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MIC4421CN Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC4421CN Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 12 page MIC4421/4422 Micrel, Inc. August 2005 9 M9999-081005 Transition Power Dissipation Transition power is dissipated in the driver each time its output changes state, because during the transition, for a very brief interval, both the N- and P-channel MOSFETs in the output totem-pole are ON simultaneously, and a current is conducted through them from V S to ground. The transition power dissipation is approximately: P T = 2 f VS (A•s) where (A•s) is a time-current factor derived from the typical characteristic curve “Crossover Energy vs. Supply Volt- age.” Total power (P D) then, as previously described is just P D = PL + PQ + PT Definitions C L = Load Capacitance in Farads. D = Duty Cycle expressed as the fraction of time the input to the driver is high. f = Operating Frequency of the driver in Hertz I H = Power supply current drawn by a driver when both inputs are high and neither output is loaded. I L = Power supply current drawn by a driver when both inputs are low and neither output is loaded. I D = Output current from a driver in Amps. P D = Total power dissipated in a driver in Watts. P L = Power dissipated in the driver due to the driver’s load in Watts. P Q = Power dissipated in a quiescent driver in Watts. P T = Power dissipated in a driver when the output changes states (“shoot-through current”) in Watts. NOTE: The “shoot-through” current from a dual transition (once up, once down) for both drivers is stated in Figure 7 in ampere-nanoseconds. This figure must be multiplied by the number of repeti- tions per second (frequency) to find Watts. R O = Output resistance of a driver in Ohms. V S = Power supply voltage to the IC in Volts. Capacitive Load Power Dissipation Dissipation caused by a capacitive load is simply the energy placed in, or removed from, the load capacitance by the driver. The energy stored in a capacitor is described by the equation: E = 1/2 C V2 As this energy is lost in the driver each time the load is charged or discharged, for power dissipation calculations the 1/2 is removed. This equation also shows that it is good practice not to place more voltage in the capacitor than is necessary, as dissipation increases as the square of the voltage applied to the capacitor. For a driver with a capacitive load: P L = f C (VS) 2 where: f = Operating Frequency C = Load Capacitance V S =Driver Supply Voltage Inductive Load Power Dissipation For inductive loads the situation is more complicated. For the part of the cycle in which the driver is actively forcing current into the inductor, the situation is the same as it is in the resistive case: P L1 = I 2 R O D However, in this instance the R O required may be either the on resistance of the driver when its output is in the high state, or its on resistance when the driver is in the low state, depending on how the inductor is connected, and this is still only half the story. For the part of the cycle when the induc- tor is forcing current through the driver, dissipation is best described as P L2 = I VD (1 – D) where V D is the forward drop of the clamp diode in the driver (generally around 0.7V). The two parts of the load dissipation must be summed in to produce P L P L = PL1 + PL2 Quiescent Power Dissipation Quiescent power dissipation (P Q, as described in the input section) depends on whether the input is high or low. A low input will result in a maximum current drain (per driver) of ≤0.2mA; a logic high will result in a current drain of ≤ 3.0mA. Quiescent power can therefore be found from: P Q = VS [D IH + (1 – D) IL] where: I H = quiescent current with input high I L = quiescent current with input low D = fraction of time input is high (duty cycle) V S = power supply voltage |
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