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MCP1623 Datasheet(PDF) 11 Page - Microchip Technology |
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MCP1623 Datasheet(HTML) 11 Page - Microchip Technology |
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11 / 26 page  2010 Microchip Technology Inc. DS41420A-page 11 MCP1623/24 4.2 Functional Description The MCP1623/24 is a compact, high-efficiency, fixed frequency, step-up DC-DC converter that provides an easy-to-use power supply solution for PIC microcontroller applications powered by either one-cell, two-cell, or three-cell alkaline, NiCd, or NiMH, or one-cell Li-Ion or Li-Polymer batteries. Figure 4-1 depicts the functional block diagram of the MCP1623/24. 4.2.1 LOW-VOLTAGE START-UP The MCP1623/24 is capable of starting from a low input voltage. Start-up voltage is typically 0.65V for a 3.3V output and 1 mA resistive load. When enabled, the internal start-up logic turns the rectifying P-Channel switch on until the output capacitor is charged to a value close to the input voltage. The rectifying switch is current limited during this time. After charging the output capacitor to the input voltage, the device starts switching. If the input voltage is below 1.6V, the device runs open-loop with a fixed duty cycle of 70% until the output reaches 1.6V. During this time, the boost switch current is limited to 50% of its nominal value. Once the output voltage reaches 1.6V, normal closed-loop PWM operation is initiated. The MCP1623/24 charges an internal capacitor with a very weak current source. The voltage on this capaci- tor, in turn, slowly ramps the current limit of the boost switch to its nominal value. The soft-start capacitor is completely discharged in the event of a commanded shutdown or a thermal shutdown. There is no undervoltage lockout feature for the MCP1623/24. The device will start-up at the lowest possible voltage and run down to the lowest possible voltage. For typical battery applications, this may result in “motor-boating” for deeply discharged batteries. FIGURE 4-1: MCP1623/24 Block Diagram. GATE DRIVE AND SHUTDOWN CONTROL LOGIC VIN EN VOUT GND ISENSE IZERO ILIMIT .3V 0V SOFT-START DIRECTION CONTROL OSCILLATOR SLOPE COMP. PWM/PFM LOGIC 1.21V INTERNAL BIAS SW FB EA |
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