Wow, almost two months since starting the last thread ! I've started updating the power distribution diagram for the new design and added more details, including maximum current flows. This is the current draft:
http://downloads.qi-hardware.com/people/werner/anelok/tmp/power-tree-20160322.pdf That's when I realized that the boost converter I had carried over from the CR2032 design (TI TLV61220) wouldn't be able to provide nearly enough current with the now much lower battery voltage. Thus, the search began ... The basic idea is that the regulator should be able to provide a continuous output (i.e., at 3.3 V) current of at least 100 mA, preferably 120 mA (i.e., like the LDO in the MCU). Anelok normally doesn't need all that, but the memory card can draw 75 mA during 1/4 second, which I'd consider close enough to "forever" as far as such things are concerned. The AAA battery has a nominal voltage of 1.5 V but this decreases with use and load. A reasonable cut-off point would be around 1.0 V. There can be further voltage losses in the path between battery and boost converter, e.g., for battery reversal protection. (To be covered in a future post.) I'm calculating a loss budget of about 100 mV for this, so the input voltage for the boost converter would be 0.9 V. AAA batteries can deliver remarkably high current, as this experiment shows: http://www.varta-microbattery.com/applications/mb_data/DOCUMENTS/GRAPHS/04103/G_04103_O_S01_en.pdf So battery current will not be a problem. But we need a boost converter that can turn 0.9 V into 3.3 V at 120 mA. It should also have not too unreasonable quiescent consumption, i.e., while Anelok is in standby. A chip with all these characteristics is surprisingly difficult to find. I looked at a lot of data sheets, and I found no more than two chip families that can do this with a reasonably simple circuit: TI TPS6102x and Exar/Sipex SP6641B and SP6648. Data sheets: http://www.ti.com/lit/ds/symlink/tps61020.pdf http://www.exar.com/common/content/document.ashx?id=20805 http://www.exar.com/Common/Content/Document.ashx?id=729 First, the various family members: Part Voltage Switch current Vin(max) ----------------------- --------------- --------------- -------- TI TPS61020 Adjustable 1.5 A 5.5 V TI TPS61025 3.3 V 1.5 A 5.5 V TI TPS61029 Adjustable 1.8 A 6.5 V Exar/Sipex SP6641B 3.3 V 1 A (inductor) 4.5 V Exar/Sipex SP6648 Adjustable 2 A (inductor) 4.5 V Switch/inductor current is much higher than continuous output current because 1) Iin = Iout * Vout / Vin (long-term average, for an ideal converter), 2) the converter has some losses, 3) the inductor is both charged and discharged, so the peak current is well above the average current. It seems that, with a regulator that can handle a switch/inductor current of about 1 A, we're on the safe side: Part Vbat Iout(max) ----------------------- ------- --------- TI TLV61220 1.0 V ~ 60 mA for comparison TI TPS6102x 0.9 V ~250 mA Exar/Sipex SP6641B 1.0 V ~160 mA Exar/Sipex SP6648 1.0 V ~130 mA Iin(peak) = 0.85 A For some reason Exar/Sipex specify the maximum current of the SP6648 for a peak of 0.85 A, not for the chip's limit of 1.6-2.0 A. I've included the TLV61220 to show that it's not quite up to the job. Next, quiescent current. Note that this is not the "off" current of the regulator. Since the battery voltage is too low to keep an MCU running, we never turn off the regulator. When idle, these regulators draw current on both sides, input and output. I also added the ams AS1310 and the Skyworks AAT1217ICA, to illustrate the range of quiescent currents one finds: Part Iin Iout ----------------------- ------- ------- TI TLV61220 500 nA 5 uA TI TPS6102x 1 uA 30 uA (from figure 10) Exar/Sipex SP6641B 250 nA 10 uA Exar/Sipex SP6648 250 nA 12 uA ams AS1310 100 nA 1 uA look at this, TI and Exar ! Skyworks AAT1217ICA-3.3 ? 300 uA from the CR2032 design None of the new regulators quite beats the TLV61220, let alone the ams AS1310 (which unfortunately can only deliver a meager 40 mA) but they're at least in the general ballpark of the standby current I've once measured for the rest of the Anelok circuit: http://downloads.qi-hardware.com/people/werner/anelok/tmp/current-session.png Now, let's look at the package and the surrounding circuit: Part number Package (size) Circuit ----------------------- ----------------------- ---------------------- TI TPS6102xDRCR 10-DFN (3 x 3 mm) Cout = large, high ESR Exar/Sipex SP6641BEK SOT-23-5 (2.8 x 2.6 mm) Diode Exar/Sipex SP6648EU 10-MSOP (4.9 x 3 mm) Cin/out = large, ESR Exar/Sipex SP6648ER 10-DFN (3 x 3 mm) = They're all available in nice and small packages. The SP6641 needs an external diode while the other chips have it integrated. TPS6102x and SP6648 both are designed for tantalum capacitors and require fairly large capacitances (1 x 47 uF for the TPS6102x and even 2 x 47 uF for the SP6648). Not nice. TI indicate that one could use ceramic capacitors if adding a series resistor to introduce a bit of ESR (at least 30 mOhm). Now, let's look at sourcing: Part number Price (USD) Stock @1000 DigiKey Mouser Newark ----------------------- --------------- ------- ------- ------- TI TPS61020DRCR 0.90 13654 8643 3308 TI TPS61025DRCR 0.90 7899 3747 4670 TI TPS61029DRCR 0.956 7909 ~1900 56 Exar/Sipex SP6641BEK*3-3 0.45 181 (300) 158 Exar/Sipex SP6648EU 0.747 813 218 - Exar/Sipex SP6648ER 0.695@3000 0 432 - (n) = on order, no stock This doesn't look too good for Exar/Sipex. The TPS6102x is a little pricey but it should be a safe sourcing choice. Does this sound reasonable so far ? - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): [email protected] Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
