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UCC2581 Datasheet(PDF) 5 Page - Texas Instruments |
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UCC2581 Datasheet(HTML) 5 Page - Texas Instruments |
5 / 16 page 5 UCC1581 UCC2581 UCC3581 The externally bypassed 4.0V reference is controlled by undervoltage lockout and chip enable circuitry. The en- able input is internally tied to a 10 µA current source which allows the pin to be driven by an open collector driver. The part is also enabled if EN floats. The UCC3581 has a soft start function which requires a user supplied external timing capacitor. When in soft start mode, the soft start capacitor, CSS, is charged with a constant current source. The soft start current is 3.75X the current in RT1. There is an on-chip control amplifier, which when driving the gate of an external depletion mode N-MOSFET, acts as a 7.5V linear preregulator supplying VDD directly from the primary input power line. The preregulator may sub- sequently be fully disabled by a tertiary bootstrap winding providing a minimum of 8.2V to the VDD pin. Computation of DCMIN DCMIN for a given duty cycle is calculated as follows: () ∆Vi DC tt C OSC ON OFF T =• • + where: • i = oscillator charge current = 9.2 . (2.0V/RT1) • DC = Duty Cycle, as a fraction of 1 • tON = 0.082 • RT1 • CT • tOFF = 0.95 • RT2 • CT • CT = Oscillator Capacitor The CT pin ramp slews from 1V to 2.5V. Therefore, add ∆V to 1V to get DCMIN voltage. Example: For 10% duty cycle with RT1 = 680k Ω, RT2 = 12k Ω, and CT = 705pF, () ∆Vi DC tt C OSC ON OFF T =• • + () = • •• • + • −− 92 20 680 0 1 4 182 10 8 55 10 56 . . . . sec . sec V k 750 10 12 • − ∆VV =018 . Therefore, DCMIN V V V =+ = 10 18 1 18 .. A Typical Micropower Application The circuit shown in Fig. 3 illustrates the use of the UCC3581 in a micropower application. The isolated 5V flyback power supply uses a minimum of parts and oper- ates over an 8:1 input voltage range (15VDC to 120VDC) while delivering a regulated 5V output with a load swing from 0W to 1W. It operates in the discontinuous mode at light load or high line, and continuous mode at heavier loads and lower line voltages. Higher input line voltages are possible by simply increasing the voltage ratings of C1, Q1, D1 and D2. The most notable feature of the design is its efficiency. With a load of 1 watt, the typical efficiency is 82%, drop- ping to 70% around 50mW. With a load of only 12.5mW, the efficiency remains as high as 50%. At this load, with an input of 50V, the total input current is only 500 µA. Note that the power supply can be disabled by pulling the UCC3581 enable pin low, in which case the input current drops to less than 150 µA. The UCC3581 achieves very low losses by means of low quiescent current and pulse skipping at light loads which reduces switching losses. The degree of pulse skipping is controlled by programming the minimum duty cycle. In this example, the frequency is 35kHz at maximum load and drops to <2kHz at 12.5mW load (minimum pulse width of around 6 µsec, or 21% duty cycle at 35kHz). Another way losses are reduced is operating with a VDD of around 10V rather than the more common 12V to 16V. At such light primary currents, the MOSFET remains in full saturation with a gate drive voltage well below 10V. Gate drive losses are minimized by choosing a MOSFET with low total gate charge, in this case only 8nC maxi- mum. By choosing a large gate drive resistor, EMI is min- imized by reducing peak currents. Due to pulse skipping, switching times are less critical for efficiency at light load. The shunt regulator (LM3411) and optocoupler (MOC8100) are also key to the efficiency at such light loads, and were chosen for their low operating current. The LM3411 has a quiescent current of only 150 µA max- imum (compared to 1mA for the more common TL431). In addition, because it is not a three terminal device, the LM3411’s quiescent current does not flow in the optocoupler LED. Since this bias current is not in the feedback control path, a higher value pull-up resistor can be used on the optocoupler output transistor, further re- ducing losses. APPLICATION INFORMATION (cont.) |
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