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1374 Datasheet(PDF) 10 Page - Linear Technology |
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1374 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 32 page  10 LT1374 1374fb APPLICATIONS INFORMATION MAXIMUM OUTPUT LOAD CURRENT Maximum load current for a buck converter is limited by the maximum switch current rating (IP) of the LT1374. This current rating is 4.5A up to 50% duty cycle (DC), decreasing to 3.7A at 80% duty cycle. This is shown graphically in Typical Performance Characteristics and as shown in the formula below: IP = 4.5A for DC ≤ 50% IP = 3.21 + 5.95(DC) – 6.75(DC)2 for 50% < DC < 90% DC = Duty cycle = VOUT/VIN Example: with VOUT = 5V, VIN = 8V; DC = 5/8 = 0.625, and; ISW(MAX) = 3.21 + 5.95(0.625) – 6.75(0.625)2 = 4.3A Current rating decreases with duty cycle because the LT1374 has internal slope compensation to prevent current mode subharmonic switching. For more details, read Ap- plication Note 19. The LT1374 is a little unusual in this regard because it has nonlinear slope compensation which gives better compensation with less reduction in current limit. 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 one-half peak-to-peak inductor current. The following formula assumes continuous mode operation, implying that the term on the right is less than one-half of IP. IOUT(MAX) = Continuous Mode For the conditions above and L = 3.3 µH, I A OUT MAX ( ) − =− () − () () =− = 43 58 5 2 3 3 10 500 10 8 43 057 3 73 63 . .• • .. . At VIN = 15V, duty cycle is 33%, so IP is just equal to a fixed 4.5A, and IOUT(MAX) is equal to: 45 515 5 2 3 3 10 500 10 15 45 1 3 5 63 . .• • .. − () − () () =− = − A Note that there is less load current available at the higher input voltage because inductor ripple current increases. This is not always the case. Certain combinations of inductor value and input voltage range may yield lower available load current at the lowest input voltage due to reduced peak switch current at high duty cycles. If load current is close to the maximum available, please check maximum available current at both input voltage extremes. To calculate actual peak switch current with a given set of conditions, use: II VV V Lf V SW PEAK OUT OUT IN OUT IN ( ) =+ − () ()( )( ) 2 For lighter loads where discontinuous operation can be used, maximum load current is equal to: IOUT(MAX) = Discontinuous mode Example: with L = 1.2 µH, VOUT = 5V, and VIN(MAX) = 15V, IA OUT MAX ( ) − = () () () − () = 4 5 500 10 1 2 10 15 2 5 15 5 182 2 36 .• . • . The main reason for using such a tiny inductor is that it is physically very small, but keep in mind that peak-to-peak inductor current will be very high. This will increase output ripple voltage. If the output capacitor has to be made larger to reduce ripple voltage, the overall circuit could actually wind up larger. CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR For most applications the output inductor will fall in the range of 3 µH to 20µH. 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 LT1374 switch, which has a 4.5A limit. Higher values also reduce output ripple voltage, and reduce core loss. Graphs in the Typical Performance Characteristics section show maximum output load current versus inductor size and input voltage. A second graph shows core loss versus inductor size for various core materials. IP − () − () ()( )( ) VV V Lf V OUT IN OUT IN 2 If L V VV V PIN OUT IN OUT () ( )( )( ) () − () 2 2 |
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