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LT1184F Datasheet(PDF) 11 Page - Linear Technology |
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LT1184F Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  11 LT1182/LT1183/LT1184/LT1184F Bulb (Pin 15): This pin connects to the low side of a 7V threshold comparator between the BAT and Bulb pins. This circuit sets the maximum voltage level across the primary side of the Royer converter under all operating conditions and limits the maximum secondary output under start-up conditions or open lamp conditions. This eases transformer voltage rating requirements. Set the voltage limit to insure lamp start-up with worst-case, lamp start voltages and cold-temperature system operating conditions. The Bulb pin connects to the junction of an external divider network. The divider network connects from the center tap of the Royer transformer or the actual battery supply voltage to the top side of the current source “tail inductor”. A capacitor across the top of the divider network filters switching ripple and sets a time constant that determines how quickly the clamp activates. When the comparator activates, sink current is generated to pull the CCFL VC pin down. This action transfers the entire regulator loop from current mode operation into voltage mode operation. BAT (Pin 14): This pin connects to the battery or battery charger voltage from which the CCFL Royer converter and LCD contrast converter operate. This voltage is typically higher than the VIN supply voltage but can be equal or less than VIN. However, the BAT voltage must be at least 2.1V greater than the internal 2.4V regulator or 4.5V minimum up to 30V maximum. This pin provides biasing for the lamp current programming block, is used with the Royer pin for floating lamp configurations, and connects to one input for the open lamp protection circuitry. For floating lamp configurations, this pin is the noninverting terminal of a high-side current sense amplifier. The typical quies- cent current is 50 µA into the pin. The BAT and Royer pins monitor the primary side Royer converter current through an internal 0.1 Ωtop side current sense resistor. A 0A to 1A primary side, center tap converter current is translated to an input signal range of 0mV to 100mV for the current sense amplifier. This input range translates to a 0 µA to 500 µAsinkcurrentattheCCFLVCpinthatnullsagainstthe source current provided by the programmer circuit. The BAT pin also connects to the top side of an internal clamp between the BAT and Bulb pins. PIN FUNCTIONS Royer (Pin 13): This pin connects to the center-tapped primary of the Royer converter and is used with the BAT pin in a floating lamp configuration where lamp current is controlled by sensing Royer primary side converter cur- rent. This pin is the inverting terminal of a high-side current sense amplifier. The typical quiescent current is 50 µA into the pin. If the CCFL regulator is not used in a floating lamp configuration, tie the Royer and BAT pins together. This pin is only available on the LT1182/LT1183/ LT1184F. VIN (Pin 12): This pin is the supply pin for the LT1182/ LT1183/LT1184/LT1184F. The ICs accept an input voltage range of 3V minimum to 30V maximum with little change in quiescent current (zero switch current). An internal, low dropout regulator provides a 2.4V supply for most of the internal circuitry. Supply current increases as switch current increases at a rate approximately 1/50 of switch current. This corresponds to a forced Beta of 50 for each switch. The ICs incorporate undervoltage lockout by sens- ing regulator dropout and lockout switching for input voltages below 2.5V. Hysteresis is not used to maximize the useful range of input voltage. The typical input voltage is a 3.3V or 5V logic supply. LT1182/LT1183 LCD VC (Pin 7): This pin is the output of the LCD contrast error amplifier and the input of the current comparator for the LCD contrast regulator. Its uses include frequency compensation and current limiting. The voltage on the LCD VC pin determines the current trip level for switch turnoff. During normal operation, this pin sits at a voltage between 0.95V (zero switch current) and 2.0V (maximum switch current). The LCD VC pin has a high impedance output and permits external voltage clamping to adjust current limit. A series R/C network to ground provides stable loop compensation. LCD PGND (Pin 8): This pin is the emitter of an internal NPN power switch. LCD contrast switch current flows through this pin and permits internal, switch-current sensing. The regulators provide a separate analog ground and power ground(s) to isolate high current ground paths from low current signal paths. Linear Technology recom- mends star-ground layout techniques. |
Similar Part No. - LT1184F_15 |
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Similar Description - LT1184F_15 |
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