where R

approximately 1M

Ω. For most applications, performance can

be optimized by choosing values within the range 5k

Ω≤ R

C

≤ 40kΩ (R

Ω if C

Output Capacitor ESR Compensation) and 680pF

≤ C

C

≤

4.7nF. Refer to the Typical Application Circuit and the Appli-

cations Information section for recommended values for spe-

cific circuits and conditions. Refer to the Compensation sec-

tion for other design requirement.

COMPENSATION FOR BOOST DC/DC

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 (Inductor current never reaches

zero), 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 produce stability.

INDUCTOR AND DIODE SELECTION

Although the inductor size mentioned earlier is fine for most

applications, a more exact value can be calculated. To en-

sure 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

R

the graph "R

acteristics section. This equation is only good for duty cycles

recommended values may be used. The corresponding in-

ductor current ripple as shown in Figure 2 (a) is given by:

The inductor ripple current is important for a few reasons.

One reason is because the peak switch current will be the

average inductor current (input current or I

∆i

As a side note, discontinuous operation occurs when the

inductor current falls to zero during a switching cycle, or

∆i

L

is greater than the average inductor current. Therefore, con-

tinuous conduction mode occurs when

∆i

average inductor current. Care must be taken to make sure

that the switch will not reach its current limit during normal

operation. The inductor must also be sized accordingly. It

should have a saturation current rating higher than the peak

inductor current expected. The output and input voltage

ripples are also affected by the total ripple current.

The output diode for a boost regulator must be chosen

correctly depending on the output voltage and the output

current. The typical current waveform for the diode in con-

tinuous conduction mode is shown in Figure 2 (b). The diode

must be rated for a reverse voltage equal to or greater than

the output voltage used. The average current rating must be

greater than the maximum load current expected, and the

peak current rating must be greater than the peak inductor

current. During short circuit testing, or if short circuit condi-

tions are possible in the application, the diode current rating

must exceed the switch current limit. Using Schottky diodes

with lower forward voltage drop will decrease power dissipa-

tion and increase efficiency.

DC GAIN AND OPEN-LOOP GAIN

Since the control stage of the converter forms a complete

feedback loop with the power components, it forms a closed-

loop system that must be stabilized to avoid positive feed-

back and instability. A value for open-loop DC gain will be

required, from which you can calculate, or place, poles and

zeros to determine the crossover frequency and the phase

margin. A high phase margin (greater than 45˚) is desired for

the best stability and transient response. For the purpose of

stabilizing the LM2702, choosing a crossover point well be-

low where the right half plane zero is located will ensure

sufficient phase margin. A discussion of the right half plane

zero and checking the crossover using the DC gain will

follow.

INPUT AND OUTPUT CAPACITOR SELECTION

The switching action of a boost regulator causes a triangular

voltage waveform at the input. A capacitor is required to

reduce the input ripple and noise for proper operation of the

regulator. The size used depends on the application and

board layout. If the regulator will be loaded uniformly, with

very little load changes, and at lower current outputs, the

input capacitor size can often be reduced. The size can also

be reduced if the input of the regulator is very close to the

source output. The size will generally need to be larger for

applications where the regulator is supplying nearly the

maximum rated output or if large load steps are expected. A

minimum value of 10µF should be used for the less stressful

conditions while a 22µF to 47µF capacitor may be required

for higher power and dynamic loads. Larger values and/or

lower ESR may be needed if the application requires very

low ripple on the input source voltage.

The choice of output capacitors is also somewhat arbitrary

and depends on the design requirements for output voltage

ripple. It is recommended that low ESR (Equivalent Series

Resistance, denoted R

ceramic, polymer electrolytic, or low ESR tantalum. Higher

ESR capacitors may be used but will require more compen-

sation which will be explained later on in the section. The

ESR is also important because it determines the peak to

peak output voltage ripple according to the approximate

equation:

∆V

OUT

) 2

∆i

A minimum value of 10µF is recommended and may be

increased to a larger value. After choosing the output capaci-

tor you can determine a pole-zero pair introduced into the

control loop by the following equations:

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