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LT3595 Datasheet(PDF) 8 Page - Linear Technology |
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LT3595 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LT3595 8 3595f OPERATION The LT3595 uses a constant-frequency, current mode control scheme to provide excellent line and load regula- tion. Operation is best understood by referring to the Block Diagram. The oscillator, V-I converter and internal regulator are shared by the sixteen converters. The control circuitry, power switches, PWM comparators and dimming control (DFC) are identical for all converters. The LT3595 enters shutdown mode when the ⎯S⎯H⎯D⎯N pin is lower than 400mV. If the ⎯S⎯H⎯D⎯N pin is above 1.6V, then the LT3595 turns on. At the start of each oscillator cycle, the power switch is turned on. Current ramps up through the output capacitor, the inductor, and the switch to ground. When the voltage across the output capacitor is larger than the LEDs’ forward voltage, current flows through the LEDs. When the switch is 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 compara- tor. 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 comparator is set by the error amplifier output. This voltage is set by the LED current and the bandgap refer- ence. In this manner, the error amplifier sets the correct peak current level in the inductor to keep the LED output current in regulation. The external RSET resistor is used to program the LED current from 10mA to 50mA. Input Voltage Range The minimum input voltage required to generate a specific output voltage in an LT3595 application is limited by its 4.5V input voltage or by its maximum duty cycle. The duty cycle is the fraction of time that the internal switch is on divided by the total period. It is determined by the input voltage and the voltage across the LEDs: DC VV VV V LED D VIN CESAT D = + −+ where VLED is the voltage drop across the LEDs, VD is the Schottky forward drop, and VCESAT is the saturation voltage of the internal switch. This leads to a minimum input voltage of: V VV DC VV IN MIN LED D MAX CESAT D () = + +− where DCMAX is the minimum rating of maximum duty cycle. The maximum input voltage is limited by the absolute maximum rating of 45V. Pulse-Skipping At low duty cycles, the LT3595 may enter pulse-skipping mode. Low duty cycle occurs at higher input voltages and lower LED count. The LT3595 can drive currents without pulse-skipping provided the voltage across the LED string is greater than 15% of the input supply voltage. If the current decreases to the point that the LED voltage is less than 15% of the input supply, the device may begin skipping pulses. This will result in some low frequency ripple, although the LED current remains regulated on an average basis down to 10mA. |
Similar Part No. - LT3595_15 |
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Similar Description - LT3595_15 |
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