pwsafe: boost converter

Felix sucotronic at gmail.com
Mon Sep 16 03:46:51 EDT 2013


Nice selection. The sky one (in the  3.3 fixed voltage option) seems to
need less parts. According to efficiency curves, with the near 3v voltage
of a CR2032 it should handle very well.


On Fri, Sep 13, 2013 at 5:56 PM, Werner Almesberger
<werner at almesberger.net>wrote:

> 1) The converter chip
>
> Requirements:
> - can convert 1.8-3.0 V to 3.3 V,
> - Imax about 200 mA if we have a partcularly nasty memory card
> - enable/disable input
> - no center pad (to make manual soldering easier)
>
> The usual Digi-Key search:
>
> Product Index > Integrated Circuits (ICs) >
>     PMIC - Voltage Regulators - DC DC Switching Regulators -> 19092
> Mounting Type: SMT -> 17,593
> In stock -> 7,022
> Packaging: Cut Tape/Bulk/Tray/Tube -> 3,739
> Voltage - Input: min <= 1.8 V -> 498
> Current - Output: >= 200 mA -> 262
> Number of Outputs: 1 ->  251
> Synchronous Rectifier: Yes -> 163
> Voltage - Output: min <= 3.3 V -> 142
> Type: Step-Up (or Step-Up and Step-Down) -> 116
> Voltage - Output: eliminate max < 3.3 V -> 107
> Supplier Device Package: < 10 pins -> 50
>
> Order by Unit Price @ 100:
> 0.39    Skyworks AAT1217ICA-1.2-T1      TSOT-23-6, up, adj, 500 mA, 1.2 MHz
> 0.39    Skyworks AAT1217ICA-3.3-T1      TSOT-23-6, up, 3.3 V, 500 mA, 1.2
> MHz
> 0.55    TI TLV61220DBVR                 SOT-23-6, up, adj, 400 mA, ?
> 0.67    TI TPS61220DCKR                 SOT-363, up, adj, 200 mA, 2 MHz
> 0.77    TI TPS61221DCKT                 SOT-363, up, 3.3 V, 200 mA, 2 MHz
> 0.77    TI TPS61220DCKT                 SOT-363, up, adj, 200 mA, 2 MHz
> 0.79    TI TPS61071DDCR                 TSOT-23-6, up, adj, 600 mA, 1.2 MHz
> ... more TPS6107xDDCR ...
> ... skip WDFN ...
> ... skip 8-VSSOP ...
> 0.79    TI LMR61428XMM/NOPB             8-VSSOP, up, adj, 2.85 A, 2 MHz
> 1.02    Exar SP6641BEK-L-3-3            SOT-23-5, up, 3.3 V, 500 mA, ?
> 1.10    TI TLV61225DCKT                 SOT-363, up, 3.3 V, 200 mA, 2 MHz
> 1.13    ON NCP1421DMR2G                 8-TSSOP, up, adj, 600 mA, 1.2 MHz
> 1.13    ON NCP1410DMR2G                 8-TSSOP, up, adj, 200 mA, 0.6 MHz
>
> AAT1217ICA-3.3-T1       2.9 x 2.2 mm    EN, 2 x 4.7 uF, 4.7 uH
> TLV61220DBVR            ?               EN, 2 x 10 uF, 4.7 uH
> TPS61221DCKT            2 x 2 mm        EN, 2 x 10 uF, 4.7 uH
> TPS61071DDCR            ?               EN, 2 x 10 uF, 4.7 uH
> LMR61428XMM     external diode
> SP6641BEK-L-3-3 **huge** caps
> TLV61225DCKT    no data sheet
> NCP1421DMR2G    complex external circuit
>
> Selecting parts with the same package and where the package isn't
> an extreme size:
>
> Pin assignment, starting at 1:
>
> AAT1217ICA      SW, GND, (FB), nSHDN, VOUT, VIN
> TLV61220DBVR    SW, GND, EN, FB, VOUT, VBAT
>
> So they're similar but not the same. There are probably a number of
> very similar parts on the Chinese market, so 3rd sources and such
> shouldn't be a problem.
>
> -> design for AAT1217ICA series,
>    leave room for voltage-setting resistors (BOM option),
>    use 10 uF caps,
>    allow for modification for TLV61220DBVR
>
>
> 2) The inductor
>
> according to the formulas on page 8 of
> http://www.skyworksinc.com/uploads/documents/AAT1217_202050B.pdf
>
> Vin = 1.8 V
> Vout = 3.3 V
> Iout = 200 mA
> Fsw = 1.2 MHz
> L = 4.7 uH
> n = 0.7
>
> Idc = Iout*Vout/(n*Vin) = 0.524
> dIl = Vin*(Vout-Vin)/(L*Fsw*Vout) = 0.145
>
> Ipeak = Idc + dIl/2
> Ipeak = 0.6 A
>
> Let's go to Digi-Key:
>
> Product Index > Inductors, Coils, Chokes > Fixed Inductors -> 102964
> In stock -> 38020
> Shielding: Shielded -> 15519
> Packaging: Bulk / Cut Tape / Tray -> 5835
> Mounting Type: SMT -> 5581
> Current Rating: >= 590 mA -> 4295
> Current Rating: <= 10 A -> 3848
> Inductance: 4.5 uH - 4.9 uH -> 298
> Package / Case: 1008 (2520 Metric) -> 14
>
> Order by price @100 units:
> 0.11900 Samsung CIG22L4R7MNE            4.7 uH, 1.1 A, 20%, 110 mOhm, H
> 2.5 mm
> 0.14630 Bourns CVH252009-4R7M           4.7 uH, 1.1 A, 20%, 110 mOhm, H
> 0.9 mm
> 0.16650 Sumida 252010CDMCDS-4R7MC       4.7 uH, 1.4 A, 20%, 240 mOhm, H
> 1.0 mm
> ...
>
> All three look quite suitable.
>
> - Werner
>
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-- 
Felix
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