The XC9215/16/17 Series Synchronous-Rectification Type Step-Down DC-DC Converters

Published:2011/7/26 1:57:00 Author:Phyllis From:SeekIC

The XC9215/16/17 series of synchronous-rectification type step-down DC-DC converters, which are in USB-6B (2.0×1.8×0.88mm) package, are an ideal choice for noise sensitive RF applications.

These highly integrated DC-DC converters can realize a stable power supply with 400 mA output current with a built-in 0.5-_ P-channel driver transistor and 0.6-_ N-channel switching transistor and an externally connected chip inductor and two small ceramic capacitors. The XC9215/16/17 series are designed for use with an output ceramic capacitor. However, because of the potential difference between input and output is too large, a ceramic capacitor may fail to absorb the resulting high switching energy and oscillation could occur on the output. If the input-output potential difference is large, connect an electrolytic capacitor in parallel to compensate for insufficient capacitance.

The XC9215 provides synchronous-rectified PWM switching control, the XC9216 automatically switches between PWM/PFM depending on output load, and the switching operation of the XC9217 can be manually controlled between PWM and PFM/PWM using the device’s CE/MODE pin. This MODE pin function gives the designer ultimate control over the switching mode of the IC, which is a big advantage in RF applications. However, regardless of which version is selected, the IC provides fast response, low ripple output and high efficiency over a broad range of output loads. Input voltage range is 2.0V to 6.0V. Output voltage is internally programmable between 0.9V and 4.0V in increments of 0.1V. The switching frequency is 600 KHz or 1.2MHz, the higher frequency allowing the designer to use small chip style inductors.

Each unit of the XC9215/16/17 series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM comparator, phase compensation circuit, output voltage adjustment resistors, P-channel MOS driver transistor, N-channel MOS synchronous rectification switch, current limited circuit, 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 resistor R1 and R2. 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 LX pin to output switching duty cycle. This process is continuously performed to ensure stable output voltage. The current feedback circuit monitors the P-channel MOS driver transistor current for each switching operation, and modulates the error amplifier output signal to provide multiple feedback signals. This enables a stable feedback loop even when a low ESR capacitor, such as a ceramic capacitor, is used, ensuring stable output voltage.

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