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Low-dropout (LDO) Linear Regulators

Two LDOs will be required for the DAC and ADC supply. If I choose to use the WM8524 & PCM1808 DAC/ADC combination, I will require a 3.3V, >50mA LDO for the DAC and a 5V, >77mA LDO for the ADC’s analog supply.

If I select the PCM1754 DAC, it’ll similarly require a 5V analog supply capable of supplying > 200mA for both DAC+ADC supplies.

Selection

Requirements

While there aren’t too many strict requirements yet, I am looking to maximize PSRR and minimize quiescent current. The only strict requirement are the following:

  • > 200 mA
  • 3.3V and 5V (separately)
  • High PSRR

Since I already have them available, I can use the Texas Instruments LP5907 3.3V, 250mA LDO for the WM8524 design and two Texas Instruments TPS7A2450 5V, 200mA LDO for the PCM1754 & PCM1808. While pretty low current, the LP5907 and TPS7A2450 possess very good characteristics, such as PSRR of 82dB and 62dB @ 1kHz and a very low quiescent current of 12uA and 2.0 uA, respectively.

Texas Instruments, also recommends the TPS7A20 series ($0.32), as a drop-in replacement for the LP5907, which is capable of outputting 300mA and has similarly high PSRR of 95 dB @ 1kHz. If higher currents are required, TI recommends the adjustable output TLV758P ($0.27), capable of outputting 0.55-5.5V @ 500mA, however the TLV758P has much worse PSRR compared to the TPS7A20.

Summary:

Overall, the TPS7A20 series is best for both 3.3V and 5V supplies due to the higher current capability compared to the LP5907 & TPS7A2450. These SOT-23-5 packages of the three LDOs are pin-compatible, so I shouldn’t need to design variants for each version even if I decide to reuse the parts I have available.

Note: w.r.t. PSRR, the PSRR vs Frequency charts provided by the datasheets for the TPS7A20 and TPS7A24 series are very difficult to compare accurately. The frequency regimes at which each LDO outperforms the other varies greatly depending on I~OUT~. However, overall the TPS7A20 has better PSRR over a larger portion of frequencies.

Switch Mode Power Supplies

Dual Rail Op-Amp Supply Selection:

Requirements:

An SMPS will be required for supplying the op amps with both positive and negative voltages.

  • Split rail +-5V
  • Low noise within audible frequency ranges
  • Does not need to supply a lot of current, probably minimum 200 mA
  • Low quiescent current

Discrete DC-DC Converters, such as the Recom RB-0505D, provide dual rail +- 5V at 1W, however, are quite pricey at $5-10 for each unit.

There are not really many choices for dual rail switching DC-DC converter regulators, however, Texas Instruments offers three similar products, the TPS65135, TPS65132 ($0.84), and TPS65133 ($1.89), with TI recommending the latter two for audio purposes.

Comparing the two, the -132 has a maximum output current of 150 mA and has variable output voltages up to +- 6V (.1 V step), while the -133 has a maximum output current of 250 mA with fixed output voltages of +- 5V. While the -133 is more expensive, to meet the current needs of the 3 amplifiers I have in my system, I think I will implement one -133 for the output amplifier stage and one -132 for the input preamp.

Single Output 3.3V Digital Supply Selection:

Requirements:
  • Single output 3.3V, 5V
  • Input range 2.5-4.2V

For 3.3V, I’ve selected the TPS613221 ($0.49) and for 5V, the TPS613222 ($0.49). Both have a input range of 0.9-5.5V and an output of 3.3V @ 1.6 A and 5V @ 2.5V respectively.

Battery Management System

A battery management system is required to handle charging, discharging, safety, and monitoring of one or more batteries. For my purposes, I need a BMS for just one LiFePO4, Li-ion, or LiPo cell to safely charge, safely discharge, and compute the charge level of the battery.

I will be selecting the Texas Instruments BQ29700DSER Li-ion/LiPo BMS IC for 1 cell battery protection due to its availability.

N-channel MOSFET

The BQ297XX implementation requires two N-channel MOSFETs. The example they provide is the CSD16406Q3 ($1.65), which features V~DS~ = 25V, Q~G~ = 5.8 nC, Q~gd~ = 1.5 nC, R~DS~ = 5.9 mOhm @ V~GS~ = 4.5V, and V~th~ = 1.8V.

Requirements:

Requirements are very loose in this case since the max V~DS~ it could possibly experience is around 3V and it isn’t being used for high frequency switching. Thus, the priority is to minimize on-resistance and threshold voltage to minimize energy lost at a low cost. I’ll set the following requirements for MOSFET selection:

  • Low R~DS~ < 50 mOhm
  • Low V~th~ < 3V

I’ve selected the IRLML6244 N-channel MOSFET ($0.43), a V~DS~=20V, R~DS~=27 mOhm @ V~GS=2.5V, V~th~=0.5-1.1V.

Citations

  1. Horowitz, P., & Hill, W., (2015) The Art of Electronics
  2. Self, D. (2010) Small Signal Audio Design