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TC429 Datasheet(PDF) 5 Page - Microchip Technology |
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TC429 Datasheet(HTML) 5 Page - Microchip Technology |
5 / 14 page 2002 Microchip Technology Inc. DS21416B-page 5 TC429 3.0 APPLICATIONS INFORMATION 3.1 Supply Bypassing Charging and discharging large capacitive loads quickly requires large currents. For example, charging a 2500pF load to 18V in 25nsec requires a 1.8A current from the device’s power supply. To ensure low supply impedance over a wide frequency range, a parallel capacitor combination is recom- mended for supply bypassing. Low-inductance ceramic disk capacitors with short lead lengths (< 0.5 in.) should be used. A 1 µF film capacitor in parallel with one or two 0.1 µF ceramic disk capacitors normally provides adequate bypassing. 3.2 Grounding The high-current capability of the TC429 demands careful PC board layout for best performance. Since the TC429 is an inverting driver, any ground lead impedance will appear as negative feedback which can degrade switching speed. The feedback is especially noticeable with slow rise-time inputs, such as those produced by an open-collector output with resistor pull- up. The TC429 input structure includes about 300mV of hysteresis to ensure clean transitions and freedom from oscillation, but attention to layout is still recommended. Figure 3-3 shows the feedback effect in detail. As the TC429 input begins to go positive, the output goes negative and several amperes of current flow in the ground lead. As little as 0.05 Ω of PC trace resistance can produce hundreds of millivolts at the TC429 ground pins. If the driving logic is referenced to power ground, the effective logic input level is reduced and oscillations may result. To ensure optimum device performance, separate ground traces should be provided for the logic and power connections. Connecting logic ground directly to the TC429 GND pins ensures full logic drive to the input and fast output switching. Both GND pins should be connected to power ground. FIGURE 3-1: INVERTING DRIVER SWITCHING TIME TEST CIRCUIT FIGURE 3-2: SWITCHING SPEED 18 45 26 7 0.1 µF +5V 18V 0V 0V 90% 10% 10% 10% 90% 90% Input VDD = 18V Input Output tD1 tF tR tD2 CL = 2500pF 1 µF Input: 100kHz, square wave, tRISE = tFALL ≤ 10nsec Output TC429 TIME (100ns/DIV) CL = 2500pF VS = 18V 5V INPUT OUTPUT 100ns TIME (100ns/DIV) CL = 2500pF VS = 7V 5V INPUT OUTPUT 100ns |
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