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UCC3588 Datasheet(PDF) 7 Page - Texas Instruments |
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UCC3588 Datasheet(HTML) 7 Page - Texas Instruments |
7 / 15 page UCC3588 5BIT PROGRAMMABLE OUTPUT BiCMOS POWER SUPPLY CONTROLLER SLUS311A – JULY 1999 – REVISED AUGUST 2000 7 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 APPLICATION INFORMATION 2) To properly approximate the full load duty cycle operating range, assumptions are made regarding the MOSFETs’ RDS(on), and the output inductor’s dc resistance. Q1 and Q2 are IRF3103s, each with an RDS(on) of 0.014 Ω. The output inductor is allowed to dissipate one watt under full load, giving a dc resistance of 6.9 mW, and R6 is 3 m Ω. The resulting duty cycle at the operating extremes is then: d MIN + V OUT IO ) I OUT R6 ) R DS on ) R V IN HI + 1.8 ) (12 0.024) 5.5 + 0.379 d MIN + V OUT HI ) I OUT R6 ) R DS on ) R V IN LO + 3.5 ) (12 0.024) 4.5 + 0.842 3) The value of the output inductor is chosen at the worst case ripple current point. L + V IN HI * V OUT LO DV OUT + (5.5 * 1.8) 0.379 3.333 m 2.4 + 1.9 mH Four turns of #16 on a micrometals T51-52C core has an inductance of 1.9 µH, has a dc resistance of 6.6 mΩ, and will dissipate about 1 W under full load conditions. With an output inductor value of 1.9 µH, the ripple current will be 1750 mA under the low-input-high-output condition. 4) To meet the output noise voltage requirement, the output capacitor(s) must be chosen so that the ripple voltage induced across the ESR of the capacitors by the output ripple current is less than 50 mV. ESR t 50 mV DI OUT + 42 mW Additionally, to meet output load transient response requirements, the capacitors’ ESL and ESR must be low enough to avoid excessive voltage transient spikes. (See Application Note U-157 for a discussion of how to determine the amount and type of load capacitance.) For this example, four Sanyo MV-GX 1500- µF, 6.3-V capacitors will be used. The ESR of each capacitor is approximately 44 m Ω so the parallel combination of four results in an equivalent ESR of 11 m Ω. 5) Q1 and Q2 are chosen to be IRF3103 N-Channel MOSFETs. Each MOSFET has an RDS(on) of approximately 0.014 Ω, a gate charge requirement of 50 nC, and a turn-off time of approximately 54 ns. To calculate the losses in the upper MOSFET, Q1, first calculate the RMS current it will be conducting. I Q1 RMS + d I OUT2 ) DI OUT2 12 Notice that with a higher output voltage, the duty cycle increases, and therefore so does the RMS current. Any heat sink design should take into account the worst case power dissipation the device will experience. (4) (5) (6) (7) (8) |
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