3A 32V Synchronous Rectified Step-Down

Converter

EC3286

Page 10 of 14

4J23N-Rev.F001

Function Description

PWM Operation

The EC3286 utilizes DEM control to improve light load efficiency. Depending on the load current,

the controller automatically operates in Diode Emulation Mode (DEM) or in Continuous

Conduction Mode (CCM) with fixed frequency PWM.

At light load condition, the EC3286 automatically operates in diode emulation mode to reduce

switching frequency to improve efficiency. As the output current decreases from heavy load

condition, the

inductor

current

decreases, and eventually the inductor valley current

decreases to zero, which is the boundary between continuous conduction mode and

discontinuous conduction mode. By emulating the behavior of diodes, the low side MOSFET

allows only

partial negative

current

to

flow

when

the

inductor freewheeling current

becomes negative. As the load current further decreases, it takes longer and longer to discharge

the output capacitor to the level that allows the next UGATE on-time to begin. When the output

current increases from light load to heavy load, the switching frequency increases to the

CCM value as the inductor current reaches the continuous conduction condition. The controller

will then operate in continuous conduction mode with 340kHz fixed PWM switching frequency.

Setting the Output Voltage

The output voltage is set using a resistive voltage divider from the output voltage to FB pin.

The voltage divider divides the output voltage down to the feedback voltage by the ratio:

is:

R2 can be as high as 100kΩ, but a typical value is 10kΩ. Using the typical value for R2, R1 is

determined by:

For example, for a 3.3V output voltage, R2 is 10kΩ, and R1 is 26.1kΩ.

Inductor

The inductor is required to supply constant current to the output load while being driven by the

switched input voltage. A larger value inductor will result in less ripple current that will

result in lower output ripple voltage. However ,the larger value inductor will have a larger

physical size, higher series resistance, and/or lower saturation current. A good rule for

determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to

be approximately 30% of the maximum switch current limit. Also, make sure that the peak

inductor current is below the maximum switch current limit. The inductance value can be

calculated by:

is the peak-to-peak inductor ripple current.

Choose an inductor that will not saturate under the maximum inductor peak current. The

peak inductor current can be calculated by:

The choice of which style inductor to use mainly depends on the price vs. size requirements and

any EMI requirements.