t6309a TM Technology Inc., t6309a Datasheet - Page 8

• Manufacturer Part Number: t6309a
• Description: Micropower Step-up Dc/dc Converter
• Manufacturer: TM Technology Inc.

tm
Applications Information
Dimming Control
There are several types of dimming control
circuit as follows:
1. Using a PWM signal to CE pin
When using a PWM signal at CE pin, the
T6309A is turned on or off by the PWM
signal. The average LED current increases
proportionally with the duty cycle of the
PWM signal. The typical frequency range of
the PWM signal is 150Hz to 250Hz. The
magnitude of the PWM signal should be
greater than the threshold voltage of CE
voltage high.
2. Using a DC voltage
control the LED brightness. The dimming
control method is shown in the figure below.
When the DC voltage is zero, the LED is fully
turned on. As the DC voltage increases, the
voltage drop on R2 increases and the voltage
drop on R1 decreases. Thus the LED current
decreases. The selection of R1 and R2 should
make the current from the variable DC source
much smaller than the LED current and much
larger than the FB pin bias current.
For V
resistors in this figure gives dimming control
of LED current from 0mA to 15mA.
Following steps are used to select the resistor
values:
• Select the voltage V
• Select 0V as default to turn the LEDs fully
TM Technology Inc. reserves the right
to change products or specifications without notice.
A DC voltage signal can be used as well to
off. (e.g. 2.0V)
on.
DC
range from 0V to 2V, the selection of
CH
TE
MAX
to turn the LEDs
P. 8
• Select the maximum LED current I
• Calculate R2 to achieve a feedback current
• Calculate R1 to meet the equation:
(e.g. V
the calculated R1 is 25KOhm)
• Calculate the sense voltage Vs at maximum
• Calculate the sense resistor Rs:
3. Using a Filtered PWM signal
The filtered PWM signal can be considered as
an adjustable DC voltage. It can be used to
replace the DC voltage source if adjustable
analog signal is not available in the system.
The circuit is shown in the figure below:
and 0mA as the minimum LED
current.
R2 = V
PWM
Signal
in the range of I1 = 3uA to 10uA as the
LEDs are fully turned on:
LED current:
R1 / (R1+R2) = V
(e.g. Vs = 0.4V x (1 + 25K/100K) = 0.5V)
(e.g. 0.5V/20mA = 25 Ohm)
Vs = V
Rs = Vs / I
REF
REF
=0.4V, V
REF
10K
0.1nF
/ I1 (e.g. 0.4V/4uA =100KOhm )
x (1 + R1/R2)
MAX
Publication Date: MAY. 2007
MAX
100K
REF
R2
T6309A
=2.0V, R2=100KOhm,
/ V
FB
MAX
25K
R1
T6309A
Revision: A
MAX
25 Ohm
R
S