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LT3754EUHPBF Datasheet(PDF) 9 Page - Linear Technology |
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LT3754EUHPBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  LT3754 9 3754f BLOCK DIAGRAM OPERATION The operation of the LT3754 is best understood by referring to the typical application circuit on the front page and the Block Diagram in Figure 1. The LT3754 drives 16 strings of LEDs by using a constant switching frequency, current mode boost controller to generate a single output voltage VOUT for the top (anode) of all LED strings. LED string current is generated and controlled by connection of the bottom LED in each string (cathode) to a current source contained in each corresponding LED pin. Each LED pin contains an accurate current sink to ground, programmable between 10mA to 50mA using a single resistor at the ISET pin. LED channels can be paralleled to achieve higher LED currents. For applications requiring less than 16 strings of LEDs, channels can be paralleled or disabled (connect LED pin to VOUT before startup). For optimum efficiency, VOUT regulates to the lowest possible voltage allowable to maintain regulated current in each LED string. Any OPEN LED fault is indicated by the FAULT pin driven low without effecting the operation of the connected LED strings. The Block Diagram in Figure 1 illustrates the key functions of the LT3754. It can be seen that two external supplies, VREF and INTVCC, are generated by the LT3754. The VREF pin provides a precision 1.485V output for use with external resistors to program the CTRL, OVPSET and TSET input pins. The INTVCC pin provides a regulated 7V output to supply the gate driver for the boost controller GATE pin. An accurate 1.476V threshold on the SHDN/UVLO pin combined with a SHDN/UVLO pin current hysteresis allows a programmable resistor divider from VIN to SHDN/UVLO Figure 1. LT3754 Block Diagram 3754 BD + + + – CHANNEL X OVERVOLTAGE AMP LED AMP 6V 1.1V + – + – + – LED LOGIC R S Q + – INTVCC_UV VIN_UV VIN SHDN_UV SHDN/UVLO PWM VREF CTRL 1V SS R 56R 600k 13 12 11 1.476V 1.485V SLOPE 100mV OVER CURRENT 52mV PEAK CURRENT HICCUP__MODE VC EN PWM EN EN 7V(REGULATED) UVLO( ) = 3.8V, UVLO( ) = 3.4V 4.2V( ) 3.7V( ) + – REF 1.485V 27 29 30 32 33 26 VPTAT + – EN GATE SYNC RT SENSE VOUT LEDx 1-8, 17-24 FAULT SOFT START 31 14 16 9 OSC 10 25 15 28 LED CURRENT CONTROL INTVCC OVPSET VC ISET TSET EXPOSED PAD (GND) |
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