Freescale Semiconductor S08 manual Current Sense Resistor, Boost Converter

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