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AAT4900IJS-T1 Datasheet(PDF) 8 Page - Advanced Analog Technology, Inc. |
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AAT4900IJS-T1 Datasheet(HTML) 8 Page - Advanced Analog Technology, Inc. |
8 / 13 page AAT4900 Buffered Power Half-Bridge 8 4900.2006.05.1.3 Synchronous Buck DC/DC Converter Application The losses associated with the AAT4900 high side switching MOSFET are due to switching losses and conduction losses. The conduction losses are asso- ciated with the RDS(ON) characteristics of the output switching device. At the full load condition, assum- ing continuous conduction mode (CCM), the on losses can be derived from the following equations. Eq. 1: D is the duty cycle. Eq. 2: ΔI is the peak-to-peak inductor ripple current. High Side Switch RMS Current Eq. 3: Low Side Switch RMS Current The low side RMS current is estimated by the fol- lowing equation. Eq. 4: Total Losses A simplified form of the above results (where the above descriptions of IRMS has been approximated with Io) is given by: Eq. 5: Substitution of the IRMS equations with IO results in very little error when the inductor ripple current is 20% to 40% of the full load current. The equation also includes switching and quiescent current loss- es where tSW is approximated at 18 nsec and IQ is the no load quiescent current of the AAT4900. Quiescent current losses are associated with the gate drive of the output stage and biasing. Since the gate drive current varies with frequency and voltage, the bias current must be checked at the frequency, voltage, and temperature of operation with no load attached to the LX node. Once the above losses have been determined, the maximum junction temperature can be calculated. Eq. 6: Using the above equations, the graph below shows the current capability for some typical applications with maximum junction temperatures of 150°C and 120°C. The increase in RDS(ON) vs. temperature is estimated at 3.75m Ω for a 10°C increase in junc- tion temperature. Step-Down Converter Limits (FS = 1MHz) 0.5 0.75 1 1.25 1.5 1.75 25 35 45 55 65 75 85 Ambient Temperature ( °C) TJMAX = 150 °C VIN = 4.2V, VO = 2.5V VIN = 5.0V, VO = 3.3V VIN = 4.2V, VO = 2.5V VIN = 5.0V, VO = 3.3V TJMAX = 120 °C T J(MAX) = PLOSS · ΘJC = TAMB P LOSS + (t sw · FS · IO + IQ) · VIN I O 2 · (R DS(ON)H · VO + RDS(ON)L · (VIN -VO)) V IN = 2 2 12 O I I ⎛⎞ Δ = + · (1 - D) ⎝⎠ IRMS(LS) 2 2 12 O I I ⎛⎞ Δ = + · D ⎝⎠ IRMS(HS) 1 ΔI = ⎛⎞ - ⎝⎠ VO VIN VO L · FS D = O IN V V |
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