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MIC4423BWM Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC4423BWM Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 12 page January 1999 9 MIC4423/4424/4425 MIC4423/4424/4425 Micrel in estimating power dissipation in the driver. Operating frequency, power supply voltage, and load all affect power dissipation. Given the power dissipation in the device, and the thermal resistance of the package, junction operating temperature for any ambient is easy to calculate. For example, the thermal resistance of the 8-pin plastic DIP package, from the datasheet, is 150 °C/W. In a 25°C ambient, then, using a maximum junction temperature of 150 °C, this package will dissipate 960mW. Accurate power dissipation numbers can be obtained by summing the three sources of power dissipation in the device: • Load power dissipation (PL) • Quiescent power dissipation (PQ) • Transition power dissipation (PT) Calculation of load power dissipation differs depending on whether the load is capacitive, resistive or inductive. Resistive Load Power Dissipation Dissipation caused by a resistive load can be calculated as: PL = I2 RO D where: I = the current drawn by the load RO = the output resistance of the driver when the output is high, at the power supply voltage used (See characteristic curves) D = fraction of time the load is conducting (duty cycle) 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: 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 = I2 RO 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 = PL1 + PL2 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 ≤2.0mA. 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 = f VS (A•s) where (A•s) is a time-current factor derived from Figure 2. Total power (PD) then, as previously described is just PD = PL + PQ +PT Examples show the relative magnitude for each term. EXAMPLE 1: A MIC4423 operating on a 12V supply driving two capacitive loads of 3000pF each, operating at 250kHz, with a duty cycle of 50%, in a maximum ambient of 60 °C. First calculate load power loss: PL = f x C x (VS)2 PL = 250,000 x (3 x 10–9 + 3 x 10–9) x 122 = 0.2160W Then transition power loss: PT = f x VS x (A•s) = 250,000 • 12 • 2.2 x 10–9 = 6.6mW Then quiescent power loss: PQ = VS x [D x IH + (1 – D) x IL] |
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