LM3224 datasheet(11/26 Pages) TI1

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2S(RC + RO)CC

fPC =

fZC =

2SRCCC

t (s)

IL_AVG

ID_AVG

=IOUT_AVG

D*Ts

IL (A)

ID (A)

OUT

(a)

(b)

LM3224

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SNVS277B – DECEMBER 2004 – REVISED SEPTEMBER 2005

INTRODUCTION TO COMPENSATION

Figure 3. (a) Inductor current. (b) Diode current.

The LM3224 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 3 (a)). If the slope of the inductor current is too great, the circuit will be unstable above

duty cycles of 50%. A 10µH to 15µH inductor is recommended for most 615 kHz applications, while a 4.7µH to

10µH inductor may be used for most 1.25 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 LM3224 provides a compensation pin (VC) to customize the voltage loop feedback. It is recommended that a

series combination of RC 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 RC and CC that will optimize the

performance of the LM3224 circuit in terms of its transient response. The series combination of RC and CC

introduces a pole-zero pair according to the following equations:

(4)

(5)

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LM3224 Datasheet(PDF) 11 Page - Texas Instruments