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LT3754IUHPBF Datasheet(PDF) 10 Page - Linear Technology |
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LT3754IUHPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 28 page  LT3754 10 3754f OPERATION to define the turn on/off voltages for VIN. SHDN/UVLO pin current switches from 2.4μA to 0μA when SHDN/UVLO pin voltage exceeds 1.476V. The LT3754 constant switching frequency is programmable from 100kHz up to 1MHz using a single resistor at the RT pin to ground. A SYNC pin is also provided to allow an external clock to define the converter switching frequency. The GATE output provides a ±0.8A peak gate drive for an external N-channel power MOSFET to generate a boosted output voltage VOUT using a single inductor, Schottky diode and output capacitor. With LED strings connected from VOUT to every LED pin, the lowest voltage on each LED pin is monitored and compared to an internal 1V reference. VOUT is regulated to ensure the lowest LED pin voltage of any connected LED string is maintained at 1V. If any of the LED strings are open, the LT3754 will ignore the open LED pin. If all of the LED strings are open VOUT charges up until a user programmable OVP (overvoltage protection) level is reached. This programmable OVP level allows the user to protect against LED damage when the LED strings are opened and then reconnected. Since the LT3754 boost controller uses a current mode topology, the VC pin voltage determines the peak current in the inductor of the converter and hence the duty cycle of the GATE switching waveform. The basic loop uses a pulse from an internal oscillator to set an RS flip-flop and turn on the external power MOSFET. Current increases in the MOSFET and inductor until the VC commanded peak switch current is exceeded and the MOSFET is then turned off. Inductor current is sensed during the GATE on period by a sense resistor RS in the source of the external N-channel power MOSFET. As with all current mode converters, slope compensation is added to the control path to ensure stability for duty cycles above 50%. Any over current fault condition in the MOSFET turns off the MOSFET and triggers soft start internally. In this fault mode the LT3754 only allows MOSFET turn-on approximately every 2ms. This hiccup mode significantly reduces the power rating required for the MOSFET. LED current programming and dimming can be achieved using the ISET, CTRL and PWM pins. A single resistor at the ISET pin programs LED current. Analog dimming of LED brightness is achieved using the CTRL pin below 1V. PWM dimming of LED brightness is achieved by control- ling the duty cycle of the PWM pin. For robust operation the LT3754 monitors system conditions and performs soft start for startup after any of the following faults: VIN, SHDN or INTVCC voltages too low or MOSFET current too high. The LT3754, when entering these faults, discharges an internal soft start node and prevents switching at the GATE pin. When exiting these faults the LT3754 ramps up an internal soft start node to control VC pin voltage rise and hence control MOSFET peak switch current rise. In addition the soft start period gradually ramps up switching frequency from approximately 33% to 100% of full scale. The LT3754 monitors each LED pin voltage. If the LED string has an open fault (V(LEDX)<0.5V) the FAULT flag is pulled low. For LED protection, the LT3754 CTRL pin allows an LED current derating curve to be programmed versus the ambient temperature of the LED strings. An NTC resistor placed close to the LEDs decreases CTRL pin voltage and hence decreases LED current as LED ambient temperature increases. The LT3754 also allows it’s own junction temperature to be monitored and regulated by derating LED currents when a junction temperature programmed by the TSET pin is exceeded. |
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