Datasheet LT3590 (Analog Devices) - 7

LT3590
OPERATION
The LT3590 uses a constant-frequency, current mode The maximum input voltage is limited by the absolute control scheme to provide excellent line and load regula- maximum VIN rating of 55V. tion. Operation can be best understood by referring to the Block Diagram.
Pulse-Skipping
At power-up, the bandgap reference, the start-up bias, and For LED strings with a low number of LEDs (1, 2, or 3), the regulator are turned on. If CTRL is pulled higher than the LT3590 can drive currents without pulse-skipping as 150mV, the switching converter sub-blocks including the long as the voltage across the LED and sense resistor is oscillator, the PWM comparator and the error amplifi er greater than roughly 15% of the input supply voltage. If are also turned on. At the start of each oscillator cycle, the LED voltage plus sense resistor is less than 15% of the power switch Q1 is turned on. Current fl ows through the input supply, the device will begin skipping pulses. the inductor and the switch to ground, ramping up as the This will result in some low frequency ripple, although switch stays on. A voltage proportional to the switch cur- the LED current remains regulated on an average basis rent is added to a stabilizing ramp and the resulting sum down to zero. is fed into the positive terminal of the PWM comparator. When this voltage exceeds the level at the negative input of
Discontinuous Current Mode
the PWM comparator, the PWM logic turns off the power The CTRL pin, in conjunction with the sense resistor, switch. The level at the negative input of the PWM com- can be used to program the LED current as discussed parator is set by the error amplifi er EAMP, and is simply under Applications Information. The LT3590 can drive a an amplifi ed version of the difference between the VIN and 10-LED string at 10mA LED current operating in continu- VLED voltage and the bandgap reference. In this manner, ous conduction mode, using the recommended external the error amplifi er sets the correct peak current level in components shown in the front page application circuit inductor L1 to keep the output in regulation. The CTRL with the sense resistor equal to 10Ω. As current is further pin is used to adjust the reference voltage. reduced, the regulator enters discontinuous conduction The LT3590 enters into shutdown when CTRL is pulled mode. The photo in Figure 2 details circuit operation driving lower than 100mV. ten LEDs at 2mA load. During the discharge phase, the inductor current reaches zero. After the inductor current
Input Voltage Range
reaches zero, the SW pin exhibits ringing due to the LC tank circuit formed by the inductor in combination with The minimum input voltage required to generate a particular the switch and the diode capacitance. This ringing is not output voltage in an LT3590 application is limited by either harmful; far less spectral energy is contained in the ringing its 4.5V limit or by its maximum duty cycle. The duty cycle than in the switch transitions. The ringing can be damped is the fraction of time that the internal switch is on and is by application of a 3kΩ resistor across the inductor, al- determined by the input and output voltages: though this will degrade effi ciency. V + V DC LED D = V – V + V IN SW D VSW 20V/DIV Where VD is the forward voltage drop of the catch diode (~0.8V) and VSW is the voltage drop of the internal switch at maximum load (~0.5V). Given DC IL MAX = 0.9, this leads 10mA/DIV to minimum input voltage of: 3590 F02 (V + V ) 400ns/DIV V LED D = + V − V VIN = 48V I IN MIN ( ) DC SW D LED = 2mA MAX 10 WHITE LEDs L = 470μH (MURATA)
Figure 2. Switching Waveforms
3590f 7