Datasheet LT3595 (Analog Devices) - 8

LT3595
OPERATION
The LT3595 uses a constant-frequency, current mode
Input Voltage Range
control scheme to provide excellent line and load regula- The minimum input voltage required to generate a specifi c tion. Operation is best understood by referring to the Block output voltage in an LT3595 application is limited by its Diagram. The oscillator, V-I converter and internal regulator 4.5V input voltage or by its maximum duty cycle. The duty are shared by the sixteen converters. The control circuitry, cycle is the fraction of time that the internal switch is on power switches, PWM comparators and dimming control divided by the total period. It is determined by the input (DFC) are identical for all converters. voltage and the voltage across the LEDs: The LT3595 enters shutdown mode when the ⎯S⎯H⎯D⎯N pin V + V is lower than 400mV. If the ⎯S⎯H⎯D⎯N pin is above 1.6V, DC LED D = then the LT3595 turns on. At the start of each oscillator V − V + V VIN CESAT D cycle, the power switch is turned on. Current ramps up where VLED is the voltage drop across the LEDs, VD is through the output capacitor, the inductor, and the switch the Schottky forward drop, and VCESAT is the saturation to ground. When the voltage across the output capacitor voltage of the internal switch. This leads to a minimum is larger than the LEDs’ forward voltage, current fl ows input voltage of: through the LEDs. V + V LED D When the switch is on, a voltage proportional to the switch V + V − V IN MIN ( ) = CESAT D DC current is added to a stabilizing ramp and the resulting MAX sum is fed into the positive terminal of the PWM compara- where DCMAX is the minimum rating of maximum duty tor. When this voltage exceeds the level at the negative cycle. input of the PWM comparator, the PWM logic turns off The maximum input voltage is limited by the absolute the power switch. The level at the negative input of the maximum rating of 45V. PWM comparator is set by the error amplifi er output. This voltage is set by the LED current and the bandgap refer-
Pulse-Skipping
ence. In this manner, the error amplifi er sets the correct peak current level in the inductor to keep the LED output At low duty cycles, the LT3595 may enter pulse-skipping current in regulation. The external R mode. Low duty cycle occurs at higher input voltages and SET resistor is used to program the LED current from 10mA to 50mA. 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. 3595f 8