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18

LM60440, LM60430

SNVSBN5A – FEBRUARY 2020 – REVISED JUNE 2020

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Product Folder Links: LM60440 LM60430

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Copyright © 2020, Texas Instruments Incorporated

9.2.2.2 Choosing the Switching Frequency

The choice of switching frequency is a compromise between conversion efficiency and overall solution size.

Lower switching frequency implies reduced switching losses and usually results in higher system efficiency.

However, higher switching frequency allows the use of smaller inductors and output capacitors, and hence a

more compact design. For this example, the LM604x0 fixed 400-kHz switching frequency was chosen.

9.2.2.3 Setting the Output Voltage

The output voltage of the LM604x0 is externally adjustable using a resistor divider network. The range of

recommended output voltage is found in the Recommended Operating Conditions table. The divider network is

The resistance of the divider is a compromise between excessive noise pick-up and excessive loading of the

output. Smaller values of resistance reduce noise sensitivity but also reduce the light-load efficiency. The

feedforward capacitor must be used across this resistor to provide adequate loop phase margin (see the CFF

Electrical Characteristics for limits).

(3)

9.2.2.4 Inductor Selection

The parameters for selecting the inductor are the inductance and saturation current. The inductance is based on

the desired peak-to-peak ripple current and is normally chosen to be in the range of 20% to 40% of the maximum

output current. Experience shows that the best value for inductor ripple current is 30% of the maximum load

current. Note that when selecting the ripple current for applications with much smaller maximum load than the

maximum available from the device, the maximum device current should be used. Equation 4 can be used to

determine the value of inductance. The constant K is the percentage of inductor current ripple. For this example,

K = 0.3 was chosen and an inductance was found; the next standard value of 6.8 µH was selected.

(4)

Ideally, the saturation current rating of the inductor must be at least as large as the high-side switch current limit,

circuit on the output. When the inductor core material saturates, the inductance falls to a very low value, causing

current run-away, a saturated inductor can cause the current to rise to high values very rapidly. This can lead to

component damage; do not allow the inductor to saturate. Inductors with a ferrite core material have very hard

saturation characteristics, but usually have lower core losses than powdered iron cores. Powered iron cores

exhibit a soft saturation, allowing for some relaxation in the current rating of the inductor. However, they have

more core losses at frequencies typically above 1 MHz. In any case, the inductor saturation current must not be

limited by the minimum current ripple required for the current mode control to perform correctly. As a rule-of-

thumb, the minimum inductor ripple current must be no less than about 10% of the device maximum rated

current under nominal conditions.

9.2.2.5 Output Capacitor Selection

The value of the output capacitor and the ESR of the capacitor determine the output voltage ripple and load

transient performance. The output capacitor bank is usually limited by the load transient requirements, rather

than the output voltage ripple. Equation 5 can be used to estimate a lower bound on the total output capacitance

and an upper bound on the ESR, which is required to meet a specified load transient.