I'm not sure how this circuit works. The circuit is a boost for low voltage, low impedance power sources. That's fine, I understand that basically. But I asked LT about a way to shut it down and this is what they sent me, the addition of a pair of back to back PMOS FETs. The simulation does stop oscillating, but I'm not clear on how the PMOS FETs are working.
Can someone explain this? With nearly 0V between gate and source, the FETs should both be off. The point C2A swings freely. The gate voltages seem to swing with the drive at about half amplitude. I suppose that is capacitance of the gate-drain and the high impedance of the resistor R1. When the drive is 2 volts the oscillations stop.
I just can't follow what is happening. Mostly I don't understand the use of two FETs back to back. But I also don't understand the use of P channel devices when the gate drive is positive. The FETs don't actually appear to be turning on or off.
As to the switcher operation, I don't see how they can make this boost circuit work at such low voltages. The oscillations start when Vin is just 52 uV!!! How dem do dat?
Version 4 SHEET 1 3380 1564 WIRE 1120 96 816 96 WIRE 1296 96 1168 96 WIRE 1408 96 1296 96 WIRE 1440 96 1408 96 WIRE 1568 96 1504 96 WIRE 1616 96 1568 96 WIRE 2432 96 1872 96 WIRE 816 128 816 96 WIRE 1120 128 1120 96 WIRE 1168 128 1168 96 WIRE 1408 192 1408 96 WIRE 1440 192 1408 192 WIRE 1552 192 1504 192 WIRE 1584 192 1552 192 WIRE 1616 192 1584 192 WIRE 2320 192 1872 192 WIRE 1168 240 1168 208 WIRE 816 272 816 208 WIRE 832 272 816 272 WIRE 1120 288 1120 208 WIRE 1584 288 1120 288 WIRE 1616 288 1584 288 WIRE 2208 288 1872 288 WIRE 2208 304 2208 288 WIRE 816 320 816 272 WIRE 1296 368 1264 368 WIRE 1408 368 1392 368 WIRE 1424 368 1408 368 WIRE 1552 368 1552 192 WIRE 1552 368 1520 368 WIRE 1264 384 1264 368 WIRE 1616 384 1584 384 WIRE 2128 384 1872 384 WIRE 2208 384 2208 368 WIRE 1584 400 1584 384 WIRE 2128 400 2128 384 WIRE 1088 432 1024 432 WIRE 1376 432 1376 416 WIRE 1376 432 1088 432 WIRE 1440 432 1440 416 WIRE 1440 432 1376 432 WIRE 816 480 816 400 WIRE 1120 480 816 480 WIRE 1296 480 1168 480 WIRE 1408 480 1296 480 WIRE 1440 480 1408 480 WIRE 1568 480 1504 480 WIRE 1616 480 1568 480 WIRE 1936 480 1872 480 WIRE 1968 480 1936 480 WIRE 2048 480 2032 480 WIRE 2128 480 2128 464 WIRE 1120 512 1120 480 WIRE 1168 512 1168 480 WIRE 1408 576 1408 480 WIRE 1440 576 1408 576 WIRE 1552 576 1504 576 WIRE 1584 576 1552 576 WIRE 1616 576 1584 576 WIRE 1936 576 1872 576 WIRE 1968 576 1936 576 WIRE 2048 576 2032 576 WIRE 1168 624 1168 592 WIRE 1120 672 1120 592 WIRE 1584 672 1120 672 WIRE 1616 672 1584 672 WIRE 1888 672 1872 672 WIRE 1984 672 1888 672 WIRE 2000 672 1984 672 WIRE 2000 688 2000 672 WIRE 1296 752 1264 752 WIRE 1408 752 1392 752 WIRE 1424 752 1408 752 WIRE 1552 752 1552 576 WIRE 1552 752 1520 752 WIRE 1264 768 1264 752 WIRE 1616 768 1584 768 WIRE 1888 768 1888 672 WIRE 1888 768 1872 768 WIRE 2000 768 2000 752 WIRE 1584 784 1584 768 WIRE 1024 816 1024 432 WIRE 1376 816 1376 800 WIRE 1376 816 1024 816 WIRE 1440 816 1440 800 WIRE 1440 816 1376 816 WIRE 1024 832 1024 816 WIRE 1616 864 1584 864 WIRE 1888 864 1872 864 WIRE 1584 880 1584 864 WIRE 1024 928 1024 912 WIRE 1104 928 1024 928 WIRE 1168 928 1104 928 WIRE 1168 944 1168 928 WIRE 1168 1040 1168 1024 WIRE 1168 1040 1024 1040 WIRE 1024 1056 1024 1040 FLAG 1168 240 0 FLAG 1584 880 0 FLAG 1168 624 0 FLAG 832 272 0 FLAG 1888 864 0 FLAG 2000 768 0 FLAG 2048 480 0 FLAG 2048 576 0 FLAG 2128 480 0 FLAG 816 96 IN+ FLAG 816 480 IN- FLAG 2128 384 OUT FLAG 1584 400 0 FLAG 1584 784 0 FLAG 1264 384 0 FLAG 1264 768 0 FLAG 1024 1056 0 FLAG 1568 96 C1A FLAG 1584 192 C2A FLAG 1584 288 SWA FLAG 1568 480 C1B FLAG 1584 576 C2B FLAG 1584 672 SWB FLAG 1984 672 VAUX FLAG 1088 432 SW FLAG 1296 96 TA FLAG 1296 480 TB FLAG 1408 752 MIDB FLAG 1408 368 MIDA FLAG 1936 576 Vst FLAG 1936 480 VLDO FLAG 2208 384 0 FLAG 2208 288 OUT2 FLAG 1104 928 V3+ SYMBOL PowerProducts\\LTC3109 1744 480 R0 SYMATTR InstName U1 SYMBOL cap 1504 80 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 1n SYMBOL cap 1504 176 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 470p SYMBOL ind2 1104 112 R0 WINDOW 0 0 43 Right 2 WINDOW 3 1 76 Right 2 SYMATTR InstName L1
SYMATTR Type ind SYMATTR SpiceLine Rser=85m SYMBOL ind2 1184 112 M0 WINDOW 0 0 41 Right 2 WINDOW 3 0 75 Right 2 SYMATTR InstName L2 SYMATTR Value 75m SYMATTR Type ind SYMATTR SpiceLine Rser=305 SYMBOL cap 1504 464 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value 1n SYMBOL cap 1504 560 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C4 SYMATTR Value 470p SYMBOL ind2 1104 496 R0 WINDOW 0 1 41 Right 2 WINDOW 3 1 75 Right 2 SYMATTR InstName L3
SYMATTR Type ind SYMATTR SpiceLine Rser=85m SYMBOL ind2 1184 496 M0 WINDOW 0 -2 41 Right 2 WINDOW 3 1 75 Right 2 SYMATTR InstName L4 SYMATTR Value 75m SYMATTR Type ind SYMATTR SpiceLine Rser=305 SYMBOL voltage 816 112 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PWL(0 0 1 30m) SYMBOL voltage 816 416 M180 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PWL(0 0 1 30m) SYMBOL cap 1984 688 R0 SYMATTR InstName C5
SYMBOL cap 1968 496 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName C6
SYMBOL cap 1968 592 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName C7
SYMBOL cap 2112 400 R0 SYMATTR InstName C8
SYMBOL pmos 1296 416 R270 WINDOW 0 69 19 VRight 2 WINDOW 3 102 14 VRight 2 SYMATTR InstName Q1A SYMATTR Value DI_DMP2240UDM SYMBOL pmos 1520 416 M270 WINDOW 0 67 28 VRight 2 WINDOW 3 72 -41 Invisible 2 SYMATTR InstName Q1B SYMATTR Value DI_DMP2240UDM SYMBOL pmos 1296 800 R270 WINDOW 0 69 19 VRight 2 WINDOW 3 102 14 VRight 2 SYMATTR InstName Q2A SYMATTR Value DI_DMP2240UDM SYMBOL pmos 1520 800 M270 WINDOW 0 67 28 VRight 2 WINDOW 3 72 -41 Invisible 2 SYMATTR InstName Q2B SYMATTR Value DI_DMP2240UDM SYMBOL res 1008 816 R0 SYMATTR InstName R1 SYMATTR Value 100K SYMBOL voltage 1168 928 M0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 WINDOW 3 34 65 Left 2 SYMATTR Value PULSE(0 2 0.050) SYMATTR InstName V3 SYMBOL cap 2192 304 R0 SYMATTR InstName C9
TEXT 1144 96 Bottom 2 !K1 L1 L2 .98 TEXT 1144 480 Bottom 2 !K2 L3 L4 .98 TEXT 2096 776 Left 2 !.tran 0.051 TEXT 2040 -168 Left 2 !*SRC=DMP2240UDM;DI_DMP2240UDM;MOSFETs Enh;20.0V
2.00A 0.150ohms Diodes Inc MOSFET\n.MODEL DI_DMP2240UDM PMOS( LEVEL=1 VTO=1.00 KP=11.9u GAMMA=1.24\n+ PHI=.75 LAMBDA=514u RD=21.0m RS=21.0m\n+ IS=1.00p PB=0.800 MJ=0.460 CBD=81.1p \n+ CBS=97.4p CGSO=720n CGDO=600n CGBO=1.88u )\n* -- Assumes default L=100U W=100U --