The LM2700 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 2 (a)). If the slope of the inductor current is too

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

A 4.7µH inductor is recommended for most 600 kHz appli-

cations, while a 2.2µH inductor may be used for most 1.25

MHz applications. If the duty cycle is approaching the maxi-

mum 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 LM2700 provides a compensation pin (V

the voltage loop feedback. It is recommended that a series

combination of R

network, as shown in

Figure 3. For any given application,

there exists a unique combination of R

optimize the performance of the LM2700 circuit in terms of

its transient response. The series combination of R

introduces a pole-zero pair according to the following equa-

tions:

where R

approximately 850k

Ω. For most applications, performance

can be optimized by choosing values within the range 5k

Ω≤

R

Output Capacitor ESR Compensation) and 680pF

≤ C

4.7nF. Refer to the

Applications Information section for rec-

ommended values for specific circuits and conditions. Refer

to the

Compensation section for other design requirement.

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 (loads greater than approximately

100mA), 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 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

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

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 voltage ripple is 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 LM2700, 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 is dependant on the application and

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

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