LM5166DRCRG4 datasheet(39/57 Pages) TI2

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LM5166

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SNVSA67B – DECEMBER 2016 – REVISED JUNE 2017

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8.2.4.2.3

Switching Frequency – LF

Tie RT to GND to select PFM mode of operation. The inductor, input voltage, output voltage and peak current

determine the pulse switching frequency of a PFM-configured LM5166. For a given input voltage, output voltage

and peak current, the inductance of LF sets the switching frequency when the output is in regulation. Use

Equation 28 to select an inductance of 22 μH based on the target PFM converter switching frequency of 100 kHz

at 12-V input.

IPK(PFM) in this example is the peak current limit setting of 1.25 A plus the inductor current overshoot resulting

from the 80-ns peak current comparator delay. An additional constraint on the inductance is the 180-ns minimum

on-time of the high-side MOSFET. Therefore, to keep the inductor current well controlled, choose an inductance

that is larger than LF(min) using Equation 29.

Choose an inductor with saturation current rating above the peak current limit setting, and allow for derating of

the saturation current at the highest expected operating temperature.

8.2.4.2.4

Output Capacitors – COUT

The output capacitor, COUT, filters the ripple current of the inductor and stores energy to meet the load current

requirement when the LM5166 is in sleep mode. The output ripple has a base component of amplitude VOUT/123

related to the typical feedback comparator hysteresis in PFM. The wake-up time from sleep to active mode adds

a ripple voltage component that is a function of the output current. Approximate the total output ripple by

Equation 30.

Also, the output capacitance must be large enough to accept the energy stored in the inductor without a large

deviation in output voltage. Setting this voltage change equal to 1% of the output voltage results in a COUT

requirement defined with Equation 31.

In general, select the capacitance of COUT to limit the output voltage ripple at full load current, ensuring that it is

rated for worst-case RMS ripple current given by IRMS = IPK(PFM)/2. In this design example, select two 100-μF, 10-

V capacitors with a high-quality dielectric.

8.2.4.2.5

Input Capacitor – CIN

The input capacitor, CIN, filters the triangular current waveform of the high-side MOSFET (see Figure 89). To

prevent large ripple voltage, use a low-ESR ceramic input capacitor sized for the worst-case RMS ripple current

given by IRMS = IOUT/2. In this design example, choose a 4.7-μF, 50-V ceramic input capacitor with a high-quality

dielectric.

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