12.1 Layout Guidelines
The switching node rise and fall times should be minimized for minimum switching loss. Proper layout of the components to minimize high-frequency current path loop (see Figure 25) is important to prevent electrical and magnetic field radiation and high-frequency resonant problems. The following procedure shows a PCB layout priority list for proper layout. Layout PCB according to this specific order is essential.
- Place the input capacitor as close as possible to the supply and ground connections of the switching MOSFET and use shortest copper trace connection. These parts should be placed on the same layer of PCB instead of on different layers and using vias to make this connection.
- The IC should be placed close to the gate terminals of the switching MOSFET and keep the gate drive signal traces short for a clean MOSFET drive. The IC can be placed on the other side of the PCB of switching MOSFETs.
- Place the inductor input terminal as close as possible to the output terminal of the switching MOSFET. Minimize the copper area of this trace to lower electrical and magnetic field radiation but make the trace wide enough to carry the charging current. Do not use multiple layers in parallel for this connection. Minimize parasitic capacitance from this area to any other trace or plane.
- Place the charging current-sensing resistor right next to the inductor output. Route the sense leads connected across the sensing resistor back to the IC in same layer, close to each other (minimize loop area) and do not route the sense leads through a high-current path (see Figure 26 for Kelvin connection for best current accuracy). Place the decoupling capacitor on these traces next to the IC.
- Place the output capacitor next to the sensing resistor output and ground
- Output capacitor ground connections must be tied to the same copper that connects to the input capacitor ground before connecting to system ground.
- Use a single ground connection to tie charger power ground to charger analog ground. Use analog ground copper pour just beneath the IC, but avoid power pins to reduce inductive and capacitive noise coupling.
- Route analog ground separately from power ground. Connect analog ground and connect power ground separately. Connect analog ground and power ground together, using power pad as the single ground connection point, or using a 0-Ω resistor to tie analog ground to power ground (power pad should tie to analog ground in this case if possible).
- Place the decoupling capacitors next to the IC pins and make trace connection as short as possible.
- It is critical that the exposed power pad on the backside of the IC package be soldered to the PCB ground. Ensure that there are sufficient thermal vias directly under the IC, connecting to the ground plane on the other layers.
- The via size and number should be enough for a given current path.
See the EVM design for the recommended component placement with trace and via locations. For the QFN information, see SCBA017 and SLUA271.
To prevent unintentional charger shut down in normal operation, MOSFET RDS(on) selection and PCB layout is very important. Figure 23 shows a improvement PCB layout example and its equivalent circuit. In this layout, the system current path and charger input current path is not separated, as a result, the system current causes voltage drop in the PCB copper and is sensed by the IC. The worst layout is when a system current pull point is after charger input; as a result all system current voltage drops are counted into overcurrent protection comparator. The worst case for IC is when the total system current and charger input current sum equals the DPM current. When the system pulls more current, the charger IC tries to regulate the RAC current as a constant current by reducing the charging current.
Figure 23. Improvement PCB Layout Example
Figure 24 shows the optimized PCB layout example. The system current path and charge input current path is separated, and as a result, the IC only senses charger input current caused PCB voltage drop and minimized the possibility of unintentional charger shutdown in normal operation. This also makes PCB layout easier for high system current application.
Figure 24. Optimized PCB Layout Example
The total voltage drop sensed by IC can be expressed as the following equation:
Equation 15. Vtop
) x k) + RDS(on)
- RAC is the AC adapter current sensing resistance.
- IDPM is the DPM current set point.
- RPCB is the PCB trace equivalent resistance.
- ICHRGIN is the charger input current.
- k is the PCB factor.
- RDS(on) is the high-side MOSFET turnon resistance.
- IPEAK is the peak current of inductor.
Here, the PCB factor k = 0 means the best layout shown in Figure 24, where the PCB trace only goes through charger input current, while k = 1 means the worst layout shown in Figure 23, where the PCB trace goes through all the DPM current. The total voltage drop must below the high-side short-circuit protection threshold to prevent unintentional charger shutdown in normal operation.
The low-side MOSFET short circuit voltage drop threshold can be adjusted through SMBus command. ChargeOption() bit  =0, 1 sets the low-side threshold, 135 mV and 230 mV, respectively. The high-side MOSFET short circuit voltage drop threshold can be adjusted through SMBus command. ChargeOption() bit  = 0, 1 disables the function and set the threshold, 750 mV, respectively. For a fixed PCB layout, host should set proper short-circuit protection threshold level to prevent unintentional charger shutdown in normal operation.