+

G

=

2

1

2

1

2S

=

4 x 250 kHz x 7 PF

7A

= 1V

1

8 x 250 kHz x 320

0.4 m:

2

PF

1

ESR

€

2

LM5116

www.ti.com

SNVS499G – FEBRUARY 2007 – REVISED MARCH 2013

(15)

With typical values for the 5V design example:

(16)

INPUT CAPACITORS

The regulator supply voltage has a large source impedance at the switching frequency. Good quality input

capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current

during the on-time. When the buck switch turns on, the current into the switch steps to the valley of the inductor

current waveform, ramps up to the peak value, and then drops to zero at turn-off. The input capacitors should be

selected for RMS current rating and minimum ripple voltage. A good approximation for the required ripple current

Quality ceramic capacitors with a low ESR were selected for the input filter. To allow for capacitor tolerances and

voltage rating, four 2.2 µF, 100V ceramic capacitors were used for the typical application circuit. With ceramic

capacitors, the input ripple voltage will be triangular and peak at 50% duty cycle. Taking into account the

capacitance change with DC bias, the input ripple voltage is approximated as:

(17)

When the converter is connected to an input power source, a resonant circuit is formed by the line impedance

and the input capacitors. If step input voltage transients are expected near the maximum rating of the LM5116, a

careful evaluation of the ringing and possible overshoot at the device VIN pin should be completed. To minimize

(18)

The converter exhibits a negative input impedance which is lowest at the minimum input voltage:

(19)

The damping factor for the input filter is given by:

where

•

•

ESR is the series resistance of the input capacitors

(20)

When

δ = 1, the input filter is critically damped. This may be difficult to achieve with practical component values.

With

δ < 0.2, the input filter will exhibit significant ringing. If δ is zero or negative, there is not enough resistance

in the circuit and the input filter will sustain an oscillation. When operating near the minimum input voltage, an

parallel capacitor should be evaluated for its RMS current rating. The current will split between the ceramic and

aluminum capacitors based on the relative impedance at the switching frequency.

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