Datasheet XCL303, XCL304 (Torex) - 9

XC9140
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XCL303/XCL304 Series
■OPERATIONAL EXPLANATION (Continued) <Normal Operation> The FB terminal voltage divided by the output voltage is compared with the VREF voltage by the error amp. Phase compensation is applied to the error amp output, which is then forwarded to the PWM comparator. At the PWM comparator the error amp output and ramp wave are compared to determine the ON time during PWM control. The XCL303 series (PWM control) is switched using a constant switching frequency (fOSC) independent of the output current. During light load current, the ON time is short, and the IC operates in a non-continuous mode. As the output current increases, the ON time becomes longer, and the IC operates in a continuous mode. At high load currents, the ON time depends heavily on the input voltage, output voltage, and output current, and the maximum ON time (tONMAX) restriction determines the maximum output current that can flow under the conditions of each input voltage and output voltage. Refer to the typical performance characteristics for the maximum output current under each condition. fOSC tON tON Lx Lx 0V 0V IPFM Coil Curren t Coil Current IOUT IOUT 0mA 0mA XCL303 Series: Example of operation at light load current XCL303 Series: Example of operation at high load currents The XCL304 series (PWM/PFM automatic switching control) turns ON the Pch driver transistor until the coil current reaches the PFM current (IPFM) and to lower the switching frequency during light load current. This operation reduces loss during light loads to achieve high efficiency from light to high load currents. As the output current grows larger, the switching frequency increases proportional to the output current, and when the switching frequency reaches the fOSC to switch from PFM control to PWM control the switching frequency is fixed. fOSC tON tON Lx Lx 0V 0V I PFM Coil Curren t Coil Current I OUT IOUT 0mA 0mA XCL304 Series: Example of operation at light load current XCL304 Series: Example of operation at high load currents Further, the phase compensation circuit optimizes the error amp frequency characteristics and is used to phase compensate the Pch driver transistor current feedback signal. This achieves output voltage stability even when low ESR capacitors, such as ceramic capacitors are used. 9/24