This document describes the hardware considerations for the schematic and layout of the MKW36A512VFT4 device. This MCU is packaged into a 48-pin HVQFN - 7x7 mm, dissimilar to MKW36Z512VHT4 which comes packaged into a 48-pin LQFN - 7x7 mm (the last one takes part of FRDM-KW36).
The MKW36A512VFT4 MCU is pin to pin compatible with the MKW36Z512VHT4 (FRDM-KW36) MCU, except for the DCDC pins. The following figure shows the distribution of the pins in the MKW36A512VFT4 MCU (left), compared with the MKW36Z512VHT4 (FRDM-KW36 MCU, right). Surely, this is the most important consideration for MKW36A512VFT4, since you can not simply move the FRDM-KW36 layout on your design.
The footprint and layout are critical for RF performance, hence if the recommended design is followed exactly in the RF region of the PCB, sensitivity, output power, harmonic and spurious radiation, and range, you will succeed. For more information of layout recommendations, please visit Hardware Design Considerations for MKW35A/36A/35Z/36Z Bluetooth Low Energy Devices. The footprint recommended for the MKW36A512VFT4 is shown in the figure below.
NXP prefers to use a top layer thickness of no less than 8-10 mils. The use of a correct substrate like the FR4 with a dielectric constant of 4.3 will assist you in achieving a good RF design. Other recommendations during EMC certification stages are:
- Specific attention must be taken on 4 pins PTC1, 2, 3 & 4 if they are used on the application. - 4 decoupling capacitors of 3pF are mandatory on those pins and be positioned as close as possible. - Wires from those 4 pins must be underlayer. - NXP recommends putting the vias under the package in case the customer HW design rules allow it.
Some recommendations for a good Vdd_RF supply layout are:
- Vdd_RF1 and Vdd_RF2 lines must have the same length as possible, linked to pointA (‘Y’ connection). - 12pF decoupling capacitor from Vdd_RF wire must be connected to the Ground Antenna. The purpose is to get the path as short as possible from Vdd_RF1/Vdd_RF2 to the ground antenna. - 12pF decoupling capacitor from the Vdd_RF3 pin must be as close as possible. Return to ground must be as short as possible. So vias (2 in this below picture) must be placed near to the decoupling capacitor to get close connection to the ground layer.
The recommended RF stage is shown in the following figure. The MKW36A512VFT4 has a single-ended RF output with a 2 components matching network composed of a shunt capacitor and a series inductor. Both elements transform the device impedance to 50 ohms. The value of these components may vary depending on your board layout. Avoid routing traces near or parallel to RF transmission lines or crystal signals. Maintain a continuous ground under an RF trace is critical to keep unaltered the characteristic impedance of the transmission line. Avoid routing on the ground layer that will result in disrupting the ground under RF traces. For more information about RF considerations please visit: Freescale IEEE 802.15.4 / ZigBee Package and Hardware Layout Considerations.