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LT1680 Datasheet(PDF) 11 Page - Linear Technology |
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LT1680 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LT1680 clamped to 2.5V through a 20k series input resistance and will therefore draw 0.5mA when tied directly to 12V. This additional current can be minimized by making the con- nection through an external resistor (100k is typically used). Oscillator Synchronization The LT1680 oscillator generates a modified sawtooth waveform at the CT pin between low and high thresholds of 0.8V (vl) and 2.5V (vh) respectively. The oscillator can be synchronized by driving a TTL level pulse into the SYNC pin. This pin connects to a one shot circuit that reduces the oscillator high threshold to 2V for about 200ns. The SYNC input signal should have minimum on/off times of ≥1µs. The inductor core type is determined by peak current and efficiency requirements. The inductor core must with- stand this peak current without saturating, and the series winding resistance and core losses should be kept as small as is practical to maximize conversion efficiency. The LT1680 peak current threshold is 40% greater than the average limit threshold. Slope compensation effects reduce this margin as duty cycle increases. This margin must be maintained to prevent peak current limit from corrupting the programmed value for average current limit. Programming the peak ripple current to less than 15% of the desired average current limit value will assure proper operation of the average current limit feature through 90% duty cycle (see Slope Compensation). Slope Compensation Current mode switching regulators that operate with a duty cycle greater than 50% and have continuous inductor current can exhibit duty cycle instability. While a regulator will not be damaged and may even continue to function acceptably during this type of subharmonic oscillation, an irritating high-pitched squeal is usually produced. The criterion for current mode duty cycle instability is met when the increasing slope of the inductor ripple current is less than the decreasing slope, which is the case at duty cycles greater than 50%. This condition is illustrated in Figure 5a. The inductor ripple current starts at I1, the beginning of each oscillator switch cycle. Current increases at a rate S1 until the current reaches the control trip level I2. The controller servo loop then disables the switch and inductor current begins to de- crease at a rate S2. If the current switch point (I2) is perturbed slightly and increased by ∆I, the cycle time ends such that the minimum current point is increased by a factor of 1 + (S2/S1) to start the next cycle. On each successive cycle, this error is multiplied by a factor of S2/ S1. Therefore, if S2/S1 is ≥1, the system is unstable. Subharmonic oscillations can be eliminated by augment- ing the increasing ripple current slope (S1) in the control loop. This is accomplished by adding an artificial ramp on the inductor current waveform internal to the IC (with a slope SX) as shown in Figure 5b. If the sum of the slopes 0.8V 1680 F04 2V 2.5V (vl) SYNC VCT (vh) FREE RUN SYNCHRONIZED Figure 4. Free Run and Synchronized Oscillator Waveforms (at CT Pin) Inductor Selection The inductor for an LT1680 converter is selected based on output power, operating frequency and efficiency require- ments. Generally, the selection of inductor value can be reduced to desired maximum ripple current in the inductor ( ∆I). For a boost converter, the minimum inductor value for a given operating ripple current can be determined using the following relation: L VV V If V MIN IN OUT IN O OUT = () ()( )( ) – ∆ Given an inductor value (L), the peak inductor current is the sum of the average inductor current (IAVG) and half the inductor ripple current ( ∆I), or: II VV V Lf V PK AVG IN OUT IN O OUT =+ () ()()( )( ) – 2 APPLICATIO S I FOR ATIO |
Similar Part No. - LT1680_15 |
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Similar Description - LT1680_15 |
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