Time

Time

D =

PVIN

SW

GND

FB

PGOOD

EN

SS/TRK

AVIN

COMP

LM20124

L

VCC

PGND

LM20124, LM20124-Q1

www.ti.com

SNVS507F – OCTOBER 2007 – REVISED MARCH 2013

Figure 25. Typical Application Circuit

The first equation to calculate for any buck converter is duty-cycle. Ignoring conduction losses associated with

the FETs and parasitic resistances it can be approximated by:

(2)

INDUCTOR SELECTION (L)

The inductor value is determined based on the operating frequency, load current, ripple current, and duty cycle.

The inductor selected should have a saturation current rating greater than the peak current limit of the device.

Keep in mind the specified current limit does not account for delay of the current limit comparator, therefore the

current limit in the application may be higher than the specified value. To optimize the performance and prevent

the device from entering current limit at maximum load, the inductance is typically selected such that the ripple

current,

inductor ripple current waveforms. Once the input voltage, output voltage, operating frequency, and desired ripple

current are known, the minimum value for the inductor can be calculated by the formula shown below:

(3)

Figure 26. Switch and Inductor Current Waveforms

be kept below the peak current limit of the device. In general, the inductor ripple current,

than 10% of the rated output current to provide adequate current sense information for the current mode control

loop. If the ripple current in the inductor is too low, the control loop will not have sufficient current sense

information and can be prone to instability.

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