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

Rev.1.4 Sep.27, 2019

AX2020



APPLICATION INFORMATION

OPERATION DESCRIPTION

The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a

self oscillating continuous-mode buck converter.

When input voltage VIN is first applied, the initial current in L1 and RS is zero and there

is no output from the current sense circuit. Under this condition, the output of CS comparator

is high. This turns on an internal switch and switches the SW pin low, causing current to flow

from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN

and L1 to produce a voltage ramp (VCSN) across RS. When (VIN-VCSN) > 150mV, the output of

CS comparator switches low and the switch turns off. The current flowing on the RS

decreases at another rate. When (VIN-VCSN) < 110mV, the switch turns on again and the

mean current on the LED is determined by (

2

150

110

+

mV) / RS= 130mV / RS.

The high-side current-sensing scheme and on-board current-setting circuitry minimize

the number of external components while delivering LED current with ±5% accuracy, using a

1% sense resistor.

The AX2020 allow dimming with a PWM signal at the DIM input. A logic level below

0.3V at DIM forces AX2020 to turn off the LED and the logic level at DIM must be at least

2.0V to turn on the full LED current. The frequency of PWM dimming ranges from 100Hz to

more than 20 kHz.

The DIM pin is pulled high current approximately 70μA. It can be floated at normal

working. When a voltage applied to DIM falls below the threshold (0.3V nom.), the output

switch is turned off. The internal regulator and voltage reference remain powered during

shutdown to provide the reference for the shutdown circuit.

Additionally, to ensure the reliability, the AX2020 is built with a thermal shutdown (TSD)

protection and a thermal pad. The TSD protests the IC from over temperature (150°C). Also

the thermal pad enhances power dissipation. As a result, the AX2020 can handle a large

amount of current safely.

Setting nominal average output current with external resistor RS

The nominal average output current in the LED(s) is determined by the value of the

external current sense resistor (RS) connected between VIN and CSN and is given by:

IOUT = 0.130/RS (RS ≧ 0.16Ω)

This equation is valid when DIM pin is float. Actually, RS sets the maximum average

current which can be adjusted to a less one by dimming.