10.1.1 Output Capacitor Placement

The output capacitor is in the path of the inductor current discharge path. As a result C_OUT detects a high current step from 0 to I_PEAK each time the switch turns off and the Schottky diode turns on. Any inductance along this series path from the cathode of the diode through C_OUT and back into the LM3630A GND pin will contribute to voltage spikes (V_SPIKE = L_{P_} × dI/dt) at SW and OUT which can potentially overvoltage the SW pin, or feed through to GND. To avoid this, C_OUT+ must be connected as close as possible to the Cathode of the Schottky diode and C_OUT– must be connected as close as possible to the device GND bump. The best placement for C_OUT is on the same layer as the LM3630A so as to avoid any vias that can add excessive series inductance (see Figure 94).

10.1.2 Schottky Diode Placement

The Schottky diode is in the path of the inductor current discharge. As a result the Schottky diode detects a high current step from 0 to I_PEAK each time the switch turns off and the diode turns on. Any inductance in series with the diode will cause a voltage spike (V_SPIKE = L_{P_} × dI/dt) at SW and OUT which can potentially overvoltage the SW pin, or feed through to V_OUT and through the output capacitor and into GND. Connecting the anode of the diode as close as possible to the SW pin and the cathode of the diode as close as possible to C_OUT+ will reduce the inductance (L_{P_}) and minimize these voltage spikes (see Figure 94).

10.1.3 Inductor Placement

The node where the inductor connects to the LM3630A SW bump has 2 issues. First, a large switched voltage (0 to V_OUT + V_{F_SCHOTTKY}) appears on this node every switching cycle. This switched voltage can be capacitively coupled into nearby nodes. Second, there is a relatively large current (input current) on the traces connecting the input supply to the inductor and connecting the inductor to the SW bump. Any resistance in this path can cause large voltage drops that will negatively affect efficiency.

To reduce the capacitively coupled signal from SW into nearby traces, the SW bump to inductor connection must be minimized in area. This limits the PCB capacitance from SW to other traces. Additionally, the other traces need to be routed away from SW and not directly beneath. This is especially true for high impedance nodes that are more susceptible to capacitive coupling such as (SCL, SDA, HWEN, PWM, and possibly ASL1 and ALS2). A GND plane placed directly below SW will dramatically reduce the capacitive coupling from SW into nearby traces

To limit the trace resistance of the VBATT to inductor connection and from the inductor to SW connection, use short, wide traces (see Figure 94).

10.1.4 Input Capacitor Selection and Placement

The input bypass capacitor filters the inductor current ripple, and the internal MOSFET driver currents during turnon of the power switch.

The driver current requirement can range from 50 mA at 2.7 V to over 200 mA at 5.5 V with fast durations of approximately 10 ns to 20 ns. This will appear as high di/dt current pulses coming from the input capacitor each time the switch turns on. Close placement of the input capacitor to the IN pin and to the GND pin is critical since any series inductance between IN and C_IN+ or C_IN– and GND can create voltage spikes that could appear on the V_IN supply line and in the GND plane.

Close placement of the input bypass capacitor at the input side of the inductor is also critical. The source impedance (inductance and resistance) from the input supply, along with the input capacitor of the LM3630A, form a series RLC circuit. If the output resistance from the source (R_S) is low enough the circuit will be underdamped and will have a resonant frequency (typically the case). Depending on the size of L_S the resonant frequency could occur below, close to, or above switching frequency of the device. This can cause the supply current ripple to be:

1. Approximately equal to the inductor current ripple when the resonant frequency occurs well above the LM3630A switching frequency;
2. Greater then the inductor current ripple when the resonant frequency occurs near the switching frequency; and
3. Less then the inductor current ripple when the resonant frequency occurs well below the switching frequency.

Figure 93 shows the series RLC circuit formed from the output impedance of the supply and the input capacitor. The circuit is re-drawn for the AC case where the V_IN supply is replaced with a short to GND and the LM3630A plus inductor is replaced with a current source (ΔI_L). In Figure 93, equation 1 is the criteria for an underdamped response, equation 2 is the resonant frequency, and equation 3 is the approximated supply current ripple as a function of L_S, R_S, and C_IN.

As an example, consider a 3.6-V supply with 0.1-Ω of series resistance connected to C_IN through 50 nH of connecting traces. This results in an underdamped input filter circuit with a resonant frequency of 712 kHz. Since the switching frequency lies near to the resonant frequency of the input RLC network, the supply current is probably larger then the inductor current ripple. In this case using equation 2 from Figure 93 the supply current ripple can be approximated as 1.68 multiplied by the inductor current ripple. Increasing the series inductance (L_S) to 500 nH causes the resonant frequency to move to around 225 kHz and the supple current ripple to be approximately 0.25 multiplied by the inductor current ripple.

Figure 93. Input RLC Network