, integrated synchronous switch, internal compensation, low noise anti-ring control, inrush current limit and soft start. There are two feature options for the MCP1640/ B/C/D family: PWM/PFM mode or PWM mode only, and “true output disconnect” or input bypass. 4.1.1 PWM/PFM MODE OPTION The MCP1640/C devices use an automatic switchover from PWM to PFM mode for light load conditions to maximize efficiency over a wide range of output current. During PFM mode, higher peak current is used to pump the output up to the threshold limit. While operating in PFM or PWM mode, the P-Channel switch is used as a synchronous rectifier, turning off when the inductor current reaches 0 mA to maximize efficiency. In PFM mode, a comparator is used to terminate switching when the output voltage reaches the upper threshold limit. Once switching has terminated, the output voltage will decay or coast down. During this period, very low IQ is consumed from the device and input source, which keeps power efficiency high at light load. The disadvantages of PWM/PFM mode are higher output ripple voltage and variable PFM mode frequency. The PFM mode frequency is a function of input voltage, output voltage and load. While in PFM mode, the boost converter pumps the output up at a switching frequency of 500 kHz. 4.1.2 PWM MODE ONLY OPTION The MCP1640B/D devices disable PFM mode switching, and operate only in PWM mode over the entire load range. During periods of light load opera- tion, the MCP1640B/D continues to operate at a con- stant 500 kHz switching frequency keeping the output ripple voltage lower than PFM mode. During PWM only mode, the MCP1640B/D P-Channel switch acts as a synchronous rectifier by turning off to prevent reverse current flow from the output cap back to the input in order to keep efficiency high. For noise immunity, the N-Channel MOSFET current sense is blanked for approximately 100 ns. With a typical minimum duty cycle of 100 ns, the MCP1640B/D continues to switch at a constant frequency under light load conditions. Figure 2-12 represents the input voltage versus load current for the pulse skipping threshold in PWM only mode. At lighter loads, the MCP1640B/D devices begin to skip pulses. 4.1.3 TRUE OUTPUT DISCONNECT OPTION The MCP1640/B devices incorporate a true output disconnect feature. With the EN pin pulled low, the output of the MCP1640/B is isolated or disconnected from the input by turning off the integrated P-Channel switch and removing the switch bulk diode connection. This removes the DC path typical in boost converters, which allows the output to be disconnected from the input. During this mode, less than 1 µA of current is consumed from the input (battery). True output discon- nect does not discharge the output; the output voltage is held up by the external COUT capacitance. 4.1.4 INPUT BYPASS OPTION The MCP1640C/D devices incorporate the input bypass shutdown option. With the EN input pulled low, the output is connected to the input using the internal P-Channel MOSFET. In this mode, the current draw from the input (battery) is less than 1 µA with no load. The Input Bypass mode is used when the input voltage range is high enough for the load to operate in Sleep or low IQ mode. When a higher regulated output voltage is necessary to operate the application, the EN input is pulled high enabling the boost converter. TABLE 4-1: PART NUMBER SELECTION Part Number PWM/ PFM PWM True Dis Bypass MCP1640 X X MCP1640B X X MCP1640C X X MCP1640D X X |
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