Datasheet LT3595A (Analog Devices) - 8

LT3595A
OPERATION
The LT3595A uses a constant-frequency, current mode 4.5V input voltage or by its maximum duty cycle. The duty control scheme to provide excellent line and load regula- cycle is the fraction of time that the internal switch is on tion. Operation is best understood by referring to the Block divided by the total period. It is determined by the input Diagram. The oscillator, V-I converter and internal regulator voltage and the voltage across the LEDs: are shared by the sixteen converters. The control circuitry, V + V power switches, PWM comparators and Dimming Feedback DC LED D = Control (DFC) blocks are identical for all converters. V − V + V VIN CESAT D The LT3595A enters shutdown mode when the SHDN where VLED is the voltage drop across the LEDs, VD is pin is lower than 400mV. If the SHDN pin is above 1.6V, the Schottky forward drop, and VCESAT is the saturation then the LT3595A turns on. At the start of each oscillator voltage of the internal switch. This leads to a minimum cycle, the power switch is turned on. Current ramps up input voltage of: through the output capacitor, the inductor, and the switch V + V to ground. When the voltage across the output capacitor V LED D + V − V IN MIN ( ) = CESAT D is larger than the LEDs’ forward voltage, current fl ows DC MAX through the LEDs. where DCMAX is the minimum rating of maximum duty When the switch is on, a voltage proportional to the switch cycle. current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM compara- The maximum input voltage is limited by the absolute tor. When this voltage exceeds the level at the negative maximum rating of 45V. input of the PWM comparator, the PWM logic turns off
Pulse-Skipping
the power switch. The level at the negative input of the PWM comparator is set by the error amplifi er output. This At low duty cycles, the LT3595A may enter pulse-skipping voltage is set by the LED current and the bandgap refer- mode. Low duty cycle occurs at higher input voltages and ence. In this manner, the error amplifi er sets the correct lower LED count. The LT3595A can drive currents with- peak current level in the inductor to keep the LED output out pulse-skipping provided the voltage across the LED current in regulation. The external RSET resistor is used string is greater than 15% of the input supply voltage. If to program the LED current from 10mA to 50mA. the current decreases to the point that the LED voltage is less than 15% of the input supply, the device may begin
Input Voltage Range
skipping pulses. This will result in some low frequency The minimum input voltage required to generate a specifi c ripple, although the LED current remains regulated on an output voltage in an LT3595A application is limited by its average basis down to 10mA. 3595af 8