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SG2644R Datasheet(PDF) 5 Page - Microsemi Corporation |
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SG2644R Datasheet(HTML) 5 Page - Microsemi Corporation |
5 / 8 page 9/91 Rev 1.2 6/97 LINFINITY Microelectronics Inc. Copyright © 1997 11861 Western Avenue ∞ ∞ ∞ ∞ ∞ Garden Grove, CA 92841 5 (714) 898-8121 ∞ ∞ ∞ ∞ ∞ FAX: (714) 893-2570 SG1644/SG2644/SG3644 APPLICATION INFORMATION POWER DISSIPATION The SG1644, while more energy-efficient than earlier gold-doped driver IC’s, can still dissipate considerable power because of its high peak current capability at high frequencies. Total power dissipation in any specific application will be the sum of the DC or steady-state power dissipation, and the AC dissipation caused by driving capacitive loads. The DC power dissipation is given by: P DC = +VCC · ICC [1] where I CC is a function of the driver state, and hence is duty-cycle dependent. The AC power dissipation is proportional to the switching fre- quency, the load capacitance, and the square of the output voltage. In most applications, the driver is constantly changing state, and the AC contribution becomes dominant when the frequency exceeds 100-200KHz. The SG1644 driver family is available in a variety of packages to accommodate a wide range of operating temperatures and power dissipation requirements. The Absolute Maximums section of the data sheet includes two graphs to aid the designer in choosing an appropriate package for his design. The designer should first determine the actual power dissipation of the driver by referring to the curves in the data sheet relating operating current to supply voltage, switching frequency, and capacitive load. These curves were generated from data taken on actual devices. The designer can then refer to the Absolute Maximum Thermal Dissipation curves to choose a package type, and to determine if heat-sinking is required. DESIGN EXAMPLE Given: Two 2500pF loads must be driven push-pull from a +15 volt supply at 100KHz. The application is a commercial one in which the maximum ambient temperature is +50°C, and cost is impor- tant. 1. From Figure 11, the average driver current consumption under these conditions will be 18mA, and the power dissipation will be 15volts x 18mA, or 270mW. 2. From the ambient thermal characteristic curve, it can be seen that the M package, which is an 8-pin plastic DIP with a copper lead frame, has more than enough thermal conductance from junction to ambient to support operation at an ambient tempera- ture of +50°C. The SG36446M driver would be specified for this application. SUPPLY BYPASSING Since the SG1644 can deliver peak currents above 3amps under some load conditions, adequate supply bypassing is essential for proper operation. Two capacitors in parallel are recommended to guarantee low supply impedance over a wide bandwidth: a 0.1µF ceramic disk capacitor for high frequencies, and a 4.7µF solid tantalum capacitor for energy storage. In military applications, a CK05 or CK06 ceramic operator with a CSR-13 tantalum capaci- tor is an effective combination. For commercial applications, any low-inductance ceramic disk capacitor teamed with a Sprague 150D or equivalent low ESR capacitor will work well. The capacitors must be located as close as physically possible to the V CC pin, with combined lead and pc board trace lengths held to less than 0.5 inches. GROUNDING CONSIDERATIONS The ability of the SG1644 to deliver high peak currents into capacitive loads can cause undesirable negative transients on the logic and power grounds. To avoid this, a low inductance ground path should be considered for each output to return the high peak currents back to it’s own ground point. A ground plane is recommended for best performance. If space for a ground plane is not available, make the paths as short and as wide as possible. The logic ground can be returned to the supply bypass capacitor and be connected at one point to the power grounds. LOGIC INTERFACE The logic input of the 1644 is designed to accept standard DC- coupled 5 volt logic swings, with no speed-up capacitors required. If the input signal voltage exceeds 6 volts, the input pin must be protected against the excessive voltage in the HIGH state. Either a high speed blocking diode must be used, or a resistive divider to attenuate the logic swing is necessary. LAYOUT FOR HIGH SPEED The SG1644 can generate relatively large voltage excursions with rise and fall times around 20-30 nanoseconds with light capacitive loads. A Fourier analysis of these time domain signals will indicate strong energy components at frequencies much higher than the basic switching frequency. These high frequen- cies can induce ringing on an otherwise ideal pulse if sufficient inductance occurs in the signal path (either the positive signal trace or the ground return). Overshoot on the rising edge is undesirable because the excess drive voltage could rupture the gate oxide of a power MOSFET. Trailing edge undershoot is dangerous because the negative voltage excursion can forward- bias the parasitic PN substrate diode of the driver, potentially causing erratic operation or outright failure. Ringing can be reduced or eliminated by minimizing signal path inductance, and by using a damping resistor between the drive output and the capacitive load. Inductance can be reduced by keeping trace lengths short, trace widths wide, and by using 2oz. copper if possible. The resistor value for critical damping can be calculated from: R D = 2√L/CL [2] where L is the total signal line inductance, and C L is the load capacitance. Values between 10 and 100ohms are usually sufficient. Inexpensive carbon composition resistors are best because they have excellent high frequency characteristics. They should be located as close as possible to the gate terminal of the power MOSFET. |
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