LM3311 datasheet(21/29 Pages) NSC

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Operation (Continued)

INTRODUCTION TO COMPENSATION (BOOST

CONVERTER)

The LM3311 is a current mode PWM boost converter. The

signal flow of this control scheme has two feedback loops,

one that senses switch current and one that senses output

voltage.

To keep a current programmed control converter stable

above duty cycles of 50%, the inductor must meet certain

criteria. The inductor, along with input and output voltage,

will determine the slope of the current through the inductor

(see Figure 5 (a)). If the slope of the inductor current is too

great, the circuit will be unstable above duty cycles of 50%.

A 10µH inductor is recommended for most 660 kHz applica-

tions, while a 4.7µH inductor may be used for most 1.28 MHz

applications. If the duty cycle is approaching the maximum of

85%, it may be necessary to increase the inductance by as

much as 2X. See Inductor and Diode Selection for more

detailed inductor sizing.

The LM3311 provides a compensation pin (V

C) to customize

the voltage loop feedback. It is recommended that a series

combination of R

C and CC be used for the compensation

network, as shown in the typical application circuit. For any

given application, there exists a unique combination of R

and C

C that will optimize the performance of the LM3311

circuit in terms of its transient response. The series combi-

nation of R

C and CC introduces a pole-zero pair according to

the following equations:

where R

O is the output impedance of the error amplifier,

approximately 900k

Ω. For most applications, performance

can be optimized by choosing values within the range 5k

Ω≤

≤ 100kΩ (R

C can be higher values if CC2 is used, see

High Output Capacitor ESR Compensation) and 68pF

≤ C

≤ 4.7nF. Refer to the Applications Information section for

recommended values for specific circuits and conditions.

Refer to the Compensation section for other design require-

ment.

COMPENSATION

This section will present a general design procedure to help

insure a stable and operational circuit. The designs in this

datasheet are optimized for particular requirements. If differ-

ent conversions are required, some of the components may

need to be changed to ensure stability. Below is a set of

general guidelines in designing a stable circuit for continu-

ous conduction operation, in most all cases this will provide

for stability during discontinuous operation as well. The

power components and their effects will be determined first,

then the compensation components will be chosen to pro-

duce stability.

INDUCTOR AND DIODE SELECTION

Although the inductor sizes mentioned earlier are fine for

most applications, a more exact value can be calculated. To

ensure stability at duty cycles above 50%, the inductor must

have some minimum value determined by the minimum

input voltage and the maximum output voltage. This equa-

tion is:

where fs is the switching frequency, D is the duty cycle, and

DSON is the ON resistance of the internal power switch.

This equation is only good for duty cycles greater than 50%

(D>0.5), for duty cycles less than 50% the recommended

values may be used. The value given by this equation is the

inductance necessary to supress sub-harmonic oscillations.

In some cases the value given by this equation may be too

small for a given application. In this case the average induc-

tor current and the inductor current ripple must be consid-

ered.

The corresponding inductor current ripple, average inductor

current, and peak inductor current as shown in Figure 5 (a) is

given by:

Continuous conduction mode occurs when

∆i

L is less than

the average inductor current and discontinuous conduction

mode occurs when

∆i

L is greater than the average inductor

current. Care must be taken to make sure that the switch will

not reach its current limit during normal operation. The in-

ductor must also be sized accordingly. It should have a

saturation current rating higher than the peak inductor cur-

rent expected. The output voltage ripple is also affected by

the total ripple current.

20126305

FIGURE 5. (a) Inductor current. (b) Diode current.

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