High-Performance PWM Controller

EC3228

Page 13 of 16

3G15N-Rev.P002

Application Information

Output Voltage Setting

The output voltage is adjustable from 0.7V to 5.5V with a

resistor-divider connected with FB, GND, and converter’s output. Using

1% or better resistors for the resistor-divider is recommended. The

output voltage is determined by:

Where 0.7 is the reference voltage, RTOP is the resistor connected from

converter’s output to FB, and RGND is the resistor connected from FB

to GND. Suggested RGND is in the range from 1k to 20k. To prevent

stray pickup, locate resistors RTOP and RGND close to EC3228.

Output Inductor Selection

The duty cycle (D) of a buck converter is the function of the input

voltage and output voltage. Once an output voltage is fixed, it can be

written as:

The inductor value (L) determines the inductor ripple current, IRIPPLE,

and affects the load transient response. Higher inductor value reduces

the inductor’s ripple current and induces lower output ripple voltage.

The ripple current and ripple voltage can be approximated by:

Where FSW is the switching frequency of the regulator. Although the

inductor value and frequency are increased and the ripple current and

voltage are reduced, a tradeoff exists between the inductor’s ripple

current and the regulator load transient response time.

A smaller inductor will give the regulator a faster load transient

response at the expense of higher ripple current. Increasing the

switching frequency (FSW) also reduces the ripple current and voltage,

but it will increase the switching loss of the MOSFETs and the power

dissipation of the converter. The maximum ripple current occurs at the

maximum input voltage. A good starting point is to choose the ripple

current to be approximately 30% of the maximum output current. Once

the inductance value has been chosen, selecting an inductor which is

capable of carrying the required peak current without going into

saturation. In some types of inductors, especially core that is made of

ferrite, the ripple current will increase abruptly when it saturates.

This results in a larger output ripple voltage. Besides, the inductor

needs to have low DCR to reduce the loss of efficiency.

Output Capacitor Selection

Output voltage ripple and the transient voltage deviation are factors

which have to be taken into consideration when selecting an output

capacitor. Higher capacitor value and lower ESR reduce the output

ripple and the load transient drop. Therefore, selecting high

performance low ESR capacitors is recommended for switching

regulator applications. In addition to high frequency noise related to

MOSFET turn-on and turnoff, the output voltage ripple includes the

capacitance voltage drop VCOUT and ESR voltage drop VESR caused

by the AC peak-to-peak inductor’s current. These two voltages can be

represented by:

These two components constitute a large portion of the total output

voltage ripple. In some applications, multiple capacitors have to be

paralleled to achieve the desired ESR value. If the output of the

converter has to support another load with high pulsating current,

more capacitors are needed in order to reduce the equivalent ESR and

suppress the voltage ripple to a tolerable level. A small decoupling

capacitor (1F) in parallel for bypassing the noise is also recommended,

and the voltage rating of the output capacitors are also must be

considered. To support a load transient that is faster than the switching

frequency, more capacitors are needed for reducing the voltage

excursion during load step change. Another aspect of the capacitor

selection is that the total AC current going through the capacitors has to

be less than the rated RMS current specified on the capacitors in order

to prevent the capacitor from over-heating.

Input Capacitor Selection

The input capacitor is chosen based on the voltage rating and the RMS

current rating. For reliable operation, selecting the capacitor voltage

rating to be at least 1.3 times higher than the maximum input voltage.

The maximum RMS current rating requirement is approximately

IOUT/2,where IOUT is the load current. During power-up, the input

capacitors have to handle great amount of surge current. For low-duty

notebook applications, ceramic capacitor is recommended. The