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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