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AX2002 Notes

By Ian Mander 8-13 April 2012, updated 14-15 May 2012.

The AX2002 is a reasonably versatile buck driver chip, with output current set by a sense resistor.

**AX2002 specifications**
Detail	Specification
Maker	AXElite.
Datasheet	http://www.axelite.com.tw//Products/AX2002-V1.2-20080729.pdf
Input Voltage	3.6 - 18 V. ~0.5 V overhead voltage.
Suitable Batteries	4x AA NiMH. (Will be flat when it gets to 3.6 V (0.9 V per cell), which is the same as the minimum input voltage of the AX2002.) 2-3x Li-ion cells in series. (To stay in regulation while driving an XM-L at 2 A requires almost 3.7 V. This is a little too high for most Li-ion cells.)
Output Current	2.5 A maximum (may need heat sinking over 2 A). Output current determined by sense resistor with a 0.25 V sense voltage, so a 1 A output current needs a 0.25 Ω resistor.
Efficiency	With 12 V input and output of 2 A @ 5 V it's an impressive 91% efficient. With 12 V input, driving a Cree XR-E at 1 amp will give an efficiency of about 87-88%. Efficiency is very poor at low currents.
Other features	Thermal protection (140°C). Over current protection. Short circuit protection. Chip enable pin (a little hard to get to in available drivers). Can be rewired to provide a constant voltage, for example to power AMC7135 drivers.
Disadvantages	No easy way to get multiple modes, either PWM or continuous. Cannot be paralleled for higher current – one driver ends up doing most of the work. (Using a separate battery for each driver might work.) Cannot easily switch between drivers. Cannot be used with a single Li-ion cell.

Mode Switching

The simplest way to provide modes with an AX2002 is to switch the sense resistor directly. This has theoretical problems, the biggest of which being unknown and varying switch contact resistance. My favourite 3 position switch has contact resistance too small for me to measure directly but I have been able to calculate it by seeing how much difference including a switch makes to the output current: ~0.1 Ω. The resistance may not be particularly stable in the short or long term (especially if switching large DC current), but switch resistance is very unlikely to decrease over time, so the output current should only decrease as the total set resistance increases.

Constant Current Supply with AMC7135

AX2002 drivers can easily be rewired as a constant voltage power supply, which could be used to power a multimode AMC7135 driver.

**XM-L Forward Voltage**
Current	Approx. LED Vf	AMC7135 overhead	Minimum AX2002 Output Voltage Required
1050 mA	2.99 V	0.12 V	3.11 V
2800 mA	3.31 V	0.12 V	3.43 V

So for a 3x AMC7135 driver if we set the output of the AX2002 to 3.25 V it should give enough wiggle room for the AMC7135 driver to run well. This means FB pin to ground is 0.25 V and output to FB is 3.0 V, or 12 times the feedback voltage will be dropped on that resistor. So a pair of resistors with values 10 kΩ and 120 kΩ should do nicely. The LED and AMC7135 driver is wired between the AX2002 output and ground (bypassing the feedback pin completely).

This greatly increases the effective input voltage range of the AMC7135, and may significantly improve its efficiency with a 4x NiMH battery.

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