Table 2. Design Parameters

8.2.1.2.1 Inductor Selection

There are two main considerations when choosing an inductor: the inductor current must not saturate, and the inductor current ripple is small enough to achieve the desired output voltage ripple.

There are two methods to choose the inductor current rating.

8.2.1.2.1.1 Method 1

The total current is the sum of the load and the inductor ripple current. This can be written as

Equation 6.

Equation 7.

where

• I_LOAD = load current
• V_IN = input voltage
• L = inductor
• ƒ = switching frequency
• I_RIPPLE = peak-to-peak current

8.2.1.2.1.2 Method 2

A more conservative approach is to choose an inductor that can handle the current limit of 700 mA.

Given a peak-to-peak current ripple (I_PP) the inductor needs to be at least

Equation 8.

A 10-µH inductor with a saturation current rating of at least 800 mA is recommended for most applications. Resistance of the inductor resistance must be less than around 0.3 Ω for good efficiency. Table 3 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 must 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.

8.2.1.2.2 Input Capacitor Selection

A ceramic input capacitor of 4.7 µF is sufficient for most applications. A larger value may be used for improved input voltage filtering. The input filter capacitor supplies current to the PFET switch of the LM3670 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 an input filter capacitor with a surge current rating sufficient for the power-up surge from the input power source. The power-up surge current is approximately the value of the capacitor (µF) times the voltage rise rate (V/µs). The input current ripple can be calculated by :

Equation 9.

8.2.1.2.3 Output Capacitor Selection

The output filter capacitor smooths out current flow from the inductor to the load, maintaining 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 ripple current can be calculated as:

Voltage peak-to-peak ripple due to capacitance =

Voltage peak-to-peak ripple due to ESR =

Voltage peak-to-peak ripple, root mean squared =

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

Because these two components are out-of-phase the RMS value is used. The R_ESR is frequency dependent (as well as temperature dependent); make sure the frequency of the R_ESR given is the same order of magnitude as the switching frequency.

Table 5. Design Parameters

8.2.2.2.1 Output Voltage Selection for Adjustable LM3670

The output voltage of the adjustable parts can be programmed through the resistor network connected from V_OUT to V_FB then to GND. V_OUT is adjusted to make V_FB equal to 0.5 V. The resistor from V_FB to GND (R₂) must be at least 100 KΩ to keep the current sunk through this network well below the 15-µA quiescent current level (PFM mode with no switching) but large enough that it is not susceptible to noise. If R₂ is 200 KΩ, and V_FB is 0.5 V, then the current through the resistor feedback network is 2.5 µA (I_FB = 0.5 V / R₂). 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 V_FB (Ω)
• R₂ Resistor from V_OUT to GND (Ω)

For output voltage greater than or equal to 0.7 V a frequency zero must be added at 10 kHz for stability.

Equation 11.

For any output voltages equal to 0.7 V or 2.5 V, a pole must also be placed at 10 kHz (see Table 6).

Table 6. Adjustable LM3670 Configurations for Various V_OUT