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ISL97691IRTZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL97691IRTZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 19 page  ISL97691 11 FN7840.1 February 1, 2013 Theory of Operation PWM Boost Converter The current mode PWM boost converter produces the minimal voltage needed to enable the LED stack with the highest forward voltage drop to run at the programmed current. The ISL97691 employs current mode control boost architecture that has a fast current sense loop and a slow voltage feedback loop. This architecture achieves the fast transient response which is essential for portable product backlight applications where the backlight must not flicker when the power source is changed from a drained battery to an AC/DC adapter. The number of LEDs that can be driven by the ISL97691 depends on the type of LED chosen in the application. The maximum output is 26V at 40mA from 2.7V input, or 21V at 60mA from 2.7V input with 30% dimming duty. AutoShutdown The ISL97691 simplifies the implementation of low power shutdown by detecting that both PWM inputs 1-WIRE_1 and 1-WIRE_2 have been low for 120ms (typical). When this timeout is detected on both 1-WIRE_1 and 1-WIRE_2, the boost converter and LED string drivers are turned off while retaining the value of the two internal 6-bit 1-Wire registers which store the analog dimming values. The 1-Wire interfaces may be written during shutdown mode subject to the following constraints: • Any interface (1-WIRE_1 or 1-WIRE_2) to be written during shutdown should idle high between transmissions • The first bit of every 6-bit 1-Wire transmission must be sent with a tighter timing tolerance than normal (25µs minimum for logic 1, 100µs minimum for logic 0). The extra 10µs allows the ISL97691 to wakeup and measure the transmission. It is recommended that applications requiring 1-Wire transmissions during shutdown simply apply the constraints described above to all transmissions. Dimming Controls The ISL97691 provides two groups, channels 1 and 2 as one group and channels 3 and 4 as the other group, with independent analog and PWM dimming. The maximum LED current is set globally for all channels by RSET in the range 15mA to 60mA per Equation 1: Where: RSET is resistor from ISET pin to GND (Ω) ILEDmax is the peak current set by resistor RSET (A) For example, if the maximum required LED current (ILEDmax) is 60mA, then the RSET value needed is: Choose nearest standard resistor: 17.8kΩ , 0.1% ANALOG DIMMING The channel peak current for the two groups may be reduced from the maximum LED current set by RSET by analog dimming through two 6-bit DACs. The DACs are updated with simple negative pulse duration and pulse counting 1-Wire interfaces. Channels 1 and 2 are set through the 1-WIRE_1 pin, and channels 3 and 4 are set through the 1-WIRE_2 pin. The two 1-Wire interfaces are independent of the two PWM inputs PWMI_1 and PWMI_2, and any combination of these 4 inputs may be active at the same time. The 1-Wire interface provides 6-bit (64-level) analog dimming resolution. The interface uses a normally high connection for use with open-drain driving schemes and Intersil’s 1-Wire interface. When held low for between 15µs and 45µs, the interface reads a logic 1. When held low for between 90µs and 120µs the interface reads a logic 0. When held low for greater that 215µs, the interface loads the last 6 bits into the brightness control register and updates the peak current. The required minimum high time is 3µs. The 1-Wire programming is summarized as follows: • Logic 0 = Negative pulse >90µs and <120µs • Logic 1 = Negative pulse >15µs and <45µs • Load = Negative pulse >215µs Figure 18 shows an example of transmitting and loading the value b’100101’. The serial interface defaults to b’111111’ (63) on power-up. The maximum LED current ILEDmax set by resistor RSET for each of the two channel groups 1 and 2 is digitally adjusted by each channel’s brightness control register per Equation 3: Where: N is the integer value 1 to 63 in the channel groups’ brightness control register ILEDmax is the peak current set by resistor RSET (A) I LEDmax 1066 R SET --------------- = (EQ. 1) R SET 1066 0.06 ⁄ 17.8k Ω == (EQ. 2) + - + - PWM SIGNAL REF + - 1-WIRE DATA 6-BIT DAC FIGURE 18. I LED I LEDmax N 63 ------ × = (EQ. 3) |
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