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LT3756EUD-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LT3756EUD-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3756/LT3756-1 10 37561f APPLICATIONS INFORMATION INTVCC Regulator Bypassing and Operation The INTVCC pin requires a capacitor for stable operation and to store the charge for the large GATE switching cur- rents. Choose a 10V rated low ESR, X7R or X5R ceramic capacitor for best performance. The value of the capacitor is determined primarily by the stability of the regulator rather than the gate charge, QG, of the switching NMOS—a 4.7μF capacitor will be adequate for many applications. Place the capacitor close to the IC to minimize the trace length to the INTVCC pin and also to the IC ground. An internal current limit on the INTVCC output protects the LT3756 from excessive on-chip power dissipation. The minimum value of this current should be considered when choosing the switching NMOS and the operating frequency. IINTVCC can be calculated from the following equation: IINTVCC = QG • fOSC Careful choice of a lower QG FET will allow higher switch- ing frequencies, leading to smaller magnetics. The INTVCC pin has its own undervoltage disable (UVLO) set to 4.3V (typical) to protect the external FETs from excessive power dissipation caused by not being fully enhanced. If the INTVCC pin drops below the UVLO threshold, the GATE and PWMOUT pins will be forced to 0V and the soft-start pin will be reset. If the input voltage, VIN, will not exceed 7V, then the INTVCC pin should be connected to the input supply. Be aware that a small current (typically less than 10μA) will load the INTVCC in shutdown. If VIN is normally above, but occasionally drops below the INTVCC regulation voltage, then the minimum operating VIN will be close to 6V. This value is determined by the dropout voltage of the linear regulator and the 4.5V (4.3V typical) INTVCC undervoltage lockout threshold mentioned above. SHDN/UVLO LT3756 VIN R2 3756 F01 R1 Figure 1 Programming the Turn-On and Turn-Off Thresholds with the SHDN/UVLO Pin The falling UVLO value can be accurately set by the resis- tor divider. A small 2μA pull-down current is active when SHDN/UVLO is below the 1.24V threshold. The purpose of this current is to allow the user to program the rising hysteresis. The following equations should be used to determine the values of the resistors: V RR R IN FALLING , .• = + 124 12 2 VμA R IN RISING HYST , • = 21 LED Current Programming The LED current is programmed by placing an appropriate value current sense resistor between the ISP and ISN pins. Typically, sensing of the current should be done at the top of the LED string. If this option is not available, then the current may be sensed at the bottom of the string, but take caution that the minimum ISN value does not fall below 3V, which is the lower limit of the LED current regulation function. The CTRL pin should be tied to a voltage higher than 1.1V to get the full-scale 100mV (typical) threshold across the sense resistor. The CTRL pin can also be used to dim the LED current to zero, although relative accuracy decreases with the decreasing voltage sense threshold. When the CTRL pin voltage is less than 1.1V, the LED current is: I VmV R LED CTRL LED = − 100 10 • |
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