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XC9221 Datasheet(PDF) 6 Page - Torex Semiconductor |
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XC9221 Datasheet(HTML) 6 Page - Torex Semiconductor |
6 / 22 page 6/22 XC9220/XC9221 Series ■TYPICAL APPLICATION CIRCUITS ● External Components Pch MOSFET: 2SJ646 (SANYO) SBD: DE5PC3 (SHINDENGEN) L: CDRH8D28-4R7 (4.7μH, SUMIDA / Fosc=1.0MHz) CDRH8D43-100 (10μH, SUMIDA / Fosc=500kHz) CDRH127-220 (22μH, SUMIDA / Fosc=300kHz) CIN: 47μF (OS-CON, SANYO) CL: 47μF (OS-CON, SANYO) ■OPERATIONAL EXPLANATION The XC9220/XC9221 series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM comparator, phase compensation circuit, protection circuits, U.V.L.O. circuit and others. The series ICs compare, using the error amplifier, the voltage of the internal voltage reference source with the feedback voltage from the VOUT pin through split resistors. Phase compensation is performed on the resulting error amplifier output, to input a signal to the PWM comparator to determine the turn-on time during PWM operation. The PWM comparator compares, in terms of voltage level, the signal from the error amplifier with the ramp wave from the ramp wave circuit, and delivers the resulting output to the buffer driver circuit to cause the EXT pin to output a switching duty cycle. This process is continuously performed to ensure stable output voltage. <Reference Voltage Source> The reference voltage source provides the reference voltage to ensure stable output voltage of the DC/DC converter. <Oscillator> The oscillator determines switching frequency. The frequency is fixed internally and can be selected from 300kHz, 500kHz and 1.0MHz. Clock pulses generated in this circuit are used to produce ramp waveforms needed for PWM operation, and to synchronize all the internal circuits. <Error Amplifier> The error amplifier is designed to monitor output voltage. The amplifier compares the reference voltage with the feedback voltage (FB pin voltage) divided by the internal split resistors. When a voltage lower than the reference voltage is fed back, the output voltage of the error amplifier increases. The gain and frequency characteristics of the error amplifier output are fixed internally to deliver an optimized signal to the PWM comparator. <Control Methods> The XC9220 series is PWM control, and the XC9221 series is PWM/PFM automatic switching mode. In the XC9220 series, it is controlled at a constant frequency from light load to heavy load. When a noise etc. is concerned, it is easy to set up a filter etc. since the frequency is fixed. On the other hand, the efficiency at the time of light load may fall. In the XC9221 series, the high efficiency can be drawn from PFM control at the time of light load. In PWM/PFM automatic switching mode, a control method is automatically changed from PWM control to PFM control at the time of light load. If coil current becomes discontinuous at the time of light load, ON time duty intends to reduce less than 25%. Therefore, the PFM circuit operates to output the pulse, which ON time duty fixed to 25% from the EXT/pin. The ON time duty is fixed when PFM operation. The pulse is outputted with the cycle suitable for the conditions at that time. In order that the number of times of switching per unit time may decrease, the efficiency, which it is at the light load time is improved. However, output cycle of the pulse cannot be fixed. For this, the circuit should be designed with this point in mind when using a noise filter etc. The conditions shifting to the PFM operation is depend on values of input voltage, load current, coil and so on. |
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