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LT1977 ๋ฐ์ดํฐ์ํธ(HTML) 13 Page  Linear Technology 

LT1977 ๋ฐ์ดํฐ์ํธ(HTML) 13 Page  Linear Technology 
13 / 24 page LT1977 13 1977f APPLICATIO S I FOR ATIO The LT1977 is able to maintain peak switch current limit over the full duty cycle range by using patented circuitry to cancel the effects of slope compensation on peak switch current without affecting the frequency compensation it provides. Maximum load current would be equal to maximum switch current for an infinitely large inductor, but with finite inductor size, maximum load current is reduced by onehalf peaktopeak inductor current. The following formula assumes continuous mode operation, implying that the term on the right (IPP/2) is less than IOUT. II VV V Lf V I I OUT MAX PK OUT IN OUT IN PK P () โ โ โ = ()( ) ()( )( ) = 2 P 2 Discontinuous operation occurs when: I VV V Lf V OUT DIS OUT IN OUT IN () โ ()( )( ) โค () 2 For VOUT = 5V, VIN = 8V and L = 15ยตH: I ee A OUT MAX () .โ โ โ .โ . . = ()( ) ()( )( ) == 15 58 5 2 15 6 500 3 8 1 5 0 125 1 375 Note that there is less load current available at the higher input voltage because inductor ripple current increases. At VIN = 15V, duty cycle is 33% and for the same set of conditions: I ee A OUT MAX () .โ โ โ .โ . . = ()( ) ()( )( ) == 15 515 5 2 15 6 500 3 15 15 022 1 28 To calculate actual peak switch current in continuous mode with a given set of conditions, use: II VV V Lf V SW PK OUT OUT IN OUT IN () โ =+ () ()( )( ) 2 If a small inductor is chosen which results in discontinous mode operation over the entire load range, the maximum load current is equal to: I If L V VV V OUT MAX PK IN OUT IN OUT () โ = ()( )( ) ()( ) 22 2 CHOOSING THE INDUCTOR For most applications the output inductor will fall in the range of 5ยตH to 33ยตH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1977 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage and reduce core loss. When choosing an inductor you might have to consider maximum load current, core and copper losses, allow able 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 han dling these somewhat complicated and conflicting requirements. 1. Choose a value in microhenries such that the maximum load current plus half the ripple current is less than the minimum peak switch current (IPK). Choosing a small inductor with lighter loads may result in discontinuous mode of operation, but the LT1977 is designed to work well in either mode. Assume that the average inductor current is equal to load current and decide whether or not the inductor must withstand continuous fault conditions. If maxi mum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 2A overload condition. For applications with a duty cycle above 50%, the inductor value should be chosen to obtain an inductor ripple current of less than 40% of the peak switch current. 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, especially with smaller inductors and lighter loads, so donโt omit this step. Powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. Other core materials fall somewhere in between. The following formula assumes continuous mode of opera tion, but it errs only slightly on the high side for discon tinuous mode, so it can be used for all conditions. 
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