written 7.1 years ago by | • modified 6.9 years ago |
Subject: Liner Integrated Circuits
Topic: Special Purpose Integrated Circuits
Difficulty: Low
written 7.1 years ago by | • modified 6.9 years ago |
Subject: Liner Integrated Circuits
Topic: Special Purpose Integrated Circuits
Difficulty: Low
written 6.9 years ago by |
The LT1070 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.
Finally, it allows simple pulse-by-pulse current limiting to provide maximum switch protection under output overload or short-circuit conditions. A low dropout internal regulator provides a 2.3V supply for all internal circuitry of the LT1070. 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 LT1070 to disconnect the main error amplifier output and connects the output of the flyback amplifier to the comparator input.
The LT1070 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 LT1070 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 LT1070 in an idle mode. Pulling the VC pin below 0.15V causes total regulator shutdown, with only 50mA supply current for shutdown circuitry biasing.