8.2.1.2.1 Inductor Selection

There are two main considerations when choosing an inductor:

• The inductor should not saturate.
• The inductor current ripple should be small enough to achieve the desired output voltage ripple.

Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of the application should be requested from the manufacturer. The minimum value of inductance to ensure good performance is 1.76 µH at I_LIM (typical) DC current over the ambient temperature range. Shielded inductors radiate less noise and should be preferred.

There are two methods to choose the inductor saturation current rating:

Method 1:

The saturation current is greater than the sum of the maximum load current and the worst case average to peak inductor current. This can be written as:

Equation 4.

Equation 5.

where

• I_RIPPLE is average-to-peak inductor current
• I_OUTMAX is maximum load current (600 mA)
• V_IN is maximum input voltage in application
• L is minimum inductor value including worst case tolerances (30% drop can be considered for method 1
• f is minimum switching frequency (1.6 MHz)
• V_OUT is output voltage

Method 2:

A more conservative and recommended approach is to choose an inductor that has saturation current rating greater than the maximum current limit of 1200 mA.

A 2.2-µH inductor with a saturation current rating of at least 1200 mA is recommended for most applications. The resistance of the inductor should be less than 0.3 Ω for good efficiency. Table 1 lists suggested inductors and suppliers. For low-cost applications, an unshielded bobbin inductor is suggested. For noise critical applications, a toroidal or shielded-bobbin inductor should be used. A good practice is to lay out the board with overlapping footprints of both types for design flexibility. This allows substitution of a low-noise toroidal inductor in the event that noise from low-cost bobbin models is unacceptable.

Table 1. Suggested Inductors and Their Suppliers

8.2.1.2.2 Input Capacitor Selection

A ceramic input capacitor of 4.7 µF, 6.3 V is sufficient for most applications. Place the input capacitor as close as possible to the V_IN pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. The minimum input capacitance to ensure good performance is 2.2 µF at 3-V DC bias; 1.5 µF at 5-V DC bias including tolerances and over ambient temperature range. The input filter capacitor supplies current to the PFET switch of the LM3674 in the first half of each cycle and reduces voltage ripple imposed on the input power source. The low equivalent series resistance (ESR) of a ceramic capacitor provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select a capacitor with sufficient ripple current rating. The input current ripple can be calculated as:

8.2.1.2.3 Output Capacitor Selection

A ceramic output capacitor of 10 µF, 6.3 V is sufficient for most applications. Use X7R or X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. DC-bias characteristics vary from manufacturer to manufacturer and DC-bias curves should be requested from them as part of the capacitor selection process.

The minimum output capacitance to ensure good performance is 5.75 µF at 1.8 V DC bias including tolerances and over ambient temperature range. The output filter capacitor smoothes out current flow from the inductor to the load, helps maintain a steady output voltage during transient load changes, and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR to perform these functions.

The output voltage ripple is caused by the charging and discharging of the output capacitor and by the R_ESR and can be calculated as:

Voltage peak-to-peak ripple due to capacitance can be expressed as:

Equation 7.

Voltage peak-to-peak ripple due to ESR:

Equation 8.

Because these two components are out of phase, the root mean squared (rms) value can be used to get an approximate value of peak-to-peak ripple.

Voltage peak-to-peak ripple, rms:

Equation 9.

Note that the output ripple is dependent on the current ripple and the equivalent series resistance of the output capacitor (R_ESR).

The R_ESR is frequency-dependent (as well as temperature-dependent); make sure the value used for calculations is at the switching frequency of the part.

Table 3. Related Plots

8.2.2.2.1 Output Voltage Selection for Adjustable (LM3674-ADJ)

The output voltage of the adjustable parts can be programmed through the resistor network connected from V_OUT to FB then to GND. V_OUT will be adjusted to make FB equal to 0.5 V. The resistor from FB to GND (R2) should be 200 kΩ to keep the current drawn through this network small but large enough that it is not susceptible to noise. If R₂ is 200 kΩ, and given the V_FB is 0.5 V, then the current through the resistor feedback network will be 2.5 µA. The output voltage formula is:

Equation 10.

where

• V_OUT = Output voltage (V)
• V_FB = Feedback voltage (0.5 V typical)
• R₁ = Resistor from V_OUT to FB (Ω)
• R₂ = Resistor from FB to GND (Ω)

For any output voltage greater than or equal to 1.0 V, a frequency zero must be added at 45 kHz for stability. The formula is:

Equation 11.

For output voltages greater than or equal to 2.5 V, a pole must also be placed at 45 kHz as well. If the pole and zero are at the same frequency the formula for calculation of C2 is:

Equation 12.

The formula for location of zero and pole frequency created by adding C1,C2 are given below. It can be seen that by adding C1, a zero as well as a higher frequency pole is introduced.

Equation 13.

See Table 4.

Table 5. Related Plots