Datasheet LT1072 - 7

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