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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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ISL97678 Fiches technique(PDF) 11 Page - Renesas Technology Corp |
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ISL97678 Fiches technique(HTML) 11 Page - Renesas Technology Corp |
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11 / 17 page  ISL97678 FN6998 Rev.3.00 Page 11 of 17 Sep 8, 2017 Theory of Operation PWM Boost Converter The current mode PWM boost converter produces the minimal voltage needed to enable the LED string with the highest forward voltage drop to run at the programmed current. The ISL97678 employs current mode control boost architecture, which has a fast current sense loop and a slow voltage feedback loop. This architecture achieves a fast transient response that is essential for notebook backlight applications in which the power can be several Li-ion cell batteries that instantly change to an AC/DC adapter without rendering a noticeable visual nuisance. The number of LEDs that can be driven by the ISL97678 depends on the type of LED chosen in the application. The ISL97678 is capable of boosting up to 45V and drive eight channels of LEDs at a maximum of 45mA per channel. Current Matching and Current Accuracy Each channel of the LED current is regulated by the current source circuit, as shown in Figure 26. The LED peak current is set by translating the RSET current to the output with a scaling factor of 707.9/RSET. The sink 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 0.1% tolerance resistor is recommended. . Dynamic Headroom Control The ISL97678 features a proprietary Dynamic Headroom Control circuit that detects the highest forward voltage string or effectively the lowest voltage from any of the CH pins. When this lowest IIN 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 pin is at the target headroom voltage. Because all LED strings 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 ensures that each channel has the same programmed current. The output voltage regulates cycle-by-cycle and is always referenced to the highest forward voltage string in the architecture. OVP and VOUT Requirement The Overvoltage Protection (OVP) pin sets the overvoltage trip level and limits the VOUT regulation range. The ISL97678 OVP threshold is set by RUPPER and RLOWER as shown in Equation 1: VOUT can regulate only between 64% and 100% of the VOUT_OVP such that: Allowable VOUT = 64% to 100% of VOUT_OVP For example, if 10 LEDs are used with the worst case VOUT of 35V. If RUPPER and RLOWER are chosen such that the OVP level is set at 40V, then the VOUT is allowed to operate between 25.6V and 40V. If the requirement is changed to a 6 LEDs 21V VOUT application, then the OVP level must be reduced and users should follow VOUT = (64% ~100%) OVP requirement. Otherwise, the headroom control will be disturbed such that the channel voltage can be much higher than expected and sometimes it can prevent the driver from operating properly. The ratio of the OVP capacitors should be the inverse of the OVP resistors. For example, if RUPPER/RLOWER = 33/1, then CUPPER/CLOWER = 1/33 with CUPPER = 100pF and CLOWER =3.3nF. Dimming Controls The ISL97678 provides 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. In any dimming controls, the EN pin must be high. EN is a high voltage pin that can be applied with a digital signal or tied directly to VIN for enable function. MAXIMUM DC CURRENT SETTING The initial brightness should be set by choosing an appropriate value for RSET. This should be chosen to fix the maximum possible LED current: Alternatively, the RSET can be replaced by a digital potentiometer for adjustable current. FIGURE 26. SIMPLIFIED CURRENT SOURCE CIRCUIT REF + - + - PWM DIMMING RSET + REF - + - RSET - VOUT_OVP 1.21V RUPPER RLOWER + R LOWER = (EQ. 1) ILEDmax 707.9 RSET --------------- = (EQ. 2) |
Numéro de pièce similaire - ISL97678 |
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Description similaire - ISL97678 |
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