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LT1370 Datasheet(PDF) 6 Page - Linear Technology |
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LT1370 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LT1370 sn1370 1370fs BLOCK DIAGRAM OPERATION The LT1370 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 voltage mode 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 frequency compensation under widely vary- ing 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 pro- vides a 2.3V supply for all internal circuitry. This low dropout design allows input voltage to vary from 2.7V to 25V with virtually no change in device performance. 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 1.245V bandgap reference biases the positive input of the error amplifier. The negative input of the amplifier is brought out for positive output voltage sensing. The error amplifier has nonlinear transconductance to reduce out- put overshoot on start-up or overload recovery. When the feedback voltage exceeds the reference by 40mV, error amplifier transconductance increases 10 times, which reduces output overshoot. The feedback input also invokes oscillator frequency shifting, which helps pro- tect components during overload conditions. When the feedback voltage drops below 0.6V, the oscillator fre- quency is reduced 5:1. Lower switching frequency allows full control of switch current limit by reducing minimum switch duty cycle. – + NFBA NFB S/S FB 100k 50k 0.005Ω EA VC VIN GND LT1370 • BD GND SENSE 1.245V REF 5:1 FREQUENCY SHIFT OSC SYNC SHUTDOWN DELAY AND RESET LOW DROPOUT 2.3V REG ANTI-SAT LOGIC DRIVER SW SWITCH – + IA AV ≈ 20 COMP |
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