Peak current definition
The peak current rating is a design parameter whose
value is dependent upon the overall application. For
the high brightness reference designs, a peak current
of 190mA was chosen to ensure that the converter
could provide the required output power to the LED.
In general, the I
PK
value must be chosen to ensure that
the switching transistor, Q1, is in full saturation with
maximum output power conditions, assuming
worse-case input voltage and transistor gain under all
operating temperature extremes.
Once I
PK
is decided the value of R
SENSE
can be
determined by:
A selection guide of sense resistor and inductor values
for given input voltages, output currents and number
of LED connected in series is provided in the table
below.
ZXSC310
ISSUE 3 - SEPTEMBER 2007
10
R
V
I
SENSE
ISENSE
PK
=
Input Voltage
(V)
LED current
(mA)
No. of LED’s R
SENSE
(m )
Inductor
( H)
Efficiency
(%)
3.3V 10 3 510 68 80
3.3V 10 4 330 68 81
3.3V 10 6 150 68 79
3.3V 20 3 220 68 84
3.3V 20 4 150 68 93
3.3V 20 6 77 68 79
3.3V 30 3 170 68 84
3.3V 30 4 100 68 84
3.3V 30 6 47 68 77
5V 10 3 750 68 83
5V 10 4 510 68 84
5V 10 6 330 68 79
5V 20 3 440 68 85
5V 20 4 250 68 85
5V 20 6 150 68 82
5V 30 3 330 68 86
5V 30 4 170 68 85
5V 30 6 100 68 83
