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LT1371 Datasheet(PDF) 8 Page - Linear Technology |
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LT1371 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page 8 LT1371 Shutdown and Synchronization The 7-pin R and T7 package devices have a dual function S/S pin which is used for both shutdown and synchroni- zation. The SW package device has both a Shutdown (SHDN) pin and a Synchronization (SYNC) pin which can be used separately or tied together. These pins are logic level compatible and can be pulled high, tied to VIN or left floating for normal operation. A logic low on the S/S pin or SHDN pin activates shutdown, reducing the part’s supply current to 12 µA. Typical synchronization range is from 1.05 to 1.8 times the part’s natural switching frequency, but is only guaranteed between 600kHz and 800kHz. A 12 µs resetable shutdown delay network guarantees the part will not go into shutdown while receiving a synchro- nization signal when the functions are combined. Caution should be used when synchronizing above 700kHz because at higher sync frequencies the amplitude of the internal slope compensation used to prevent subharmonic switching is reduced. This type of subharmonic switching only occurs when the duty cycle of the switch is above 50%. Higher inductor values will tend to eliminate problems. Thermal Considerations Care should be taken to ensure that the worst-case input voltage and load current conditions do not cause exces- sive die temperatures. Typical thermal resistance is 30 °C/W for the R package and 50°C/W for the SW and T7 packages but these numbers will vary depending on the mounting techniques (copper area, air flow, etc.). Heat is transferred from the R and T7 packages via the tab and from the SW package via pins 4 to 7 and 14 to 17. Average supply current (including driver current) is: IIN = 4mA + DC [ISW/60 + ISW (0.004)] ISW = switch current DC = switch duty cycle Switch power dissipation is given by: PSW = (ISW) 2 (RSW)(DC) RSW = output switch ON resistance Total power dissipation of the die is the sum of supply current times supply voltage, plus switch power: PD(TOTAL) = (IIN)(VIN) + PSW Surface mount heat sinks are also becoming available which can lower package thermal resistance by 2 or 3 times. One manufacturer is Wakefield Engineering who offers surface mount heat sinks for both the R package (DD) and SW package (SW20) and can be reached at (617) 245-5900. Choosing the Inductor For most applications the inductor will fall in the range of 2.2 µH to 22µH. Lower values are chosen to reduce physi- cal size of the inductor. Higher values allow more output current because they reduce peak current seen by the power switch, which has a 3A limit. Higher values also reduce input ripple voltage and reduce core loss. When choosing an inductor you might have to consider maximum load current, core and copper losses, allowable component height, output voltage ripple, EMI, fault current in the inductor, saturation and, of course, cost. The following procedure is suggested as a way of handling these somewhat complicated and conflicting requirements. 1. Assume that the average inductor current for a boost converter is equal to load current times VOUT/VIN and decide whether or not the inductor must withstand continuous overload conditions. If average inductor current at maximum load current is 1A, for instance, a 1A inductor may not survive a continuous 3A overload condition. Also be aware that boost converters are not short-circuit protected and that, under output short conditions, inductor current is limited only by the available current of the input supply. 2. Calculate peak inductor current at full load current to ensure that the inductor will not saturate. Peak current can be significantly higher than output current, espe- cially with smaller inductors and lighter loads, so don’t omit this step. Powdered iron cores are forgiving because they saturate softly, whereas ferrite cores APPLICATIO S I FOR ATIO |
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