LM3489 datasheet(13/23 Pages) TI1

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Ipk = IOUT +

x 1.1

Ipk = IOUT +

x 1.1

VIN - VDS - VOUT

L =

ISENSE

ADJ

VIN

PGATE

LM3489

GND

PGND

VIN

VEN

VIN(UVLO_HYST) = VEN_HYST x 1+

LM3489

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SNVS443A – MAY 2006 – REVISED JUNE 2008

(8)

where VEN is the enable rising threshold voltage and VEN_HYST is the enable threshold hysteresis.

Figure 7. Adjustable UVLO

Design Information

Hysteretic control is a simple control scheme. However the operating frequency and other performance

characteristics highly depend on external conditions and components. If either the inductance, output

capacitance, ESR, VIN, or Cff is changed, there will be a change in the operating frequency and output ripple.

The best approach is to determine what operating frequency is desirable in the application and then begin with

the selection of the inductor and COUT ESR.

INDUCTOR SELECTION (L)

The important parameters for the inductor are the inductance and the current rating. The LM3489 operates over

a wide frequency range and can use a wide range of inductance values. A rule of thumb is to use the equations

used for National's Simple Switchers®. The equation for inductor ripple (

Δi) as a function of output current (IOUT)

is:

for Iout < 2.0Amps

Δi ≤ Iout x Iout

−0.366726

for Iout > 2.0Amps

Δi ≤ Iout x 0.3

The inductance can be calculated based upon the desired operating frequency where:

(9)

And

(10)

where D is the duty cycle, VD is the diode forward voltage, and VDS is the voltgae drop across the PFET.

The inductor should be rated to the following:

(11)

The inductance value and the resulting ripple is one of the key parameters controlling operating frequency. The

second is the inductor ESR that contribute to the steady state power loss due to current flowing through the

inductor.

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