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LCPB Datasheet(PDF) 7 Page - Linear Technology |
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LCPB Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  LT3590 7 3590f OPERATION The LT3590 uses a constant-frequency, current mode control scheme to provide excellent line and load regula- tion. Operation can be best understood by referring to the Block Diagram. At power-up, the bandgap reference, the start-up bias, and the regulator are turned on. If CTRL is pulled higher than 150mV, the switching converter sub-blocks including the oscillator, the PWM comparator and the error amplifier are also turned on. At the start of each oscillator cycle, the power switch Q1 is turned on. Current flows through the inductor and the switch to ground, ramping up as the switch stays on. A voltage proportional to the switch cur- rent is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator. When this voltage exceeds the level at the negative input of the PWM comparator, the PWM logic turns off the power switch. The level at the negative input of the PWM com- parator is set by the error amplifier EAMP, and is simply an amplified version of the difference between the VIN and VLED voltage and the bandgap reference. In this manner, the error amplifier sets the correct peak current level in inductor L1 to keep the output in regulation. The CTRL pin is used to adjust the reference voltage. The LT3590 enters into shutdown when CTRL is pulled lower than 100mV. Input Voltage Range The minimum input voltage required to generate a particular output voltage in an LT3590 application is limited by either its 4.5V limit or by its maximum duty cycle. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC VV VV V LED D IN SW D = + + – 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 DCMAX = 0.9, this leads to minimum input voltage of: V VV DC VV IN MIN LED D MAX SW D () () = + +− The maximum input voltage is limited by the absolute maximum VIN rating of 55V. Pulse-Skipping For LED strings with a low number of LEDs (1, 2, or 3), the LT3590 can drive currents without pulse-skipping as long as the voltage across the LED and sense resistor is greater than roughly 15% of the input supply voltage. If the LED voltage plus sense resistor is less than 15% of the input supply, the device will begin skipping pulses. This will result in some low frequency ripple, although the LED current remains regulated on an average basis down to zero. Discontinuous Current Mode The CTRL pin, in conjunction with the sense resistor, can be used to program the LED current as discussed under Applications Information. The LT3590 can drive a 10-LED string at 10mA LED current operating in continu- ous conduction mode, using the recommended external components shown in the front page application circuit with the sense resistor equal to 10Ω. As current is further reduced, the regulator enters discontinuous conduction mode. The photo in Figure 2 details circuit operation driving ten LEDs at 2mA load. During the discharge phase, the inductor current reaches zero. After the inductor current reaches zero, the SW pin exhibits ringing due to the LC tank circuit formed by the inductor in combination with the switch and the diode capacitance. This ringing is not harmful; far less spectral energy is contained in the ringing than in the switch transitions. The ringing can be damped by application of a 3kΩ resistor across the inductor, al- though this will degrade efficiency. Figure 2. Switching Waveforms 400ns/DIV VSW 20V/DIV IL 10mA/DIV 3590 F02 VIN = 48V ILED = 2mA 10 WHITE LEDs L = 470μH (MURATA) |
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