(in Amps)

2Lfs

0.144 fs

L >

(

)

D

D'

2

-1

(

)

D

D'

+1

(in H)

LM2700Q

www.ti.com

SNVS794A – MARCH 2012 – REVISED MARCH 2013

•

(4)

For most applications, performance can be optimized by choosing values within the range 5k

can be up to 200k

Refer to the Applications Information section for recommended 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 different 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 continuous 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 equation is:

where

•

fs is the switching frequency

•

D is the duty cycle

•

(5)

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 corresponding inductor current ripple as shown in Figure 17 (a) is given

by:

(6)

The inductor ripple current is important for a few reasons. One reason is because the peak switch current will be

when the inductor current falls to zero during a switching cycle, or

current. Therefore, continuous conduction mode occurs when

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 continuous conduction mode is shown in Figure 17 (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 conditions 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 dissipation and increase efficiency.

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