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TPS92661QPHPRQ1 Fiches technique(PDF) 11 Page - Texas Instruments |
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TPS92661QPHPRQ1 Fiches technique(HTML) 11 Page - Texas Instruments |
11 / 53 page LEDxOFF 800 LEDxON 250 TCNT 0 1023 LEDx current Time ( )( ) ( )( ) DS on x _ channels ALL on max n R R 238 m n 12 æ ö = ´ = W ´ ç ÷ è ø 11 TPS92661-Q1 www.ti.com SLUSBU2A – SEPTEMBER 2014 – REVISED FEBRUARY 2016 Product Folder Links: TPS92661-Q1 Submit Documentation Feedback Copyright © 2014–2016, Texas Instruments Incorporated Feature Description (continued) 7.3.2 Internal Switch Resistance Each single switch (connected between LEDn and LEDn–1) has a measurable typical RDS(on) value of 225 mΩ. This measurement includes the actual on-resistance (RDS(on)) of the switch and the resistance of the two internally connected bond wires. When multiple series switches are on, the effective resistance is not simply the number of channels multiplied by 225 m Ω because there are not two conducting bond wires for every series- connected switch. For this reason the all-switches on-resistance (RALL(on)) is specified in the Electrical Characteristics table. This value includes the twelve RDS(on) on-resistances and the resistance of the bond wires at each end of the series connected switches. The dominant power loss mechanism In the TPS92661 device, is I2R loss through the switches. Other power loss sources are always less than 50 mW. When calculating the power dissipation of the TPS92661 device switches, use Equation 1 for the best estimation of this power loss. • where n is the number of channels (1) See the 6 LED, 1.5-A Application section for a sample calculation. 7.3.3 PWM Dimming The TPS92661 device provides 10-bit PWM dimming of each individual LED. The LED turn-on and turn-off times are separately programmed for each LED. The LEDxON registers and LEDxOFF registers (where x = 1 to 12) determine the LED turn-on and turn-off times, respectively, within the PWM dimming period. Phase shifting can be accomplished by staggering the LEDxON times or LEDxOFF times. The 10-bit internal PWM Period Counter (TCNT) is compared against the LEDxON and LEDxOFF values. When TCNT reaches the programmed LEDxON value for a given LED, the corresponding bypass switch is turned off to force current through the LED. Similarly, when TCNT reaches the programmed LEDxOFF value, the bypass switch is turned on to turn off the LED. TCNT counts continuously from 0 to 1023 and returns to 0 again. The LED PWM dimming period equals 1024 times the internally divided, programmable PWM clock period. Figure 9 shows an example of LED PWM using values of LEDxON = 250 and LEDxOFF = 800. Figure 9. LED PWM Example Because the LEDxON and LEDxOFF times are completely programmable, this allows the system flexibility to phase shift the leading edge (LED On), the trailing edge (LED Off), or double-edge PWM. The comparison circuitry consists of a digital comparator, along with an AND gate to allow that particular comparison to propagate to the LED switch. The TCNT counter value is continuously compared against the value programmed into the LED On/Off registers. The ENON bit that corresponds to that particular LED determines whether or not that comparison has any effect at the LED. The logic is represented in Figure 10. |
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