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ISL97682 データシート(PDF) 10 Page - Renesas Technology Corp |
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ISL97682 データシート(HTML) 10 Page - Renesas Technology Corp |
10 / 18 page ISL97682, ISL97683, ISL97684 FN7689 Rev.2.00 Page 10 of 18 Sep 19, 2017 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 ISL97682, ISL97683, and ISL97684 employ current mode control boost architecture that has a fast current sense loop and a slow voltage feedback loop. Such architecture achieves a fast transient response that is essential for the notebook backlight application where the power can be a series of drained batteries or instantly change to an AC/DC adapter without rendering a noticeable visual nuisance. The number of LEDs that can be driven by the ISL97682, ISL97683, ISL97684 depends on the type of LED chosen in the application. The ISL97682, ISL97683, ISL97684 are capable of boosting up to 45V and typically driving 13 LEDs in series for each of the 4 channels, enabling a total of 52 pieces of the 3.2V/20mA type of LEDs. OVP The Overvoltage Protection (OVP) pin has a function of setting the overvoltage trip level as well as limiting the VOUT regulation range. The ISL97682, ISL97683, ISL97684 OVP threshold is set by RUPPER and RLOWER shown by Equation 1: VOUT can only regulate between 42% and 100% of the VOUT_OVP such that: Allowable VOUT = 42% to 100% of VOUT_OVP For example, if 10 LEDs are used with the worst case being VOUT of 35V. If R1 and R2 are chosen such that the OVP level is set at 40V, then the VOUT is allowed to operate between 16.8V and 40V. If the requirement is changed to 4 LEDs of 14V VOUT application, then the OVP level must be reduced and users should follow the VOUT = (42% ~100%) OVP level requirement. Otherwise, the headroom control will be disturbed such that the channel voltage can be much higher than expected and sometimes can prevent the driver from operating properly. The ratio of the OVP capacitor should be the inverse of the OVP resistor. For example: if RUPPER/RLOWER = 33/1, then CUPPER/CLOWER = 1/33 with CUPPER = 100pF and CUPPER =3.3nF. Enable An EN signal is required to enable the internal regulator for normal operation. If there is no signal for longer than 28ms, the device will enter shutdown. Power Sequence There is no specific power sequence requirement for the ISL97682, ISL97683, ISL97684. The EN signal can be tied to VIN but not the VDC that will prevent the device from powering up. Current Matching and Current Accuracy Each channel of the LED current is regulated by the current source circuit, as shown in Figure 19. The LED peak current is set by translating the RSET current to the output with a scaling factor of 401.8/RSET. The source terminals of the current source MOSFETs are designed to operate within a range at about 500mV to optimize power loss versus accuracy requirements. The sources of errors of the channel-to-channel current matching come from the op amps offset, internal layout, reference, and current source resistors. These parameters are optimized for current matching and absolute current accuracy. However, the absolute accuracy is additionally determined by the external RSET. A 1% tolerance resistor is recommended. Dynamic Headroom Control The ISL97682, ISL97683, ISL97684 feature a proprietary Dynamic Headroom Control circuit that detects the highest forward voltage string or the lowest voltage from any of the CH pins digitally. When the lowest CH voltage is lower than the short circuit threshold (VSC), such voltage will be used as the feedback signal for the boost regulator. The boost makes the output to the correct level such that the lowest CH is at the target headroom voltage. Since all LED stacks are connected to the same output voltage, the other CH pins will have a higher voltage, but the regulated current source circuit on each channel will ensure that each channel has the same current. The output voltage will regulate cycle-by-cycle and it is always referenced to the highest forward voltage string in the architecture. Dimming Controls The ISL97682, ISL97683, ISL97684 allow two ways of controlling the LED current, and therefore, the brightness. They are: 1. DC current adjustment 2. PWM chopping of the LED current defined in Step 1. There are various ways to achieve DC or PWM current control, which will be described in the following. (EQ. 1) VOUT_OVP = 1.22V * (RUPPER + RLOWER)/RLOWER FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT REF + - PWM DIMMING RSET +- |
同様の部品番号 - ISL97682 |
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同様の説明 - ISL97682 |
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