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 (L1)

The important parameters for the inductor are the induc-

tance and the current rating. The LM3485 operates over a

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

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

used for National’s Simple Switchers

inductor 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 and V

voltage.

The inductor should be rated to the following:

Ipk = (Iout+

The inductance value and the resulting ripple is one of the

key parameters controlling operating frequency. The second

is the ESR.

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, then operating

frequency increases as well. If ESR is reduced then the

operating 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 provide

highly accurate control over the output voltage ripple. The

other types capacitor, such as Sanyo POS CAP and

OS-CON, Panasonic SP CAP, 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.

Input Capacitor Selection (C

A bypass capacitor is required between the input source and

ground. It must be located near the source pin of the external

PFET. The input capacitor prevents large voltage transients

at the input and provides the instantaneous current when the

PFET turns on.

The important parameters for the input capacitor are the

voltage rating and the RMS current rating. Follow the manu-

facturer’s recommended voltage derating. For high input

voltage application, low ESR electrolytic capacitor, the Nichi-

con ’UD’ series or the Panasonic ’FK’ series, is available.

The RMS current in the input capacitor can be calculated.

The input capacitor power dissipation can be calculated as

follows.

P

CIN

The input capacitor must be able to handle the RMS current

and the P

parallel to handle large RMS currents. In some cases it may

be much cheaper to use multiple electrolytic capacitors than

a single low ESR, high performance capacitor such as

OS-CON or Tantalum. The capacitance value should be

selected such that the ripple voltage created by the charge

and discharge of the capacitance is less than 10% of the

total ripple across the capacitor.

Programming the Current Limit (R

The current limit is determined by connecting a resistor

(R

R

where:

R

I

I

Catch Diode Selection (D1)

The important parameters for the catch diode are the peak

current, the peak reverse voltage, and the average power

dissipation. The average current through the diode can be

calculated as following.

I

The off state voltage across the catch diode is approximately

equal to the input voltage. The peak reverse voltage rating

must be greater than input voltage. In nearly all cases a

shottky diode is recommended. In low output voltage appli-

cations a low forward voltage provides improved efficiency.

For high temperature applications, diode leakage current

may become significant and require a higher reverse voltage

rating to achieve acceptable performance.

www.national.com

12