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LMR14003, LMR14006

www.ti.com

SNVSA10 – NOVEMBER 2013

Output Capacitor

modulator pole, the output voltage ripple, and how the regulator responds to a large change in load current. The

output capacitance needs to be selected based on the most stringent of these three criteria.

The desired response to a large change in the load current is the first criteria. The regulator usually needs two or

more clock cycles for the control loop to see the change in load current and output voltage and adjust the duty

cycle to react to the change. The output capacitance must be large enough to supply the difference in current for

2 clock cycles while only allowing a tolerable amount of droop in the output voltage. Equation 5 shows the

minimum output capacitance necessary to accomplish this. For this example, the transient load response is

0.03 = 0.57A and

ceramic capacitors, the ESR is usually small enough to ignore in this calculation. Aluminum electrolytic and

tantalum capacitors have higher ESR that should be taken into account.

The stored energy in the inductor will produce an output voltage overshoot when the load current rapidly

decreases. The output capacitor must also be sized to absorb energy stored in the inductor when transitioning

from a high load current to a lower load current. Equation 6 is used to calculate the minimum capacitance to

capacitor voltage. For this example, the worst case load step will be from 0.6A to 0.03A. The output voltage will

increase during this load transition and the stated maximum in our specification is 3% of the output voltage. This

of 5V. Using these numbers in Equation 6 yields a minimum capacitance of 2.36µF.

Equation 7 calculates the minimum output capacitance needed to meet the output voltage ripple specification.

the inductor ripple current. Equation 7 yields 0.21µF.

Equation 8 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple

specification. Equation 8 indicates the ESR should be less than 277m

Ω.

Additional capacitance de-ratings for aging, temperature and dc bias should be factored in which will increase

this minimum value. For this example, 10µF ceramic capacitors will be used. Capacitors in the range of 4.7µF-

100µF are a good starting point with an ESR of 0.1

Ω or less.

(5)

(6)

(7)

(8)

Schottky Diode

The breakdown voltage rating of the diode is preferred to be 25% higher than the maximum input voltage. The

current rating for the diode should be equal to the maximum output current for best reliability in most

applications. In cases where the input voltage is much greater than the output voltage the average diode current

however the peak current rating should be higher than the maximum load current. A 0.5A to 1A rated diode is a

good starting point.

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