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Step-up DC/DC Converter with Li-Ion

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Newbie level 2
Jan 11, 2003
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I need to drive a LUXEON 5W Led from a baterry source to build an electronic flashlight..
The led needs 6.5-7Volts, at 700mA.
I have some U18650 Li-Ion cells (3.7V 1800mAh), and i need to build
a step-up/step-down??? converter for the current in the output to be constant 700mA, even if the battery discharges.
I cannot find an IC to do this.
I tried a design with LT1302 and with MAX1676, for the 1W Leds (350mA@3.6V) but cannot even reach this current.
If someone out there has experience with DC/DC converter or SMPS
please help me.

It is possible to build an SMPS with an Atmel Tiny part, with A/D and PWM??

I think you should use a high frequency transformer to step up the voltage to get the best result. But I don't have enough experience with that.

Another solution might be using a voltage doubler circuit with non-polar capacitors and fast diodes. The driver circuit should work at about 10-50 Khz??. You should choose the capacitor values according to the charge and frequency response.

Last solution may be using two battery packs in series...


Have a look to TI ref :
UC2577-ADJ : Vin 3 to 40V, Vout 5 to 60V, Iout 3A

**broken link removed**

You have also DC/DC controllers with external switch.



download the free program SwitcherCAD from here
Start the software, put in your needs and you'll get the schematic together with power consumption efficiency etc.

greetings from Germany

Single Cell Lithium-Ion Charge Management Controller with Charge Complete Indicator and Temperature Monitor

The MCP73828 is a linear charge management controller for use in space-limited, cost sensitive applications. The MCP73828 combines high accuracy constant voltage, controlled current regulation, cell preconditioning, cell temperature monitoring, and charge complete indication in a space saving 8-pin MSOP package. The MCP73828 provides a stand-alone charge management solution. The MCP73828 charges the battery in three phases: preconditioning, controlled current, and constant voltage. If the battery voltage is below the internal low-voltage threshold, the battery is preconditioned with a foldback current. The preconditioning phase protects the lithium-ion cell and minimizes heat dissipation. Following the preconditioning phase, the MCP73828 enters the controlled current phase. The MCP73828 allows for design flexibility with a programmable charge current set by an external sense resistor. The charge current is ramped up, based on the cell voltage, from the foldback current to the peak charge current established by the sense resistor. This phase is maintained until the battery reaches the charge-regulation voltage. Then, the MCP73828 enters the final phase, constant voltage. The accuracy of the voltage regulation is better than ±1% over the entire operating temperature range and supply voltage range. The MCP73828-4.1 is preset to a regulation voltage of 4.1V, while the MCP73828- 4.2 is preset to 4.2V. The charge complete output, CD10, indicates when the charge current has diminished to approximately 10% of the peak charge current established by the sense resistor. The MCP73828 operates with an input voltage range from 4.5V to 5.5V. The MCP73828 is fully specified over the ambient temperature range of -20°C to +85°C.

Single Cell Lithium-Ion Battery Chargers

Personal Data Assistants

Cellular Telephones

Hand Held Instruments

Cradle Chargers

Digital Cameras

Linear Charge Management Controller for Single Lithium-Ion Cells

High Accuracy Preset Voltage Regulation: +1% (max)

Two Preset Voltage Regulation Options:
- 4.1V - MCP73828-4.1
- 4.2V - MCP73828-4.2

Programmable Charge Current

Automatic Cell Preconditioning of Deeply Depleted Cells, Minimizing Heat Dissipation During Initial Charge Cycle


Use the lt1618 from linear , it can directly drive the LUXEON led up to 700 mA

I'm using it that way and it's works very good



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