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UCC3889NG4 Datasheet(PDF) 2 Page - Texas Instruments |
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UCC3889NG4 Datasheet(HTML) 2 Page - Texas Instruments |
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2 / 14 page  UCC1889 UCC2889 UCC3889 OPERATION (cont.) At the end of the on time, Q1 is turned off and the RON current into TON is again diverted to VCC. Thus the cur- rent through RON, which charges CT during the on time, contributes to supplying control power during the off time. The power switch off time is controlled by the discharge of CT which, in turn, is programmed by the regulated out- put voltage. The relationship between CT discharge cur- rent, IDCHG, and output voltage is illustrated as follows: 1. When VOUT = 0, the off time is infinite. This feature provides inherent short circuit protection. However, to ensure output voltage startup when the output is not a short, a high value resistor, RS, is placed in parallel with CT to establish a minimum switching frequency. 2. As VOUT rises above approximately 0.7V to its regu- lated value, IDCHG is defined by ROFF, and therefore is equal to: IDCHG = (VOUT - 0.7V) / ROFF As VOUT increases, IDCHG increases resulting in the reduction of off time. The frequency of operation in- creases and VOUT rises quickly to its regulated value. 3. In this region, a transconductance amplifier reduces IDCHG in order to maintain VOUT in regulation. 4. If VOUT should rise above its regulation range, IDCHG falls to zero and the circuit returns to the minimum fre- quency established by RS and CT. The range of switching frequencies is established by RON, ROFF, RS, and CT as follows: Frequency = 1/(TON + TOFF) TON = RON • CT • 4.6 V/(VIN - 4.5V) TOFF (max) = 1.4 • RS • CT Regions 1 and 4 TOFF = ROFF • CT • 3.7V /(VOUT - 0.7V) Region 2, excluding the effects of RS which have a minimal impact on TOFF. The above equations assume that VCC equals 9V. The voltage at TON increases from approximately 2.5V to 6.5V while CT is charging. To take this into account, VIN is adjusted by 4.5V in the calculation of TON. The voltage at TOFF is approximately 0.7V. DESIGN EXAMPLE The UCC3889 regulates a 12 volt, 1 Watt nonisolated DC output from AC inputs between 80 and 265 volts. In this ex- ample, the IC is programmed to deliver a maximum on time gate drive pulse width of 2.4 microseconds which occurs at 80 VAC. The corresponding switching frequency is approximately 100kHz at low line, and overall efficiency is ap- proximately 50%. Additional design information is available in Unitrode Application Note U-149. UDG-93062-3 2 |
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