Non-RF design considerations

AN3206

12/14

Doc ID 17406 Rev 3

3.3

1.8V regulator load capacitor

2.2 µF ceramic loading capacitor is required on the output of VREG_OUT of the STM32W. A

series resistor (R5) is used to keep the minimum ESR of the loading capacitor below 1 Ohm.

This is shown in Figure 8.

Figure 8.

1.8V regulator loading capacitance

3.4

1.2V regulator loading

The STM32W contains an integrated 1.2V Regulator to drive the Core Logic. By driving the

regulated voltage, a PCB designer should connect pin VDD_CORE_0 to pin VDD_CORE_1.

If you would like to drive the core at 1.8V, then the 1.8V net should be connected to both

pins VDD_CORE_0 and VDD_CORE_1.

A 1µF ceramic loading capacitor should be placed close to pin VDD_CORE_0. In addition, a

10nF decoupling capacitor should be place close to pin VDD_CORE_1.

3.5

1.8V net decoupling

To minimize noise and coupling paths into the STM32W, decoupling capacitors must be

placed on each of the VDD pins. All decoupling capacitors should be placed as close to the

STM32W as possible as well as in between the source point (via) and the STM32W pin. In

addition, all values should be maintained as defined by the STM32W Reference Design.

3.6

1.8V analog to 1.8V digital filtering

To keep the Radio Active current at 36 mA, ST recommends adding a single element filter

(resistor) between the digital and analog 1.8V nets, 1.8V_A and 1.8V_D, respectively (see

Figure 9). This resistor maintains a reduced bias current for the digital logic within the

STM32W. If this resistor is not used, the RX current will increase by 1mA to 37mA.