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TC1303B Datasheet(PDF) 18 Page - Microchip Technology |
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TC1303B Datasheet(HTML) 18 Page - Microchip Technology |
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18 / 30 page  TC1303B DS21949A-page 18 © 2005 Microchip Technology Inc. 5.0 APPLICATION CIRCUITS/ISSUES 5.1 Typical Applications The TC1303B 500 mA buck regulator + 300 mA LDO with power-good operates over a wide input voltage range (2.7V to 5.5V) and is ideal for single-cell Li-Ion battery-powered applications, USB-powered applica- tions, three-cell NiMH or NiCd applications and 3V to 5V regulated input applications. The 10-pin MSOP and 3X3 DFN packages provide a small footprint with minimal external components. 5.2 Fixed Output Application A typical VOUT1 fixed output voltage application is shown in “Typical Application Circuits” on page 3. A 4.7 µF VIN1 ceramic input capacitor, 4.7 µF VOUT1 ceramic capacitor, 1.0 µF ceramic VOUT2 capacitor and 4.7 µH inductor make up the entire external component solution for this dual-output application. No external dividers or compensation components are necessary. For this application, the input voltage range is 2.7V to 4.2V, VOUT1 = 1.5V at 500 mA, while VOUT2 =2.5V at 300 mA. 5.3 Adjustable Output Application A typical VOUT1 adjustable output application is also shown in “Typical Application Circuits” on page 3. For this application, the buck regulator output voltage is adjustable by using two external resistors as a voltage divider. For adjustable output voltages, it is recom- mended that the top resistor divider value be 200 k Ω. The bottom resistor divider can be calculated using the following formula: EQUATION 5-1: Example: RTOP =200 kΩ VOUT1 =2.1V VFB =0.8V RBOT =200 kΩ x (0.8V/(2.1V – 0.8V)) RBOT =123 kΩ (Standard Value = 121 kΩ) For adjustable output applications, an additional R-C compensation is necessary for the buck regulator control loop stability. Recommended values are: RCOMP =4.99kΩ CCOMP =33 pF An additional VIN2 capacitor can be added to reduce high frequency noise on the LDO input voltage pin (VIN2). This additional capacitor (1 µF on page 3) is not necessary for typical applications. 5.4 Input and Output Capacitor Selection As with all buck-derived dc-dc switching regulators, the input current is pulled from the source in pulses. This places a burden on the TC1303B input filter capacitor. In most applications, a minimum of 4.7 µF is recom- mended on VIN1 (buck regulator input voltage pin). In applications that have high source impedance or have long leads (10 inches) connecting to the input source, additional capacitance should be used. The capacitor type can be electrolytic (aluminum, tantalum, POSCAP, OSCON) or ceramic. For most portable electronic applications, ceramic capacitors are preferred due to their small size and low cost. For applications that require very low noise on the LDO output, an additional capacitor (typically 1 µF) can be added to the VIN2 pin (LDO input voltage pin). Low ESR electrolytic or ceramic can be used for the buck regulator output capacitor. Again, ceramic is recommended because of its physical attributes and cost. For most applications, a 4.7 µF is recommended. Refer to Table 5-1 for recommended values. Larger capacitors (up to 22 µF) can be used. There are some advantages in load step performance when using larger value capacitors. Ceramic materials X7R and X5R have low temperature coefficients and are well within the acceptable ESR range required. TABLE 5-1: TC1303B RECOMMENDED CAPACITOR VALUES R BOT R TOP V FB V OUT1 V FB – -------------------------------- ⎝⎠ ⎛⎞ × = C(VIN1)C(VIN2)COUT1 COUT2 min 4.7 µF none 4.7 µF 1 µF max none none 22 µF 10 µF |
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