(in Amps)

2Lfs

0.144 fs

L >

(in H)

)

(

D

D'

-1

LM3224

SNVS277B – DECEMBER 2004 – REVISED SEPTEMBER 2005

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performance can be optimized by choosing values within the range 5k

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, 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:

(6)

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 3 (a) is given by:

(7)

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 3 (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.

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 feedback 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 LM3224, choosing a crossover point well below where the

right half plane zero is located will ensure sufficient phase margin.

After this value is known, you can calculate the crossover visually by placing a

−20dB/decade slope at each pole,

and a +20dB/decade slope for each zero. The point at which the gain plot crosses unity gain, or 0dB, is the

crossover frequency. If the crossover frequency is less than ½ the RHP zero, the phase margin should be high

12

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