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LT1307B Datasheet(PDF) 7 Page - Linear Technology |
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LT1307B Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page 7 LT1072 1072fc The LT1072 is a current mode switcher. This means that switch duty cycle is directly controlled by switch current rather than by output voltage. Referring to the block diagram, the switch is turned “on” at the start of each oscillator cycle. It is turned “off” when switch current reaches a predetermined level. Control of output voltage is obtained by using the output of a voltage sensing error amplifier to set current trip level. This technique has several advantages. First, it has immediate response to input voltage variations, unlike ordinary switchers which have notoriously poor line transient response. Second, it reduces the 90 phase shift at midfrequencies in the energy storage inductor. This greatly simplifies closed loop fre- quency compensation under widely varying input voltage or output load conditions. Finally, it allows simple pulse- by-pulse current limiting to provide maximum switch protection under output overload or short conditions. A low-dropout internal regulator provides a 2.3V supply for all internal circuitry on the LT1072. This low-dropout design allows input voltage to vary from 3V to 60V with virtually no change in device performance. A 40kHz oscillator is the basic clock for all internal timing. It turns “on” the output switch via the logic and driver circuitry. Special adaptive antisat circuitry detects onset of saturation in the power switch and adjusts driver current instantaneously to limit switch saturation. This minimizes driver dissipation and provides very rapid turn-off of the switch. A 1.2V bandgap reference biases the positive input of the error amplifier. The negative input is brought out for output voltage sensing. This feedback pin has a second function; when pulled low with an external resistor, it programs the LT1072 to disconnect the main error amplifier output and connects the output of the flyback amplifier to the comparator input. The LT1072 will then regulate the value of the flyback pulse with respect to the supply voltage. This flyback pulse is directly proportional to output voltage in the traditional transformer coupled flyback topology regulator. By regulating the amplitude of the flyback pulse, the output voltage can be regulated with no direct connection between input and output. The output is fully floating up to the breakdown voltage of the transformer windings. Multiple floating outputs are easily obtained with additional windings. A special delay network inside the LT1072 ignores the leakage inductance spike at the leading edge of the flyback pulse to improve output regulation. The error signal developed at the comparator input is brought out externally. This pin (VC) has four different functions. It is used for frequency compensation, current limit adjustment, soft starting, and total regulator shutdown. During normal regulator operation this pin sits at a voltage between 0.9V (low output current) and 2.0V (high output current). The error amplifiers are current output (gm) types, so this voltage can be externally clamped for adjusting current limit. Likewise, a capacitor coupled external clamp will provide soft start. Switch duty cycle goes to zero if the VC pin is pulled to ground through a diode, placing the LT1072 in an idle mode. Pulling the VC pin below 0.15V causes total regulator shutdown, with only 50µA supply current for shutdown circuitry biasing. See AN-19 for full application details. Extra Pins on the MiniDIP and Surface Mount Packages The 8 and 16-pin versions of the LT1072 have the emitters of the power transistor brought out separately from the ground pin. This eliminates errors due to ground pin voltage drops and allows the user to reduce switch current limit 2:1 by leaving the second emitter (E2) disconnected. The first emitter (E1) should always be connected to the ground pin. Note that switch “on” resistance doubles when E2 is left open, so efficiency will suffer somewhat when switch currents exceed 100mA. Also, note that chip dissipation will actually increase with E2 open during normal load operation, even though dissipation in current limit mode will decrease. See “Thermal Considerations.” Thermal Considerations When Using Small Packages The low supply current and high switch efficiency of the LT1072 allow it to be used without a heat sink in most applications when the TO-220 or TO-3 package is selected. These packages are rated at 50°C/W and 35°C/W respectively. The small packages, however, are rated at greater than 100°C/W. Care should be taken with these packages to ensure that the worse case input voltage and load current conditions do not cause excessive die temperatures. The following formulas can be used as a LT1072 OPERATIO |
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