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UCC3972PWTR Datasheet(PDF) 9 Page - Texas Instruments |
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UCC3972PWTR Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 24 page  9 UCC1972/3 UCC2972/3 UCC3972/3 The current sense resistor RS provides direct control of lamp current. Since the current sense resistor voltage is controlled to a 1.5V reference, its power loss is inversely proportional to its value at a given lamp current. Synchronizing the Stages An internal comparator at the BUCK node is used to syn- chronize the PWM buck frequency to twice the resonant tank frequency. Synchronization is accomplished with sync pulse that is generated each time the BUCK node voltage is within 1.0V of VBAT; the UCC3972/3 uses this sync pulse to reset the PWM oscillator’s saw-tooth ramp. The syn circuit will operate at 2 X the transform switching frequency. Buck Stage Design The PWM output controls current in the buck inductor. The UCC3972/3’s buck power stage differs from a tradi- tional buck topology in a few respects: • The topology is inverted using a ground referenced N-Channel MOSFET rather than a VDD referenced P-Channel. • The output voltage is a full wave rectified sinewave at the switching frequency, rather than DC. Referring back to Fig. 1, when OUT turns SBUCK on, the BUCK node voltage VBUCK is placed across the inductor. This voltage is typically positive and current ramps up in the inductor (it is possible for the BUCK node voltage to go negative if VBAT is low and the lamp current is near maximum). When SBUCK is turned off, VBAT-VBUCK+VDBUCK is placed across the inductor with opposite polarity. As with any buck converter, the volt-seconds across the inductor must be reversed on each switching cycle to maintain constant current. The duty cycle (D) relationship is complicated somewhat by the fact the output voltage is changing within a switching cycle. The equations below determine the relationship between on and off times in continuous conduction mode where T is the switching period, D = tON/T, and tOFF =T- tON. () V dt VBAT V V dt BUCK t BUCK D t T ON ON ·= - + · òò 0 (8) Selecting the buck inductor: Maximum ripple current in the inductor occurs when fre- quency and duty cycle are at a minimum, which corre- sponds to VBAT and lamp current being a maximum. The average value of VBUCK at rated lamp current is equal to: VV V N VV V BUCK AVE BAT SEC BAT BAT _ . =- · · =- · · =- · 2 820 2 67 55 p p olts (9) The approximate on time using the maximum 22V input voltage (VBUCK_AVE = 16.4), a 100kHz switching fre- quency (two times the resonant frequency), and ignoring the diode drop can be calculated from the following: t Tt VBAT V V ON ON BUCK AVE BUCK AVE - = - _ _ (10) The resulting on time is 2.5 ms. A 150mH inductor will re- sult in a peak to peak ripple current of 280mA. Average inductor current (with maximum lamp current) can be cal- culated by taking the lamp power divided by the tank effi- ciency and the RMS buck voltage. I VI Efficiency N V BUCK LAMP LAMP SEC = · æ è çç ö ø ÷÷· · æ è çç ö ø ÷ 2 ÷ = ·· · · = 375 0 005 2 67 0 8 820 380 . . mA (11) The resulting inductor ripple is less than 50%. A list of possible inductors are given below along with ESR and current rating (losses in the inductor are calculated with RMS current). The choice of a MOSFET for the buck switch should take into consideration conduction and switching losses. The RDS(on) and gate charge are typically at odds, however, where minimizing one will typically result in the other in- creasing. An International Rectifier IRFL014 was se- lected (SOT-223 package) in this application with a gate charge of 11nC and RDS(on) of 0.2W. A Schottky diode should be used for the buck diode in order to minimize forward drop. APPLICATION INFORMATION (cont.) Vendor L Part Number ESR Current Rating Coilcraft (847) 639-6400 150 mH DO3316-154 0.38 1A Coiltronics (407) 241-7876 150 mH CTX150-4 0.175 0.72A Sumida (847) 956-0666 150 mH CDR125-151 0.4 0.85A Toko (847) 297-0070 150 mH 646CY-151 0.73 0.4A Table 3. Inductor Suppliers |
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