LM3477 datasheet(16/23 Pages) NSC

16 / 23 page

Design Section (Continued)

tion). A convenient indicator of how much resonance exists is

quality factor Q. If Q is too high, subharmonic oscillations

could occur, if Q is too low, the current mode architecture

begins to act like a voltage mode architecture and the nec-

essary compensation becomes more complex. This is dis-

cussed in more detail in the COMPENSATION section, but

here it is important to calculate Q to be sure the selected

inductance will not cause problems to the stability of the

converter. The calculations below call for an inductance that

results in Q between 0.15 and 2. See the COMPENSATION

section if the chosen inductance enforces Q to be out of this

range. By default, no extra slope compensation is needed,

so R

SL = 0. In general, a Q between 0.5 and 1 is optimal.

Where,

D’ = 1−D

of the MOSFET when it is conducting

OUT*RDS(ON).

SEN = Voltage across the sense resistor

) I

OUT xRSN

Back solving for L gives a range for acceptable inductances

based on a range for Q:

It is recommended that:

Q(max) = 2, and

Q(min) = 0.15

Values for V

SL can be found in the ELECTRICAL CHARAC-

TERISTICS section.

Note: Adding slope compensation with R

SL will decrease the

current limit. An iterative process may be needed to meet

current limit and stability requirements, see PROGRAM-

MING CURRENT LIMIT/HYSTERETIC THRESHOLD sec-

tion.

Output Capacitor Selection

A capacitance between 47µF - 100µF is typically used. Skip

to ’Calculations for the Output Capacitance’ for minimum

capacitance calculations.

Type of output capacitors

Different type of capacitors often have different combinations

of capacitance, equivalent series resistance (ESR), and volt-

age ratings. High-capacitance multi-layer ceramic capacitors

(MLCCs) have a very low ESR, typically 12m

Ω, but also

relatively low capacitance and low voltage ratings. Tantalum

capacitors can have fairly low ESR, such as 18m

Ω, and high

capacitance (up to 1mF) at higher voltage ratings than ML-

CCs. Aluminum capacitors offer high capacitance and rela-

tively low ESR and are available in high voltage ratings.

OSCON capacitors can achieve ESR values that are even

lower than those of MLCCs and with higher capacitance, but

the voltage ratings are low. Other tradeoffs in capacitor

technology include temperature stability, surge current capa-

bility, and capacitance density (physical size vs. capaci-

tance).

Output Capacitor Considerations

Skip to the ’Calculations for the output capacitor’ subsection

if a quick design is desired. While it is generally desired to

use as little output capacitance as possible to keep costs

down, the output capacitor should be chosen with care as it

directly affects the ripple component of the output voltage as

well as other components in the design. The output voltage

ripple is directly proportional to the ESR of the output capaci-

tor (see POWER INDUCTOR section). Therefore, designs

requiring low output voltage ripple should have an output

capacitor with low ESR. Choosing a capacitor with low ESR

has the additional benefit of requiring one less component in

the compensation network, as discussed in the Compensa-

tion section.

In addition to the output voltage ripple, the output capacitor

directly affects the output voltage overshoot in a load tran-

sient. Two transients are possible: an unloading transient

and a loading transient. An unloading transient occurs when

the load current transitions to a higher current, and charge is

unloaded from the output capacitor. A loading transient is

when the load transitions to a lower current, and charge is

loaded to the output capacitor. How the output voltage reacts

during these transitions is known as the transient response.

Both the capacitance and the ESR of the output capacitor

will affect the transient response.

The control loop of the LM3477/A can be made fast enough

to saturate the duty cycle when the worst case lode transient

occurs. This means the duty cycle jumps to D

MIN or DMAX,

depending on the type of load transient. In a loading tran-

sient, as shown in Figure 9, the duty cycle drops to D

MIN

while the inductor current falls to match the load current.

During this time, the regulator is heavily dependent on the

output capacitors to handle the load transient. The initial

overshoot is caused by the ESR of the output capacitors.

200033B9

FIGURE 9. A Loading Transient

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