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TDA9109 Datasheet(PDF) 24 Page - STMicroelectronics |
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TDA9109 Datasheet(HTML) 24 Page - STMicroelectronics |
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24 / 30 page  IV - DC/DC CONVERTER PART OPERATING DESCRIPTION (continued) This unit controls the switch-mode DC/DC con- verter. It converts a DC constant voltage into the B+ voltage (roughly proportional to the horizontal frequency) necessary for the horizontal scanning. ThisDC/DC converter must be configured in step- up mode. It operates very similarly to the well known UC3842. IV.1 - Step-up Mode Operating Description - The power MOS is switched-on at the middle of the horizontal flyback. - The power MOS is switched-off when its current reaches a predetermined value. For this purpose, a sense resistor is inserted in its source. The voltage on this resistor is sent to Pin16 (ISENSE). - The feedback (coming either from the EHV or from the flyback) is divided to a voltage close to 4.8V and compared to the internal 4.8V reference (IVREF). The difference is amplified by an error amplifier, the output of which controls the power MOS switch-off current. Main Features - Switching synchronized on the horizontal fre- quency, - B+ voltage always higher than the DC source, - Current limited on a pulse-by-pulse basis. The DC/DC converter is disabled : - when VCC or VDD are too low, - when X-Ray protection is latched, - directly through I 2C bus. When disabled, BOUT is driven to GND by a 0.5mA current source. This feature allows to implement externally a soft start circuit. 16 14 15 V B+ L + C3 C2 1/3 S S R Q 400ns Inhibit SMPS 28 12V BOUT I SENSE COMP REGIN 95dB A ± I adjust DAC 7bits I2C TDA9109/S 1M W 22k W 1.2V 1.2V 4.8V ±20% 8V Inhibit SMPS Soft Start Figure 17 : DC/DC Converter TDA9109/S 24/30 |
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