LM3224 datasheet(12/27 Pages) TI

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'iL =

(in Amps)

VIND

2Lfs

VINRDSON

0.144 fs

L >

(in H)

)

(

-1

2S(RC + RO)CC

fPC =

fZC =

2SRCCC

LM3224

SNVS277C – DECEMBER 2004 – REVISED MARCH 2013

www.ti.com

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 22 (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)

where

•

RO is the output impedance of the error amplifier (approximately 900kΩ)

(5)

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

Ω ≤ RC ≤ 100kΩ (RC

can be up to 200k

Ω if CC2 is used, see High Output Capacitor ESR Compensation) and 680pF ≤ CC ≤ 10nF.

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, 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 cycl

•

RDSON is the ON resistance of the internal switch taken from the graph "NMOS RDSON vs. Input Voltage" in the

Typical Performance Characteristics section.

(6)

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 22 (a) is given

by:

(7)

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