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LT1370 Datasheet(PDF) 8 Page - Linear Technology |
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LT1370 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1370 sn1370 1370fs Shutdown and Synchronization The device has a dual function S/S pin which is used for both shutdown and synchronization. This pin is 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 activates shutdown, reducing the part’s supply current to 12µA. 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 12µs resetable shutdown delay network guarantees the part will not go into shutdown while receiving a synchronization signal. Caution should be used when synchronizing above 700kHz because at higher sync frequencies the amplitude of the internal slope compensation used to prevent subhar- monic 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 elimi- nate this problem. 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 T7 package but these numbers will vary depending on the mounting techniques (copper area, airflow, etc.). Heat is transferred from the package via the tab. Average supply current (including driver current) is: IIN = 4.5mA + DC(ISW/45) 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 available which can lower package thermal resistance by two or three times. One manufacturer, Wakefield Engineering, offers surface mount heat sinks for the R package and can be reached at (617) 245-5900 or at www.wakefield.com. Choosing the Inductor For most applications the inductor will fall in the range of 2.2µH 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 6A limit. Higher values also reduce input ripple voltage and reduce core loss. When choosing an inductor you need to consider maxi- mum 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 3A, for instance, a 3A inductor may not survive a continuous 6A 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 saturate abruptly and other core materials fall in between. The following formula assumes continuous mode operation but it errs only slightly on the high side for discontinuous mode, so it can be used for all conditions. APPLICATIO S I FOR ATIO |
Similar Part No. - LT1370_15 |
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Similar Description - LT1370_15 |
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