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MIC4422ABM Datasheet(PDF) 10 Page - MIC GROUP RECTIFIERS |
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MIC4422ABM Datasheet(HTML) 10 Page - MIC GROUP RECTIFIERS |
10 / 13 page Micrel, Inc. MIC4421A/4422A June 2007 10 M9999-062707 Table 1. MIC4421A Maximum Operating Frequency VS Max Frequency 18V 220kHz 15V 300kHz 10V 640kHz 5V 2MHz Conditions: 1. θJA = 150°C/W 2. TA = 25°C 3. CL = 10,000pF 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 V 2 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: PL = f C (VS)2 where: f = Operating Frequency C = Load Capacitance VS = 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: PL1 = I 2 R O D However, in this instance the RO 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 inductor is forcing current through the driver, dissipation is best described as: PL2 = I VD (1 – D) where VD 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 PL: PL = P L1 + P L2 Quiescent Power Dissipation Quiescent power dissipation (PQ, 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: PQ = VS [D IH + (1 – D) IL] where: IH = Quiescent current with input high IL = Quiescent current with input low D = Fraction of time input is high (duty cycle) VS = Power supply voltage 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 VS to ground. The transition power dissipation is approximately: PT = 2 f VS (A•s) where (A•s) is a time-current factor derived from the typical characteristic curve “Crossover Energy vs. Supply Voltage.” Total power (PD) then, as previously described is just: PD = PL + PQ + PT Definitions CL = 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. IH = Power supply current drawn by a driver when both inputs are high and neither output is loaded. IL = Power supply current drawn by a driver when both inputs are low and neither output is loaded. ID = Output current from a driver in Amps. PD = Total power dissipated in a driver in Watts. PL = Power dissipated in the driver due to the driver’s load in Watts. PQ = Power dissipated in a quiescent driver in Watts. |
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