I was looking at using an LT1170 in a Positive-to-Negative Buck-Boost Converter, as per page 14 of this data sheet
Thing is, when I put this into LTSpice (using Linear Techology's own model of their part), it shows that the current through the inductor rises to 10 amps, during start-up, settling to something more reasonable later. I don't want to have to use a 10 amp inductor, and the part they suggest isn't 10 amps anyway. I can limit the current by including a clamp circuit on the Vc connection, but it increases the component count, and would probably need to include a trimpot because the relationship between current limit and clamp voltage isn't specified.
Does it matter if the inductor saturates during start-up?
Note that in the spice schematic below, I've included the switching on and off of the 2 amp load at regular intervals to look at the transient response, and regulation (the latter not being good at all, though good enough in my application).
Sylvia.
Version 4 SHEET 1 1704 788 WIRE 96 -64 64 -64 WIRE 256 -64 176 -64 WIRE 368 -64 256 -64 WIRE 528 -64 368 -64 WIRE 368 -48 368 -64 WIRE 528 -48 528 -64 WIRE 64 16 64 -64 WIRE 64 16 -144 16 WIRE 64 32 64 16 WIRE -144 96 -144 16 WIRE -144 96 -160 96 WIRE 256 112 256 -64 WIRE 256 112 192 112 WIRE -160 128 -160 96 WIRE 256 176 192 176 WIRE 320 176 256 176 WIRE 368 176 320 176 WIRE 368 224 368 176 WIRE 480 224 448 224 WIRE 544 224 480 224 WIRE 640 224 608 224 WIRE 752 224 720 224 WIRE 816 224 752 224 WIRE 896 224 896 128 WIRE 896 224 816 224 WIRE 992 224 944 224 WIRE 224 240 192 240 WIRE 240 240 224 240 WIRE 320 240 320 176 WIRE 480 256 480 224 WIRE 816 256 816 224 WIRE 752 272 752 224 WIRE 896 272 896 224 WIRE 992 272 992 224 WIRE 1136 272 1040 272 WIRE 1040 288 1040 272 WIRE 240 352 240 320 WIRE 944 352 944 224 WIRE 944 352 896 352 WIRE 1040 352 1040 336 WIRE 1072 352 1040 352 WIRE 1136 352 1072 352 WIRE -160 416 -160 192 WIRE 16 416 -160 416 WIRE 64 416 64 320 WIRE 64 416 16 416 WIRE 240 416 64 416 WIRE 320 416 320 320 WIRE 320 416 240 416 WIRE 368 416 320 416 WIRE 480 416 480 320 WIRE 480 416 368 416 WIRE 560 416 480 416 WIRE 752 416 752 336 WIRE 752 416 624 416 WIRE 816 416 816 336 WIRE 816 416 752 416 WIRE 992 416 992 352 WIRE 992 416 816 416 WIRE 368 464 368 416 WIRE 304 512 304 496 WIRE 368 560 368 544 FLAG 368 16 0 FLAG -208 16 0 FLAG 528 32 0 FLAG 368 560 0 FLAG 16 416 NOMGROUND FLAG 256 176 FB FLAG 1072 352 0 FLAG 224 240 VC FLAG 896 128 0 SYMBOL diode -208 32 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL res 192 -80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName 470 SYMATTR Value 470 SYMBOL cap -176 128 R0 SYMATTR InstName C1 SYMATTR Value 1E-6 SYMBOL cap 352 -48 R0 SYMATTR InstName C2 SYMATTR Value 100E-6 SYMBOL res 224 224 R0 SYMATTR InstName R2 SYMATTR Value 5000 SYMBOL cap 224 352 R0 SYMATTR InstName C3 SYMATTR Value 0.1E-6 SYMBOL res 304 224 R0 SYMATTR InstName R3 SYMATTR Value 1200 SYMBOL cap 464 256 R0 SYMATTR InstName C4 SYMATTR Value 2.2E-7 SYMBOL res 352 208 M90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 10000 SYMBOL diode 608 208 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D2 SYMATTR Value 1N914 SYMBOL schottky 624 400 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D3 SYMATTR Value MBR745 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 736 208 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 47 SYMBOL cap 736 272 R0 SYMATTR InstName C5 SYMATTR Value 1000E-6 SYMBOL res 800 240 R0 SYMATTR InstName R6 SYMATTR Value 470 SYMBOL res 880 256 R0 SYMATTR InstName R1 SYMATTR Value 6 SYMBOL voltage 528 -64 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 124 Left 2 SYMATTR SpiceLine Rser=0.5 SYMATTR InstName V1 SYMATTR Value 16 SYMBOL ind 352 448 R0 SYMATTR InstName L1
SYMATTR SpiceLine Ipk=7 Rser=0.013 Rpar=0 Cpar=0 mfg="Coiltronics" pn="CTX50-7-52" SYMBOL sw 992 368 R180 WINDOW 3 24 -12 Left 2 SYMATTR Value "" SYMATTR InstName SW SYMATTR SpiceModel MYSWITCH SYMBOL voltage 1136 256 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(0 2 20e-3 1e-6 1e-6 10E-3 20E-3 10) SYMBOL PowerProducts\\LT1170 64 176 R0 SYMATTR InstName U1 TEXT -232 464 Left 2 !.tran 100E-3 TEXT 688 528 Left 2 !.model MySwitch SW(Ron=.1 Roff=1Meg Vt=1)