Datasheet LT3956 (Analog Devices) - 10

LT3956 applicaTions inForMaTion
INTVCC Regulator Bypassing and Operation LED Current Programming
The INTVCC pin requires a capacitor for stable operation The LED current is programmed by placing an appropri- and to store the charge for the switch driver and PWMOUT ate value current sense resistor, RLED, between the ISP pin switching currents. Choose a 10V rated low ESR, X7R and ISN pins. Typically, sensing of the current should or X5R ceramic capacitor for best performance. A 4.7µF be done at the top of the LED string. If this option is not capacitor will be adequate for many applications. Place available, then the current may be sensed at the bottom the capacitor close to the IC to minimize the trace length of the string, but take caution that the minimum ISN value to the INTVCC pin and also to the IC ground. does not fall below 3V, which is the lower limit of the LED An internal current limit on the INTV current regulation function. The CTRL pin should be tied CC output protects the LT3956 from excessive on-chip power dissipation. to a voltage higher than 1.2V to get the full-scale 250mV The INTV (typical) threshold across the sense resistor. The CTRL pin CC pin has its own undervoltage disable (UVLO) set to 4.1V (typical) to protect the internal MOSFET from can also be used to dim the LED current to zero, although excessive power dissipation caused by not being fully en- relative accuracy decreases with the decreasing voltage hanced. If the INTV sense threshold. When the CTRL pin voltage is less than CC pin drops below the UVLO threshold, the PWMOUT pin will be forced to 0V, the power switch 1V, the LED current is: turned off and the soft-start pin will be reset. V − 10 m 0 V I CTRL = If the input voltage, V LED IN, will not exceed 7V, then the INTVCC RLED • 4 pin could be connected to the input supply. This action allows the LT3956 to operate from as low as 4.5V. Be aware When the CTRL pin voltage is between 1V and 1.2V that a small current (less than 12μA) will load the INTV the LED current varies with CTRL, but departs from the CC in shutdown. Otherwise, the minimum operating V previous equation by an increasing amount as the CTRL IN value is determined by the dropout voltage of the linear regulator voltage increases. Ultimately, above CTRL = 1.2V, the LED and the 4.4V (4.1V typical) INTV current no longer varies with CTRL. At CTRL = 1.1V, the CC undervoltage lockout threshold mentioned above. actual value of ILED is ~98% of the equation’s estimate. When VCTRL is higher than 1.2V, the LED current is regu-
Programming the Turn-On and Turn-Off Thresholds
lated to:
With the EN/UVLO Pin
25 m 0 V The falling UVLO value can be accurately set by the resistor ILED = R divider. A small 2.1µA pull-down current is active when LED EN/UVLO is below the falling threshold. The purpose of The CTRL pin should not be left open (tie to VREF if not this current is to allow the user to program the rising used). The CTRL pin can also be used in conjunction with hysteresis. The following equations should be used to a thermistor to provide overtemperature protection for determine the values of the resistors: the LED load, or with a resistor divider to VIN to reduce R1 R2 output power and switching current when VIN is low. V = + 1.22 • IN,FALLING The presence of a time varying differential voltage signal R2 (ripple) across ISP and ISN at the switching frequency V = 2. µ 1 A • R1+ V is expected. The amplitude of this signal is increased by IN R , ISING IN F , ALLING high LED load current, low switching frequency and/or a VIN smaller value output filter capacitor. Some level of ripple LT3956 R1 signal is acceptable: the compensation capacitor on the EN/UVLO VC pin filters the signal so the average difference between R2 ISP and ISN is regulated to the user-programmed value. 3956 F01 Ripple voltage amplitude (peak-to-peak) in excess of
Figure 1
3956f 0 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Typical Application Related Parts