LM2696MXAX datasheet(9/19 Pages) NSC

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Application Information (Continued)

OUTPUT CAPACITOR

The output capacitor size and ESR have a direct affect on

the stability of the loop. This is because the adaptive on-time

control scheme works by sensing the output voltage ripple

and switching appropriately. The output voltage ripple on a

buck converter can be approximated by assuming that the

AC inductor ripple current flows entirely into the output ca-

pacitor and the ESR of the capacitor creates the voltage

ripple. This is expressed as:

∆V

OUT

≈∆I

L • RESR

To ensure stability, two constraints need to be met. These

constraints are the voltage ripple at the feedback pin must be

greater than some minimum value and the voltage ripple

must be in phase with the switch pin.

The ripple voltage necessary at the feedback pin may be

estimated using the following relationship:

∆V

FB > −0.057 • fSW +35

Where f

SW is in kHz and

∆V

FB is in mV.

This minimum ripple voltage is necessary in order for the

comparator to initiate switching. The voltage ripple at the

feedback pin must be in-phase with the switch. Because the

ripple due to the capacitor charging and capacitor ESR are

out of phase, the ripple due to capacitor ESR must dominate.

The ripple at the output may be calculated by multiplying the

feedback ripple voltage by the gain seen through the feed-

back resistors. This gain H may be expressed as:

To simplify design and eliminate the need for high ESR

output capacitors, an RC network may be used to feed

forward a signal from the switchpin to the feedback (FB) pin.

See the ‘Ripple Feed Forward’ section for more details.

Typically, the best performance is obtained using POSCAPs,

SP CAPs, tantalum, Niobium Oxide, or similar chemistry

type capacitors. Low ESR ceramic capacitors may be used

in conjunction with the RC feed forward scheme; however,

the feed forward voltage at the feedback pin must be greater

than 30 mV.

RIPPLE FEED FORWARD

An RC network may be used to eliminate the need for high

ESR capacitors. Such a network is connected as shown in

Figure 1.

The value of R

ff should be large in order to prevent any

potential offset in V

OUT. Typically the value of Rff is on the

order of 1 M

Ω and the value of R

FB1 should be less than

10 k

Ω. The large difference in resistor values minimizes

output voltage offset errors in DCM. The value of the capaci-

tor may be selected using the following relationship:

Where the on-time (T

ON_MIN) is in µs, and the resistance

ff)isinM

Ω.

FEEDBACK RESISTORS

The feedback resistors are used to scale the output voltage

to the internal reference value such that the loop can be

regulated. The feedback resistors should not be made arbi-

trarily large as this creates a high impedance node at the

feedback pin that is more susceptible to noise. Typically,

FB2 is on the order of 1 k

Ω. To calculate the value of R

FB1,

one may use the relationship:

Where V

FB is the internal reference voltage that can be

found in the electrical characteristics table (1.254V typical).

The output voltage value can be set in a precise manner by

taking into account the fact that the reference voltage is

regulating the bottom of the output ripple as opposed to the

average value. This relationship is shown in Figure 2.

20153428

FIGURE 1. RC Feed Forward Network

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