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TPS92690Q1PWP Datasheet(PDF) 10 Page - Texas Instruments |
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TPS92690Q1PWP Datasheet(HTML) 10 Page - Texas Instruments |
10 / 28 page RCS I LED CCMP VCS TPS92690 COMP CSP IADJ GM EA 0-5V R 9R IS PWM ILIM ILIM Artificial Ramp GATE VLIM IT 0-400mV To GATE *RF *CF * RLIM SW 11 9 T 1 2.29 10 R 80 10 - - ¦ = ´ ´ + ´ TPS92690 SLVSBK3 – DECEMBER 2012 www.ti.com where n is the primary to secondary turns ratio of the coupled inductor, n:1. PEAK CURRENT MODE CONTROL Peak current mode control is used by the TPS92690 to regulate the average LED current through an array of HBLEDs. This method of control uses a series resistor in the LED path to sense LED current and can use either a series resistor in the MosFET path or the MosFET RDS-ON for both cycle-by-cycle current limit and input voltage feed forward. The controller has a fixed switching frequency set by an internal programmable oscillator therefore slope compensation is added to mitigate current mode instability. A detailed explanation of this control method is presented in the following sections. SWITCHING FREQUENCY AND SYNCHRONIZATION The switching frequency of the TPS92690 is programmed using an external resistor (RT) connected from the RT pin to GND. This switching frequency is defined as: (4) See Typical Characteristics for a graph of switching frequency versus resistance on RT. For maximum operational range and best efficiency a switching frequency of 1MHz or lower is recommended. Switching frequencies as high as 2MHz are possible for reduced solution size but some factors need to be considered. Higher frequencies require increased gate drive current and result in higher AC losses, both of which result in decreased efficiency. It is also possible that the minimum on time (leading edge blanking time) limits the minimum operational duty cycle and reduces the input voltage range for a given output voltage. Alternatively, an external PWM signal can be applied to the SYNC pin to synchronize the part to an external clock. If the PWM signal frequency applied is higher than the base frequency set by the RT resistor, the internal oscillator is bypassed and the switching frequency is equal to the synchronized frequency. The PWM signal should have an amplitude between 2.5V and 5V. The TPS92690 triggers a switch-on time on the falling edge of the PWM signal and operates correctly regardless of the duty cycle of the applied signal. Figure 12. Current Sensed and Control Circuitry (* optional) CURRENT SENSE/CURRENT LIMIT The TPS92690 implements peak current mode control using the circuit shown in Figure 12. The peak detection is accomplished with a comparator that monitors the main MosFET current, comparing it with the COMP pin. When the IS voltage (plus the DC level shift and the ramp discussed later) exceeds the COMP voltage, the MosFET is turned off. The MosFET is turned back on when the oscillator starts a new on-time and the cycle repeats. 10 Submit Documentation Feedback Copyright © 2012, Texas Instruments Incorporated Product Folder Links :TPS92690 |
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