cat3649 ON Semiconductor, cat3649 Datasheet - Page 11

• Manufacturer Part Number: cat3649
• Description: Cat3649 6-channel Quad-mode Led Driver With 32 Dimming Levels & Pwm
• Manufacturer: ON Semiconductor

C
proportionally to its duty cycle. When the LED driver
operates in PWM dimming mode, the C
minimizes the LED current ripple. This prevents audio noise
from the LED driver output capacitors as the PWM signal is
converted into a near DC current internally. The PWM input
is a logic input and the amplitude of the PWM signal does
not affect the LED current. An internal 4 mA current source
is charging the C
until it reaches a maximum voltage; see Figure 29 block
diagram. The internal resistor R (150 kW) and external
capacitor C
frequency f
frequency f
cut−off frequency f
PWM
The PWM input signal controls the LED current
To minimize the ripple current, we recommend the PWM
Filtering Capacitor
C
PWM
PWM
= 1 / 2π R C
PWM
to be at least 40 times greater than the
act as a low pass filter with a cut−off
C
:
capacitor when the PWM input is high
PWM
Figure 28. ADIM Dimming Timing Diagram (no C
PWM
.
Buffer
Figure 29. PWM Circuit Block Diagram
150 kW
PWM
N1
R
CPWM
capacitor
http://onsemi.com
4 mA
V
C
47 nF
11
controlled
Voltage−
current
For example for f
any PWM frequency between 1 kHz and 200 kHz. For PWM
frequency below 1 kHz, the above formula will provide the
recommended capacitor value.
the time to reach the nominal LED current. The power−up
time (t
can be calculated as follows.
For example, for C
source
G1
We recommend a 47 nF capacitor C
The C
C
PU
f
C
t
PWM
PWM
PU
PWM
) is proportional to the C
PWM
^ C
GND
w
w 40
I = g x V
I = LED
current
reference
(for LED at
max current,
g = 0.045)
PWM
capacitor affects the power−up time which is
w
(2p
PWM
(2p R f
PWM
, PWM high)
PWM
C
f
40
150
C
= 1 kHz, the capacitor value is:
3
PWM
or
= 47 nF, t
40
10
)
10
5
3
PWM
PU
10
is about 15 ms.
3
PWM
)
capacitor value and
+ 42 nF
compatible for
(eq. 2)
(eq. 3)
(eq. 4)
(eq. 5)