Datasheet LT1767, LT1767-1.8, LT1767-2.5, LT1767-3.3, LT1767-5 (Analog Devices) - 10

LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5
applicaTions inForMaTion
especially with smaller inductors and lighter loads, so Continuous Mode (V )(V ) I OUT IN − VOUT don’t omit this step. Powdered iron cores are forgiving I P − 2 L ( ) (f) (V ) OUT M ( AX IN because they saturate softly, whereas ferrite cores ) = saturate abruptly. Other core materials fall somewhere Discontinuous operation occurs when in between. V ( ) I I OUT VIN− VOUT OUT(DIS) = (VOUT ) PEAK =IOUT + 2(L)(f) 2 L ( )(f)(VIN) For VIN = 8V, VOUT = 5V and L = 3.3µH, VIN = Maximum input voltage (5 f = Switching frequency, 1.25MHz )(8−5) IOUT M ( AX) =1.5− 3. Decide if the design can tolerate an “open” core ge- 2 3.3 • 10−6 ( ) 1.25•106 ( )(8) ometry like a rod or barrel, which have high magnetic =1.5 −0.23=1.27 A field radiation, or whether it needs a closed core like a toroid to prevent EMI problems. This is a tough decision Note that the worst case (minimum output current avail- because the rods or barrels are temptingly cheap and able) condition is at the maximum input voltage. For the small and there are no helpful guidelines to calculate same circuit at 15V, maximum output current would be when the magnetic field radiation will be a problem. only 1.1A. 4. After making an initial choice, consider the secondary When choosing an inductor, consider maximum load cur- things like output voltage ripple, second sourcing, etc. rent, core and copper losses, allowable component height, Use the experts in the Linear Technology’s applications output voltage ripple, EMI, fault current in the inductor, department if you feel uncertain about the final choice. saturation, and of course, cost. The following procedure They have experience with a wide range of inductor is suggested as a way of handling these somewhat com- types and can tell you about the latest developments plicated and conflicting requirements. in low profile, surface mounting, etc. 1. Choose a value in microhenries from the graphs of maximum load current. Choosing a small inductor
Table 1
with lighter loads may result in discontinuous mode
PART NUMBER VALUE (uH) ISAT(Amps) DCR (Ω) HEIGHT (mm)
of operation, but the LT1767 is designed to work well
Coiltronics
in either mode. TP1-2R2 2.2 1.3 0.188 1.8 Assume that the average inductor current is equal to TP2-2R2 2.2 1.5 0.111 2.2 load current and decide whether or not the inductor TP3-4R7 4.7 1.5 0.181 2.2 must withstand continuous fault conditions. If maxi- TP4- 100 10 1.5 0.146 3.0 mum load current is 0.5A, for instance, a 0.5A inductor
Murata
may not survive a continuous 2A overload condition. LQH1C1R0M04 1.0 0.51 0.28 1.8 Also, the instantaneous application of input or release LQH3C1R0M24 1.0 1.0 0.06 2.0 from shutdown, at high input voltages, may cause LQH3C2R2M24 2.2 0.79 0.1 2.0 saturation of the inductor. In these applications, the LQH4C1R5M04 1.5 1.0 0.09 2.6 soft-start circuit shown in Figure 10 should be used.
Sumida
2. Calculate peak inductor current at full load current to CD73- 100 10 1.44 0.080 3.5 ensure that the inductor will not saturate. Peak cur- CDRH4D18-2R2 2.2 1.32 0.058 1.8 rent can be significantly higher than output current, CDRH5D18-6R2 6.2 1.4 0.071 1.8 CDRH5D28-100 10 1.3 0.048 2.8 1767fb 10 For more information www.linear.com/LT1767 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Application