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ISL97671A Datasheet(PDF) 12 Page - Renesas Technology Corp |
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ISL97671A Datasheet(HTML) 12 Page - Renesas Technology Corp |
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12 / 28 page  ISL97671A FN7709 Rev.4.00 Page 12 of 28 Sep 14, 2017 The default PWM dimming mode is in DPST. In all of the methods, the average LED channel current is controlled by ILED and the PWM duty cycle in percent, as shown in Equation 5: Method 1 (SMBus/I2C Controlled Dimming) The average LED channel current is controlled by the internally generated PWM signal, as shown in Equation 6: where BRT is the PWM brightness level programmed in the Register 0x00. BRT ranges from 0 to 255 in decimal and defaults to 255 (0xFF). BRT = 0 disconnects all channels. To use only the SMBus/I2C controlled PWM brightness control, set Register 0x01 to 0x05. Alternatively, the same operation can be obtained by leaving Register 0x01 at its default value of 0x01 (DPST mode) and connecting the PWM input to VDC, so that the dimming level depends only on the BRT register. The PWM dimming frequency is adjusted by a resistor at the FPWM pin. Method 2 (PWM Controlled Dimming with Settable Dimming Frequency) The average LED channel current can also be controlled by the duty cycle of the external PWM signal, as shown in Equation 7: The PWM dimming frequency is adjusted by a resistor at the FPWM pin. The PWM input cannot be low for more than 30.5ms or else the driver will enter shutdown. To use the externally applied PWM signal only for brightness control, set Register 0x01 to 0x03. Alternatively, the same operation can be obtained by leaving Register 0x01 at its default value of 0x01 (DPST mode), and not programming Register BRT, so that it contains its default value of 0xFF. A third way to obtain this mode of operation is to tie both SCL and SDA to ground. Method 3 (DPST Mode) The average LED channel current can also be controlled by the product of the SMBus/I2C controlled PWM and the external PWM signals as: where: therefore: where BRT is the value held in Register 0x00 (default setting 0xFF) controlled by SMBus/I2C and PWMI is the duty cycle of the incoming PWM signal. In this way, users can change the PWM current in a ratiometric manner to achieve DPST compliant backlight dimming. To use the DPST mode, set Register 0x01 to 0x01. The PWM dimming frequency is adjusted by a resistor at the FPWM pin. For example, if the SMBus/I2C controlled PWM duty is 80% dimming at 200Hz (see Equation 11) and the external PWM duty cycle is 60% dimming at 1kHz, the resultant PWM duty cycle is 48% dimming at 200Hz. In DPST mode, the ISL97671A features 8-bit dimming resolution; it calculates the dimming level by taking the eight most significant bits of the product of the PWMI duty cycle (digitized with 8-bit resolution) and of the BRT I2C register. Method 4 (Direct PWM Mode) Direct PWM Dimming mode is selected when fPWM is tied to VDC and SMBCLK/SMBDAT are grounded. The current of the six channels will follow the incoming PWM signal’s frequency and duty cycle. The minimum duty cycle can be as low as 0.007% at 200Hz (or an equivalent pulse width of 350ns). This ultra low duty cycle dimming performance can be achieved if no channel capacitor is present. Additionally, the Phase Shift function is disabled in Direct PWM Dimming mode. PWM Dimming Frequency Adjustment For dimming methods 1-3, the PWM dimming frequency is set by an external resistor at the FPWM pin as: where fPWM is the PWM dimming frequency and RFPWM is the setting resistor. The maximum PWM dimming frequency is 30kHz when the duty cycle is from 0.4% to 100%. Phase Shift Control For dimming methods 1-3, the ISL97671A is capable of delaying the phase of each current source to minimize load transients. By default, phase shifting is disabled as shown in Figure 20 where the channels PWM currents are switching at the same time. ILED ave ILED PWM = (EQ. 5) ILED ave ILED BRT 255 = (EQ. 6) ILILED ave ILED PWMI = (EQ. 7) ILED ave ILEDxPWMDPST = (EQ. 8) PPWMDPST BRT 255 PWMI = (EQ. 9) ILED ave ILED BRT 255 PWMI = (EQ. 10) TABLE 1. DIMMING MODE SELECTION SMBCLK/ SCL PIN SIGNAL SMBDAT/ SDA PIN SIGNAL FPWM PIN 0x01 REGISTER DIMMING METHOD SELECTION SMBUS /I2C clock SMBUS /I2C data Resistor to ground Set to 0x05, or set to 0x01 and connect PWM to VDC Method 1 (SMBUS/I2C controlled dimming) SMBUS /I2C clock SMBUS /I2C data Resistor to ground Set to 0x03, or set to 0x01 and not program register 0x00 Method 2 (PWM controlled with settable dimming frequency) Grounded Grounded Resistor to ground N/A Method 2 (PWM controlled with settable dimming frequency) SMBUS /I2C clock SMBUS /I2C data Resistor to ground Set to 0x01 Method 3 (DPST mode) Grounded Grounded Tie to VDC N/A Method 4 (Direct PWM dimming) FPWM 6.66 7 10 RFPWM ------------------------ = (EQ. 11) |
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