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RC5051 Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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RC5051 Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 16 page PRODUCT SPECIFICATION RC5051 REV. 1.0.4 4/2/01 11 developed across the sense resistor exceeds the 120mV com- parator threshold voltage, the RC5051 reduces the output duty cycle to help protect the power devices. The DC-DC converter returns to normal operation after the fault has been removed. Oscillator The RC5051 oscillator section uses a fixed current capacitor charging configuration. An external capacitor (CEXT) is used to set the oscillator frequency between 80KHz and 1MHz. This scheme allows maximum flexibility in choosing external components. In general, a higher operating frequency decreases the peak ripple current flowing in the output inductor, thus allowing the use of a smaller inductor value. In addition, operation at higher frequencies decreases the amount of energy storage that must be provided by the bulk output capacitors during load transients due to faster loop response of the controller. Unfortunately, the efficiency losses due to switching of the MOSFETs increase as the operating frequency is increased. Thus, efficiency is optimized at lower frequencies. An oper- ating frequency of 300KHz is a typical choice which opti- mizes efficiency and minimizes component size while maintaining excellent regulation and transient performance under all operating conditions. Design Considerations and Component Selection Additional information on design and component selection may be found in Fairchild Semiconductor’s Application Note 53. MOSFET Selection This application requires N-channel Logic Level Enhance- ment Mode Field Effect Transistors. Desired characteristics are as follows: • Low Static Drain-Source On-Resistance, RDS,ON < 20m Ω (lower is better) • Low gate drive voltage, VGS = 4.5V rated • Power package with low Thermal Resistance • Drain-Source voltage rating > 15V. The on-resistance (RDS,ON) is the primary parameter for MOSFET selection. The on-resistance determines the power dissipation within the MOSFET and therefore significantly affects the efficiency of the DC-DC Converter. For details and a spreadsheet on MOSFET selection, refer to Applica- tions Bulletin AB-8 MOSFET Gate Bias The high side MOSFET gate driver can be biased by one of two methods–Charge Pump or 12V Gate Bias. The charge pump method has the advantage of requiring only +5V as an input voltage to the converter, but the 12V method will real- ize increased efficiency by providing an increased VGS to the high side MOSFETs. Method 1. Charge Pump (Bootstrap) Figure 3 shows the use of a charge pump to provide gate bias to the high side MOSFET when +12V is unavailable. Capac- itor CP is the charge pump used to boost the voltage of the RC5051 output driver. When the MOSFET Q1 switches off, the source of the MOSFET is at approximately 0V because of the MOSFET Q2. (The Schottky D2 conducts for only a very short time, and is not relevent to this discussion.) CP is charged through the Schottky diode D1 to approximately 4.5V. When the MOSFET Q1 turns on, the voltage at the source of the MOSFET is equal to 5V. The capacitor voltage follows, and hence provides a voltage at VCCQP equal to almost 10V. The Schottky diode D1 is required to provide the charge path when the MOSFET is off, and reverses biases when VCCQP goes to 10V. The charge pump capaci- tor (CP) needs to be a high Q, high frequency capacitor. A 1 µF ceramic capacitor is recommended here. Figure 3. Charge Pump Configuration Method 2. 12V Gate Bias Figure 4 illustrates how a 12V source can be used to bias VCCQP. A 47 Ω resistor is used to limit the transient current into the VCCQP pin and a 1µF capacitor is used to filter the VCCQP supply. This method provides a higher gate bias voltage (VGS) to the high side MOSFET than the charge pump method, and therefore reduces the RDS,ON and the resulting power loss within the MOSFET. In designs where efficiency is a primary concern, the 12V gate bias method is recommended. A 6.2V Zener diode, D1, is used to clamp the voltage at VCCQP to a maximum of 12V and ensure that the absolute maximum voltage of the IC will not be exceeded. PWM/PFM Control 65-5051-06 VO +5V D1 D2 CP Q1 Q2 L2 RS COUT VCCQP HIDRV LODRV GNDP |
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