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TC105333ECT Datasheet(PDF) 5 Page - Microchip Technology |
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TC105333ECT Datasheet(HTML) 5 Page - Microchip Technology |
5 / 14 page © 2002 Microchip Technology Inc. DS21349B-page 5 TC105 3.8 External Switching Transistor Selection EXT is a complementary output with a maximum ON resistance of 22 Ω to VDD when high and 19Ω to ground when low. It is designed to directly drive a P-channel MOSFET or a PNP bipolar transistor through a base current limiting resistor (Figure 4-2). A PNP transistor is recommended in applications where VIN is less than 2.5V. Otherwise, a P-channel MOSFET is preferred as it affords the highest efficiency because it does not draw any gate drive current. However, P-channel MOSFETs are typically more expensive than bipolar transistors. P-channel MOSFET selection is determined mainly by the on-resistance, gate-source threshold, and gate charge requirements. Also, the drain-to-source and gate-to-source breakdown voltage ratings must be greater than VDDMAX. The total gate charge specifica- tion should be less than 100nC for best efficiency. The MOSFET must be capable of handling the required peak inductor current, and should have a very low on-resistance at that current. For example, an Si9430 MOSFET has a drain-to-source rating of -20V, and a typical on-resistance rDSON of 0.07Ω at 2A, with VGS = -4.5V. Table 4-1 lists suppliers of external components recommended for use with the TC105. 3.8.1 BOARD LAYOUT GUIDELINES As with all inductive switching regulators, the TC105 generates fast switching waveforms, which radiate noise. Interconnecting lead lengths should be mini- mized to keep stray capacitance, trace resistance and radiated noise as low as possible. In addition, the GND pin, input bypass capacitor and output filter capacitor ground leads should be connected to a single point. The input capacitor should be placed as close to power and ground pins of the TC105 as possible. The length of the EXT trace must also be kept as short as possible. |
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