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LT1976EFE Datasheet(PDF) 8 Page - Linear Technology |
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LT1976EFE Datasheet(HTML) 8 Page - Linear Technology |
8 / 24 page LT1976 8 1976f BLOCK DIAGRA 12 FB VC Σ 15 INTERNAL REF UNDERVOLTAGE LOCKOUT THERMAL SHUTDOWN SOFT-START FOLDBACK DETECT SLOPE COMP ANTISLOPE COMP 1.2V CT CLAMP 2.4V – + 1.3V 1.25V SHDN 9 CSS 14 SYNC 10 BIAS 4 VIN SHDN COMP – + – + ERROR AMP 11 13 PGFB CT 1.12V – + PG COMP 7 BURST MODE DETECT 200kHz OSCILLATOR SWITCH LATCH CURRENT COMP DRIVER CIRCUITRY R Q SW S 2 BOOST 6 PG 16 GND 17 PGND 1976 BD 8 VC CLAMP The LT1976 is a constant frequency, current mode buck converter. This means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. In addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. A switch cycle starts with an oscilla- tor pulse which sets the RS latch to turn the switch on. When switch current reaches a level set by the current compara- tor the latch is reset and the switch turns off. Output volt- age control is obtained by using the output of the error amplifier to set the switch current trip point. This technique means that the error amplifier commands current to be delivered to the output rather than voltage. A voltage fed system will have low phase shift up to the resonant fre- quency of the inductor and output capacitor, then an abrupt 180 ° shift will occur. The current fed system will have 90° phase shift at a much lower frequency, but will not have the additional 90 ° shift until well beyond the LC resonant fre- quency. This makes it much easier to frequency compen- sate the feedback loop and also gives much quicker tran- sient response. Most of the circuitry of the LT1976 operates from an internal 2.4V bias line. The bias regulator normally draws Figure 1. LT1976 Block Diagram |
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