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MCP1601 Datasheet(PDF) 10 Page - Microchip Technology |
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MCP1601 Datasheet(HTML) 10 Page - Microchip Technology |
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10 / 22 page  MCP1601 DS21762A-page 10 2003 Microchip Technology Inc. 4.0 DEVICE OPERATION The MCP1601 is a synchronous DC/DC converter with integrated switches. Developed to provide high effi- ciency across a wide line and load range, the MCP1601 integrates the three modes of operation described below. In addition to three operating modes, the MCP1601 also integrates many features that mini- mize external circuitry, saving board space and cost. With two external resistors used to set the output volt- age, the MCP1601 output is adjustable from 0.9V to VIN. 4.1 Operating Modes The MCP1601 has three distinct modes of operation, with each one optimized for a specific operating condi- tion commonly encountered in handheld portable power applications. 4.1.1 FEEDFORWARD VOLTAGE PULSE WIDTH MODULATION (PWM) MODE The Pulse Width Modulation (PWM) mode of operation is desired when operating from typical to maximum out- put currents with the proper head room voltage at the input. This mode of operation optimizes efficiency and noise by switching at a fixed frequency. Typical output ripple voltage is less than 10 mV when using a 10 µH inductor and 10 µF ceramic capacitor. The internal operating frequency of the MCP1601 is 750 kHz, nom- inal. The duty cycle, or “ON” time, of the high-side, inte- grated, P-Channel MOSFET is determined by the continuous mode BUCK transfer function. For the con- tinuous inductor current case, the duty cycle can be approximated by VOUT/VIN. The integrated high-side BUCK P-Channel switch will conduct for the “on” time. At the end of the “on” time, the high-side P-Channel switch is turned off and the integrated, low-side, N- Channel synchronous switch is turned on to freewheel the inductor current. The PWM mode architecture employed in the MCP1601 is a feedforward voltage mode control and feeds the input voltage into the PWM oscillator ramp. This information is used to quickly change the operating duty cycle in the event of a sud- den input voltage change. The effects on the output voltage are minimized. To force the MCP1601 into PWM mode, the SYNC/PWM pin should be tied to a logic high. The forced PWM mode should be used for applications that require the fastest transient response from light load to heavy load or applications that require a single switching frequency independent of load. An external oscillator between 850 kHz and 1 MHz can be connected to the SYNC/PWM pin for synchroniza- tion to an external clock source. The MCP1601 will always operate in the PWM mode when synchronized to an external oscillator. 4.1.2 PULSE FREQUENCY MODULATION (PFM) MODE The MCP1601 is also capable of operating in a pulse frequency modulation mode. This mode of operation is desired for applications that have very long periods of inactivity and the output current requirement placed on the MCP1601 is very low. By entering the PFM mode of operation, the switching frequency becomes mainly a function of load current and will decrease as the load current decreases. By switching slower, the energy used turning “on” and “off” the high-side P-Channel and low-side N-Channel is reduced, making the PFM mode more efficient with light output load currents. When load activity is encountered, the MCP1601 will auto- matically switch from the PFM mode to the fixed fre- quency PWM mode by sensing the increase in load current. The auto PWM/PFM mode is selected by plac- ing a logic low at the SYNC/PWM input pin. If an exter- nal clock is used to synchronize the MCP1601 switching frequency, the PFM mode is automatically disabled. To enter the PFM mode of operation, the SYNC/PWM pin must be held to a logic low level and the peak induc- tor current, sensed internal to the MCP1601, is below the internal PFM threshold for more than 1024 clock cycles. If both of these conditions are met, the MCP1601 will enter the PFM mode. While in the PFM mode, the MCP1601 will disable the low-side N-Chan- nel switch to optimize efficiency at low operating cur- rents. A cycle will begin by turning on the high-side P-Channel switch and will end when the output voltage exceeds a predetermined voltage set point. If the peak inductor current exceeds the internal PFM mode cur- rent threshold prior to the output voltage exceeding the voltage set point, the load current has increased and the MCP1601 will automatically switch to PWM opera- tion. The typical hysteresis on the PFM comparator is 6 mV. The typical output ripple voltage is below 40 mV when using a 10 µH inductor and 10 µF ceramic output capacitor when VIN = 4.2V. For proper PFM mode oper- ation, the value of the external inductor and the exter- nal capacitor should be the same. For example, when using a 10 µH inductor, a 10 µF capacitor should be used. When using a 22 µH inductor, a 22 µF capacitor should be used. 4.1.3 LOW DROP OUT (LDO) MODE When the input voltage to the MCP1601 is decreasing and approaches the set output voltage level, the duty cycle increases to a maximum of 90% (typically). To continue to regulate the output to as high a voltage as possible, the MCP1601 enters the low drop out mode of operation. In this mode, the high-side P-Channel MOSFET acts like a saturated LDO. This mode allows the operation of the load circuitry down to the minimum input supply that is typical in battery-powered applications. |
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