where V

EN

is the enable rising threshold voltage and

V

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, V

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 C

INDUCTOR SELECTION (L)

The important parameters for the inductor are the induc-

tance and the current rating. The LM3489 operates over a

wide frequency range and can use a wide range of induc-

tance values. A rule of thumb is to use the equations used for

National’s Simple Switchers

ripple (

∆i) as a function of output current (I

for I

∆i ≤ I

−0.366726

for I

∆i ≤ I

The inductance can be calculated based upon the desired

operating frequency where:

And

where D is the duty cycle, V

and V

The inductor should be rated to the following:

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.

OUTPUT CAPACITOR SELECTION (C

The ESR of the output capacitor times the inductor ripple

current is equal to the output ripple of the regulator. How-

ever, the V

ESR is increased with a given inductance, operating fre-

quency increases as well. If ESR is reduced then the oper-

ating frequency reduces.

The use of ceramic capacitors has become a common de-

sire of many power supply designers. However, ceramic

capacitors have a very low ESR resulting in a 90˚ phase shift

of the output voltage ripple. This results in low operating

frequency and increased output ripple. To fix this problem a

low value resistor should be added in series with the ceramic

output capacitor. Although counter intuitive, this combination

of a ceramic capacitor and external series resistance pro-

vides highly accurate control over the output voltage ripple.

Other types capacitor, such as Sanyo POS CAP and OS-

CON, Panasonic SP CAP, and Nichicon "NA" series, are

also recommended and may be used without additional

series resistance.

For all practical purposes, any type of output capacitor may

be used with proper circuit verification.

20186930

FIGURE 6. Adjustable UVLO

www.national.com

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