18V/2A High Efficiency Synchronous Rectified

Step-Down DC/DC Converter

EC3292B

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5B16N-Rev.F001

Detailed Description

The part is a current mode PWM synchronous step-down converter with a constant switching frequency. It regulates the

input voltage from 4.5V to 18V and a low output voltage of 0.6V.The supplied load current is up to 2A.

Power Saving Mode

The switching losses resulted from the Miller capacitance of the MOSFET are the dominant power dissipation parameters at

light load. The power saving mode at light load can minimize the switching loss by reducing the switching frequency.

Therefore, the part is designed as the power saving mode for high efficiency at light load.

Oscillator Frequency

Slope compensated current mode PWM control provides not only stable switching and cycle-by-cycle current limit for

superior load and line response but also protection of the internal main switch and synchronous rectifier. The part switches

at a constant frequency (500KHz) and regulates the output voltage. The PWM comparator modulates the power transferred

to the load by changing the inductor’s peak current based on the feedback error voltage during each cycle. The main switch

is turned on for a certain period to ramp the inductor’s current at each rising edge of the internal oscillator under normal

operation whereas off when the inductor’s peak current is above the error voltage. After the main switch is turned off, the

low side MOS will be turned on immediately and stay on until the next cycle starts.

Short Circuit Protection

The part provides short circuit protection. When the output is shorted to ground, the oscillator’s frequency is reduced to

prevent the inductor’s current from increasing beyond the NMOS current limit. The NMOS current limit is also reduced to

lower the short circuit current. The frequency and current limit will return to the normal values once the short circuit

condition is removed and the feedback voltage reaches 0.6V.

Maximum Load current

The part can operate down to 4.5V input voltage; however the maximum load current decreases at lower input due to large

IR voltage drop on the main switch and low side switch. The slope compensation signal reduces the inductor’s peak current

as a function of the duty cycle to prevent sub-harmonic oscillations at duty cycles greater than 50%.

Enable

The EN pin provides electrical on/off control of the regulator. Once the voltage of the EN pin exceeds the threshold voltage,

the regulator starts operation and the internal slow start begins to ramp. If the voltage of the EN pin is pulled below the

threshold, the regulator will stop switching and the internal slow start reset. If the EN pin is open, it will be pulled to high by

the internal circuit.