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UCC3888NG4 Datasheet(PDF) 2 Page - Texas Instruments |
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UCC3888NG4 Datasheet(HTML) 2 Page - Texas Instruments |
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2 / 9 page  www.ti.com THEORY OF OPERATION UCC2888 UCC3888 SLUS159B – MARCH 1997 – REVISED JUNE 2007 With reference to the application diagram below, when input voltage is first applied, the current through RON into TON is directed to VCC where it charges the external capacitor, C3, connected to VCC. As voltage builds on VCC, an internal undervoltage lockout holds the circuit off and the output at DRIVE low until VCC reaches 8.4 V. At this time, DRIVE goes high, turning on the power switch, Q1, and redirecting the current into TON to the timing capacitor, CT. CT charges to a fixed threshold with a current ICHG = 0.8 • (VIN – 4.5 V)/RON. Because DRIVE is high only as long as CT charges, the power switch on time will be inversely proportional to line voltage. This provides a constant (line voltage) • (switch on time) product. At the end of the on time, Q1 is turned off, and the current through RON is again diverted to VCC. Thus the current through RON, which charges CT during the on time, contributes to supplying power to the chip during the off time. The power switch off time is controlled by the discharge of CT, which, in turn, is programmed by the regulated output voltage. The relationship between CT discharge current, IDCHG, and output voltage is illustrated as follows: Region 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. Region 2. As VOUT rises above approximately 0.7 V to its regulated value, IDCHG is defined by ROFF, and is equal to: IDCHG = (VOUT– 0.7V) / ROFF As VOUT increases, IDCHG increases reducing off time. The operating frequency increases and VOUT rises quickly to its regulated value. Region 3. In this region, a transconductance amplifier reduces IDCHG to maintain a regulated VOUT. Region 4. If VOUT should rise above its regulation range, IDCHG falls to zero and the circuit returns to the minimum frequency 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 – 4.5 V) TOFF (max) = 1.4 • RS• CT Regions 1 and 4 TOFF = ROFF• CT• 3.7 V/(VOUT – 0.7 V) Region 2, excluding the effects of RS, which have a minimal impact on TOFF. The above equations assume that VCC equals 9 V. The voltage at TON increases from approximately 2.5 V to 6.5 V while CT is charging. To take this into account, VIN is adjusted by 4.5 V in the calculation of TON. The voltage at TOFF is approximately 0.7 V. 2 Submit Documentation Feedback |
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