Datasheet LT3519, LT3519-1, LT3519-2 (Analog Devices) - 9

LT3519/LT3519-1/LT3519-2
APPLICATIONS INFORMATION Dimming Control
When VCTRL is higher than 1.2V, the LED current is clamped to be: There are two methods to control the current source for dimming using the LT3519/LT3519-1/LT3519-2. The fi rst I method, PWM Dimming, uses the PWM pin to modulate LED = 250mV RSENSE the current source between zero and full current to achieve a precisely programmed average current. To make this When VCTRL is more than 1V but less than 1.2V, the method of current control more accurate, the switch de- LED current is in the nonlinear region of VISP-VISN mand current is stored on the internal VC node during the Threshold vs VCTRL as shown in the Typical Performance quiescent phase when PWM is low. This feature minimizes Characteristics. recovery time when the PWM signal goes high. To obtain The LED current programming feature through the CTRL best PWM dimming performance, it is necessary to use pin possibly increases the total dimming range by a factor an external disconnect switch in the LED current path to of ten. In order to have the accurate LED current, precision prevent the output capacitor from discharging during the resistors are preferred (1% is recommended). The CTRL PWM signal low phase. For best product of analog and PWM pin should not be left open. Tie to V dimming, the minimum PWM low or high time should be at REF if not used. least six switching cycles (3μs for fSW = 2MHz). Maximum
Programming Output Voltage (Constant Voltage
PWM period is determined by the system. The maximum
Regulation) or Open LED/Overvoltage Threshold
PWM dimming ratio (PWMRATIO) can be calculated from the maximum PWM period (t For a boost application, the output voltage can be set by MAX) and the minimum PWM pulse width (t selecting the values of R1 and R2 (see Figure 1) according MIN) as follows: to the following equation: PWMRATIO = tMAX t ⎛ ⎞ MIN VOUT = R1 ⎜ +1⎟•1.22V ⎝R2 ⎠ Example: tMAX = 9ms, tMIN = 3μs (fSW = 2MHz) VOUT PWM R1 RATIO = 9ms = 3000:1 LT3519/ 3µs LT3519-1/ FB LT3519-2 R2 The second method of dimming control, Analog Dim- ming, uses the CTRL pin to linearly adjust the current 3519 F01 sense threshold during the PWM high state. When the
Figure 1. FB Resistor Divider for Boost LED Driver
CTRL pin voltage is less than 1V but more than 100mV, the LED current is: V I CTRL – 100mV LED = 4 •R SENSE 3519fa 9