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TC110301ECT データシート(PDF) 7 Page - Microchip Technology |
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TC110301ECT データシート(HTML) 7 Page - Microchip Technology |
7 / 16 page © 2002 Microchip Technology Inc. DS21355B-page 7 TC110 3.7 Output Diode For best results, use a Schottky diode such as the MA735, 1N5817, MBR0520L or equivalent. Connect the diode between the FB (or SENSE) input as close to the IC as possible. Do not use ordinary rectifier diodes since the higher threshold voltages reduce efficiency. 3.8 External Switching Transistor Selection The EXT output is designed to directly drive an N-channel MOSFET or NPN bipolar transistor. N- channel MOSFETs afford the highest efficiency because they do not draw continuous gate drive current, but are typically more expensive than bipolar transistors. If using an N-channel MOSFET, the gate should be connected directly to the EXT output as shown in Figure 3-1 and Figure 3-1. EXT is a compli- mentary output with a maximum ON resistances of 43 Ω to VDD when high and 27Ω to ground when low. Peak currents should be kept below 10mA. When selecting an N-channel MOSFET, there are three important parameters to consider: total gate charge (Qg); ON resistance (rDSON) and reverse transfer capacitance (CRSS). Qg is a measure of the total gate capacitance that will ultimately load the EXT output. Too high a Qg can reduce the slew rate of the EXT output sufficiently to grossly lower operating efficiency. Transistors with typical Qg data sheet values of 50nC or less can be used. For example, the Si9410DY has a Qg (typ) of 17nC @ VGS = 5V. This equates to a gate current of: IGATEMAX =fMAX x Qg = 115kHz x 17nC = 2mA The two most significant losses in the N-channel MOSFET are switching loss and I2R loss. To minimize these, a transistor with low rDSON and low CRSS should be used. Bipolar NPN transistors can be used, but care must be taken when determining base current drive. Too little current will not fully turn the transistor on, and result in unstable regulator operation and low efficiency. Too high a base drive causes excessive power dissipation in the transistor and increase switching time due to over-saturation. For peak efficiency, make RB as large as possible, but still guaranteeing the switching transis- tor is completely saturated when the minimum value of hFE is used. 3.9 Board Layout Guidelines As with all inductive switching regulators, the TC110 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 TC110 as possible. |
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同様の説明 - TC110301ECT |
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