1. STM32WL3x Hardware & BoM flexibility
STM32WL3x SoC permits important Hardware & BoM flexibility to fits exactly with customer requirements.
Depending on customer requirements, seven key blocks can be optimized:
- Output Power Range (10, 14 or 20dBm): TX & TX-HP pins can be selected depending on output power targeted by customer.
- SMPS ON/OFF & by-pass mode: will permit to optimize the current consumption in Transmit & Receive mode.
- High Speed Crystal Internal or External (OSCout & In pins): HSE Crystal is mandatory for RF activities.
- Low Speed Crystal Internal or External (SXTAL): LSE can be used if customer needs precise RTC calendar.
- Direct-tie or RF switch: will permit to optimize the BoM or the RF performances.
- SMD or IPD solution for RF matching: ST IPD solution will permit to optimize the BoM & layout size.
- LPWAWuR feature: this Low Power Autonomous Wake-Up Radio (LPWAWuR) is an optional feature and will permit to wake up the WL3x core.
1.1. Output Power range
STM32WL3x can reach 20 dBm output power. Two dedicated pins (TX & TX_HP) can be used/combined to optimize the current consumption depending on the targeted customer output power range. Only BOM change (TX matching BOM) is required to change from one configuration to another if needed.
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| TX configuration versus output power |
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See the extract of our NucleoSTM32WL33 board: only TX/TX-HP to be selected & L4 C9 to be modified depending on targeted output power.
1.2. SMPS configuration
The SMPS has the following possible configurations:
1.2.1. SMPS_ON
- the VFBSD pin of the SMPS outputs a regulated voltage (from 1.2 V to 2.4 V). In this condition, the SMPS needs a clock.
- In this configuration, the TX & RX current consumption is optimized:
- TX current consumption @3v3: 10mA @10dBm
- RX current consumption @3v3: 5,6mA
- SMPS_ON configuration needs an extra Bill Of Material: 10µH DCDC coil + 100nH RF coil + 4,7µF
1.2.2. No SMPS
- VFBSD pin must be connected or to an external supply or to VDD, VLXSD pin must be floating. The SMPS does not need a clock
- In this configuration, the Bill Of material is optimized (No extra component), & sensitivity is improved by 1dB (versus SMPS configuration)
- However, the current consumption in RX (12,5mA) & in TX (23.5mA @10dBm) are significantly degraded.
1.2.3. SMPS can also be by-Passed by SW
- it will require the extra BOM (10µH DCDC coil + 100nH RF coil + 4,7µF) but can be passed in RX to improve sensitivity.
1.3. High Speed XTAL / TCXO
1.3.1. High Speed XTAL (HSE) or TXCO is mandatory for RF activities
- XTAL frequency should be between 46 & 50MHz.
| Symbol | Parameter | Conditions | Min. | Typ. | Max. | Unit |
|---|---|---|---|---|---|---|
| fnom | Oscillator frequency | - | 46 | 48 | 50 | MHz |
| tTOL | Frequency accuracy | Initial accuracy at 25°C | - | - | +/-10 | ppm |
| Over temperature 40°C to +105°C | - | - | +/-25 | |||
| Aging over 5 years | - | - | +/-5 | |||
| ESR | Equivalent series resitance | - | - | - | 80 | Ω |
| PD | Drive level | - | - | - | 20 | μW |
1.3.2. STM32WL3x embeds a wide-range capacitor bank for High speed Xtals
- Up to +/- 25ppm frequency correction for 8pF external Xtals
- Capacitor bank SW manageable to cope with both initial and temperature variations
- No external load capacitances & No HSE frequency trimming needed in production => BOM optimized & easy calibration.
1.4. Low speed external XTAL or low speed internal can be used as low speed clock source
1.4.1. LSE clock
- Low load cap crystal (e.g. 6pF) and low ESR ensure low drive can be used.
- Can be used for accurate RTC calendar.
- Permits to optimize the current consumption.
1.4.2. LSI clock
- In this configuration, pins can be left open or can be used as GPIO (alternate function).
- The BOM & layout is optimized since No external 32kHz XTAL used.
1.5. RF switch or Direct-tie configuration
STM32WL3x includes three different RF paths: the transmission path (TX & TX-HP pins), the reception path (RX pin) & the Low Power Asynchronous Wake Up Radio reception path. These paths can be connected directly or connected via an RF switch.
1.6. SMD or IPD solution for RF matching
To optimize the PCB layout & BoM, STMicroelectronics has developed a STM32WL3x companion chip solution for RF matching.
The MLPF-WL-XXDXX integrates an impedance matching network and harmonics filter. The matching impedance network has been tailored to maximize the RF performances of STM32WL3xxx. Several versions have been developed depending on targeted output power, frequency bands & PCB stack-up (two or four layers).
1.7. Low Power Autonomous Wake Up Receiver feature
The STM32WL33xx includes an always-on ultra-low-power wake-up receiver. It is intended to use the reception of a specific frame to trig the wake up the entire SOC while in Deepstop mode. This feature is optional. It should not be mounted/ used if it is not used.
- Wide band dedicated RX circuitry/pin (from 100MHz up to 2,4GHz)
- Based on RF detection principle using amplitude-modulated (OOK modulation) input frame
- Used as proximity detector by waking-up RF SoC on a predefined radio frame
- Short range (-50dBm of sensitivity) and ultra-low current consumption (4µA)
2. Conclusions on BoM & applications
Three main flavors of HW configuration can be combined depending on customer requirements
2.1. RF performances optimized
If customer wants to optimize RF performances, he must use this HW configuration:
- SMPS ON to optimize current consumption
- TXCO to reach maximum frequency accuracy.
- RF switch to reach maximum sensitivity performances.
2.2. Bill of Material optimized
If the customer wants to optimize the Bill of Material, they must use this HW configuration:
- No SMPS to save 10µH coil.
- No RF switch & XTAL (instead of TCXO).
- The current consumption in Transmission & Reception will significantly increase.
3. Best Trade-off
The best Trade-off is the BoM used for our STMicroelectronics Reference board Nucleo-WL33C
- SMPS ON for best current consumption
- Maximum Tx output power (up to +20dBm
- Xtal and no RF switch
