Freescale Semiconductor S08 manual Current Sense Resistor, Boost Converter

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Hardware Description

2.11.2 Current Sense Resistor

The value of the current sense resistor RS is determined by two factors: power dissipation on RS and the reference level VREF for operational amplifier non-inverting input. Smaller RS reduces power dissipation, but the detection of a feedback signal in operational amplifier is more difficult.

The voltage VRS across the current sense resistor RS is directly proportional to the current ILED through LED. In closed-loop condition, VRS is equal to the reference level VREF, so the LED current ILED is equal to the reference voltage VREF divided by the current sense resistor RS.

Setting VREF to 1V and RS equals 20Ω, the LED current ILED is equal to 50mA.

Power dissipation on RS is around 50mW, I2R = (50mA)2 × 20Ω, which is reasonable compared to total LED power.

2.11.3 Boost Converter

The switching regulator MC34063 from On Semiconductor is a monolithic circuit containing the primary functions required for DC-to-DC converters. It can be incorporated in boost converter application with minimum number of external components.

Boost Converter Calculations:

Output voltage VOUT > (VLED x 8) + VRS + VDROP (set maximum linear drop to 2 V)

Output current Iout > 50 mA x 3

Set Vin = 12 V, Vout = 30 V, and Iout = 175 mA

Refer to equations in Figure 2-4to calculate the values for inductor and other external components.

LED Lighting Control using the MC9S08AW60, Rev. 1

20	Freescale Semiconductor

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Contents LED Lighting Control Using the MC9S08AW60 Page Designer Reference Manual LED Lighting Control using the MC9S08AW60Revision History Freescale Semiconductor Table of Contents Appendix a Schematics Appendix B Bill of Materials Introduction FeaturesChapter Introduction Devices in the MC9S08AW60/48/32/16 Series System OverviewMC9S08AW60 Peripherals Available per Package Type Introduction LED Lighting Control using the MC9S08AW60, Rev Chapter Hardware Description AW60 Control BoardDEMO9S08AW60LED Features DEMO9S08AW60LED LayoutPower Reset SwitchDevelopment Support Clock SourcePushbutton Switches LED IndicatorsUser Options ADC InterfaceOther I/O Connectors IIC PortSCI Port SPI Port10. Sensor Interface Type a PWM PortLED Light Box Interface 13. User Assignable Input 12. Temperature Sensor Input11. Sensor Interface Type H LED Driving Board LED Driver Design Procedures RGB LED ChipCurrent Sense Resistor Boost ConverterEquations for Boost Converter Freescale Semiconductor Chapter Firmware Description Firmware Flow Main ProgramPC Control Mode Firmware Flow PWM AdjustmentUser Interface Menu Standalone Mode Firmware Files Freescale Semiconductor Chapter Demo Setup Hardware and Software SetupHardware Setup PC Software SetupEcho Typed Characters Setting Demo Examples Demo 1 Demonstration DisplayDemo 2 Preset Colors Display Demo 3 Auto White Balance ControlDemo 4 PWM Output Frequency Control Demo 5 Full Manual ControlTroubleshooting Program the MCU FlashDemo Setup Appendix a Schematics LED Lighting Control using the MC9S08AW60, Rev LED Lighting Control using the MC9S08AW60, Rev LED Lighting Control using the MC9S08AW60, Rev LED Lighting Control using the MC9S08AW60, Rev LED Lighting Control using the MC9S08AW60, Rev Appendix B Bill of Materials BOM for AW60 Control Board Part Description Quantity Value DesignatorsBOM for LED Driving Board Value DesignatorsLED Lighting Control using the MC9S08AW60, Rev Page How to Reach Us