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LT1513IR Datasheet(PDF) 6 Page - Linear Technology |
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LT1513IR Datasheet(HTML) 6 Page - Linear Technology |
6 / 16 page 6 LT1513/LT1513-2 BLOCK DIAGRAM – + IFBA IFB S/S VFB 4k 50k* *REMOVE ON LT1513-2 0.04 Ω EA VC VIN LT1513 • BD 1.245V REF 500kHz OSC SYNC SHUTDOWN DELAY AND RESET LOW DROPOUT 2.3V REG ANTISAT LOGIC DRIVER SW SWITCH – + IA AV ≈ 6 COMP The LT1513 is a current mode switcher. This means that switch duty cycle is directly controlled by switch current rather than by output voltage or current. 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 and current is obtained by using the output of a dual feedback voltage sensing error amplifier to set switch current trip level. This technique has the advantage of simplified loop frequency compensation. A low dropout internal regulator provides a 2.3V supply for all internal circuitry on the LT1513. This low dropout design allows input voltage to vary from 2.7V to 25V. A 500kHz 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 unique error amplifier design has two inverting inputs which allow for sensing both output voltage and current. A 1.245V bandgap reference biases the noninverting input. The first inverting input of the error amplifier is brought out for positive output voltage sensing. The second inverting input is driven by a “current” amplifier which is sensing output current via an external current sense resistor. The current amplifier is set to a fixed gain of – 12.5 which provides a – 100mV current limit sense voltage. The LT1513-2 option removes the feedback resistors around the IFB amplifier and connects its output to the FB signal. This provides a ground referenced current sense voltage suitable for external current programming and makes amplifier input and output available for external loop compensation. The error signal developed at the amplifier output is brought out externally and is used for frequency compen- sation. During normal regulator operation this pin sits at a voltage between 1V (low output current) and 1.9V (high output current). Switch duty cycle goes to zero if the VC pin is pulled below the VC pin threshold, placing the LT1513 in an idle mode. OPERATION Figure 2 |
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