User's Manual

Hardware Description
LED Lighting Control using the MC9S08AW60, Rev. 1
20 Freescale Semiconductor
2.11.2 Current Sense Resistor
The value of the current sense resistor R
S
is determined by two factors: power dissipation on R
S
and the
reference level V
REF
for operational amplifier non-inverting input. Smaller R
S
reduces power dissipation,
but the detection of a feedback signal in operational amplifier is more difficult.
The voltage V
RS
across the current sense resistor R
S
is directly proportional to the current I
LED
through
LED. In closed-loop condition, V
RS
is equal to the reference level V
REF
, so the LED current I
LED
is equal
to the reference voltage V
REF
divided by the current sense resistor R
S
.
Setting V
REF
to 1V and R
S
equals 20Ω, the LED current I
LED
is equal to 50mA.
Power dissipation on R
S
is around 50mW, I
2
R = (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 V
OUT
> (V
LED
x 8) + V
RS
+ V
DROP
(set maximum linear drop to 2 V)
Output current Iout > 50 mA x 3
Set V
in
= 12 V, V
out
= 30 V, and I
out
= 175 mA
Refer to equations in Figure 2-4 to calculate the values for inductor and other external components.