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TPS61187RTJR Datasheet(PDF) 11 Page - Texas Instruments |
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TPS61187RTJR Datasheet(HTML) 11 Page - Texas Instruments |
11 / 25 page ´ ISETH FB ISET ISETH V I = K R 8 DIM FPWM 1.818 10 F = R ´ TPS61187 www.ti.com SLVSA85D – JUNE 2010 – REVISED FEBRUARY 2012 LED CURRENT SINKS The six current sink regulators embedded in the TPS61187 can be collectively configured to provide up to a maximum of 30 mA each. These six specialized current sinks are accurate to within ±2% max for currents at 20 mA, with a string-to-string difference of ±1.5% typical. The IFB current must be programmed to the highest WLED current expected using the ISETH pin resistor and Equation 2. (2) Where: KISET = 980 (current multiple) VISETH = 1.229V (ISETH pin voltage) RISETH = ISETH pin resistor ENABLE AND STARTUP The internal regulator which provides VDDIO wakes up as soon as VIN is applied even when EN is low. This allows the IC to start when EN is tied to the VDDIO pin. VDDIO does not come to full regulation until EN is high. The TPS61187 checks the status of all current feedback channels and shuts down any unused feedback channels. It is recommended to short the unused channels to ground for faster startup. After the device is enabled, if the PWM pin is left floating, the output voltage of the TPS61187 regulates to the minimum output voltage. Once the IC detects a voltage on the PWM pin, the TPS61187 begins to regulate the IFB pin current, as pre-set per the ISETH pin resistor, according to the duty cycle of the signal on the PWM pin. The boost converter output voltage rises to the appropriate level to accommodate the sum of the white LED string with the highest forward voltage drops plus the headroom of the current sink at that current. Pulling the EN pin low shuts down the IC, resulting in the IC consuming less than 11 µA in shutdown mode. IFB PIN UNUSED The TPS61187 has open/short string detection. For an unused IFB string, simply short it to ground or leave it open. Shorting unused IFB pins to ground for faster startup is recommended. BRIGHTNESS DIMMING CONTROL The TPS61187 has auto phase shifted PWM dimming control with the PWM control interface. The internal decoder block detects duty information from the input PWM signal, saves it in an eight bit register and delivers it to the output PWM dimming control circuit. The output PWM dimming control circuit turns on/off six output current sinks at the PWM frequency set by RFPWM and the duty cycle from the decoder block. The TPS61187 also has direct PWM dimming control with the PWM control interface. In direct PWM mode, each current sink turns on/off at the same frequency and duty cycle as the input PWM signal. See the Mode Selection section for dimming mode selection. When in phase shifted PWM mode, it is recommended to insert a series resistor of 10k Ω to 20kΩ value close to PWMIN pin. This resistor together with an internal capacitor forms a low pass R-C filter with 30ns to 60ns time constant. This prevents possible high frequency noises being coupled into the input PWM signal and causing interference to the internal duty cycle decoding circuit. However, it is not necessary for direct PWM mode since the duty cycle decoding circuit is disabled during the direct PWM mode. ADJUSTBLE PWM DIMMING FREQUENCY AND MODE SELECTION (R_FPWM / MODE) The TPS61187 can operate in auto phase shift mode or direct PWM mode. Tying the RFPWM/MODE pin to VDDIO forces the IC to operate in direct PWM mode. A resistor between the RFPWM/MODE pin and ground sets the IC into auto phase shift mode and the value of the resistor determines the PWM dimming frequency. Use Equation 3 or refer to Table 2 to select the correct value: (3) Copyright © 2010–2012, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s): TPS61187 |
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