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Axelite Confidential Materials, do not copy or distribute without written consent.

Rev.1.1 Nov.05, 2019

AX3984

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 23V and a low output voltage of

0.925V.The supplied load current is up to 3A.

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 (FSW) 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 VFB.

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%.