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

Rev.1.1 Nov.01, 2019

AX3704

Enable

When input voltage is above the under voltage lock-out threshold, AX3704 can be enabled by

pulling the EN pin to above 1.21V. AX3704 is disabled if the EN pin is pulled below 1.11V. The

enable/disable threshold for EN pin is accurately designed to be 1.21V and 1.11V respectively, so

one can also use external resistor divider to program the desired input under-voltage lockout level.

Light Load Operation

In light load condition where the converter operates in discontinuous mode, AX3704 cuts down its

quiescent current to as low as 15uA and achieves excellent light load efficiency.

Theory of Operation

AX3704 is a constant on-time control synchronous step-down converter that offers excellent

transient response over a wide range of input voltage. It achieves superior light-load efficiency with

extremely low quiescent current.

Constant On-time Control

Constant on-time control step-down converters turn on HS immediately when FB droops below

reference. The HS is turned on for a pre-determined period (on-time) of time to ramp up the

inductor current, and then the LS will be turned on to ramp down the inductor current. The cycle

repeats itself if FB droops below reference again. AX3704 uses proprietary technique to take into

account the load current impact and adjusts the on-time accordingly to achieve a constant

switching frequency over entire load current range.

For AX3704, the on-time is approximately:

Due to its immediate response on FB voltage droop and simplified loop compensation, constant

on-time offers a superior transient response compare to traditional fixed frequency PWM control

step-down converters.

Soft Start

AX3704 has built-in soft start of 1msec as well as external programmable soft-start. Connect a

capacitor to SS pin to program desired soft-start time. AX3704 provides a constant charging

current of 2uA from the SS pin. External soft-start time can be programmed as:

The actual soft-start time is the longer one of the programmed value and the internal value.

During the soft start period, output voltage is ramped up linearly to the regulation voltage,

independent of the load current level and output capacitor value.

Current Limit and Hiccup Mode

AX3704 has cycle-by-cycle HS current limit protection to prevent inductor current from running

away. Once HS current limit is triggered, AX3704 will turn on LS and wait for the inductor to drop

down to a pre-determined level before the HS can be turned on again. If this current limit condition

is repeated for a sustained long period of time, AX3704 will consider it over-load or short circuit.

Either way, AX3704 will enter hiccup mode, where it stop switching for a pre-determined period of

time before automatically re-try to start up again. It always starts up with soft-start to limit inrush

current and avoid output overshoot.