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B82470-A1223-M Datasheet(PDF) 6 Page - Linear Integrated Systems |
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B82470-A1223-M Datasheet(HTML) 6 Page - Linear Integrated Systems |
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6 / 16 page  LT3591 6 3591f Figure 2. Switching Waveforms OPERATION The LT3591 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 in Figure 1. At power-up, the capacitor at the CAP pin is charged up to VIN (input supply voltage) through the inductor and the internal Schottky diode. If CTRL is pulled higher than 100mV, the bandgap reference, the start-up bias and the oscillator are turned on. At the start of each oscillator cycle, the power switch Q1 is turned on. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A2. When this voltage exceeds the level at the negative input of A2, the PWM logic turns off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the VCAP and VLED voltage and the bandgap reference. In this manner the error amplifier, A1, sets the correct peak current level in inductor L1 to keep the output in regulation. The CTRL pin is used to adjust the LED current. The LT3591 enters into shutdown when CTRL is pulled lower than 50mV. Minimum Output Current The LT3591 can drive a 2-LED string at 2mA LED current without pulse skipping using the same external compo- nents shown in the application circuit on the front page of this data sheet. As current is further reduced, the device will begin skipping pulses. This will result in some low frequency ripple, although the average LED current remains regulated down to zero. The photo in Figure 2 details circuit operation driving two white LEDs at 2mA load. Peak induc- tor current is less than 40mA and the regulator operates in discontinuous mode, meaning the inductor current reaches zero during the discharge phase. 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. VSW 5V/DIV IL 20mA/DIV VIN = 4.2V ILED = 2mA 2 LEDs 500ns/DIV 3591 F02 |
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