The inductor value for the MAX38650 affects the ripple current, the transition point from LPM to HPM and overall efficiency performance. It is recommended to use an inductor value of 2.2μH.
The input capacitor (CIN) reduces the peak current drawn from battery or input power source and reduces the switching noise in the IC. The impedance of CIN at the switching frequency should be very low. Ceramic capacitors are recommended with their small size and low ESR. For most applications, it is recommended to use a 10µF ceramic capacitor with X7R temperature characteristics. For operations where ambient temperature is less than +85°C, X5R can be used. In applications where the device will approach or go into 100% duty cycle operation, more capacitance at the input pin is required; therefore, 10μF (5μF effective capacitance) at CIN is recommended.
The output capacitor (COUT) is required to keep the output voltage ripple small and to ensure loop stability. COUT must have low impedance at the switching frequency. Ceramic capacitors are recommended due to their small size and low ESR. Make sure the capacitor does not degrade its capacitance significantly over temperature and DC bias. For most applications, it is recommended to use 22µF ceramic capacitor with X7R temperature characteristics. For operations where ambient is less than +85°C, X5R can be used. A 22µF ceramic capacitor (10μF effective capacitance) is recommended for applications.
Careful PCB layout is especially important in nanoPower DC-DC converters. Poor layout can affect the IC performance causing electromagnetic interference (EMI), electromagnetic compatibility (EMC) issues, ground bounce, voltage drops, etc. Poor layout can also affect regulation and stability.
A good layout is implemented using the following rules:
- Place the inductor, input capacitor, and output capacitor close to the IC using short traces and/or copper pours. These components carry high switching currents and long traces act like antennas. The input capacitor placement is the most important in the PCB layout and should be placed directly next to the IC. The inductor and output capacitor placement are secondary to the input capacitor’s placement but should remain close to the IC.
- The connection from the bottom plate of the input capacitor and the ground pin of the device must be extremely short, as should be that of the output capacitor.
- Similarly, the top plate of input capacitor connection to the IN pin of the device must be short as well.
- Minimize the surface area used for LX since this is the noisiest node.
- Keep the main power path from IN, LX, OUT, and GND as tight and short as possible.
- Route the output voltage sense away from the inductor and LX switching node to minimize noise and magnetic interference.
- Maximize the size of the ground metal on the component side to help with thermal dissipation. Use a ground plane with several vias connecting to the component-side ground to further reduce noise interference on sensitive circuit nodes.
- The trace used for the RSEL signal should neither be too long nor should produce a capacitance of more than 2pF.
It is also recommended to consult the MAX38650 EV kit layout.