CPC9909 Clare, Inc., CPC9909 Datasheet - Page 7

• Manufacturer Part Number: CPC9909
• Description: High Efficiency, Off-line, High Brightness Led Driver
• Manufacturer: Clare, Inc.

The switching frequency is determined by:
Where:
•
•
In general, switching frequency selection is based on
the inductor size, controller power dissipation, and the
input filter capacitor.
The typical off-line LED driver switching frequency, f
is between 30 kHz and 120 kHz.
This operating range gives the designer a reasonable
compromise between switching losses and inductor
size. The internal off-time one-shot has an accuracy of
±20%. The figure below shows the R
selection for the desired off-time.
2.8 Inductor Design
The inductor value is defined by the LED/inductor
ripple current, minimum off time, and the output
voltage. The minimum off time is determined by the
duty cycle and switching frequency. The duty cycle is
given by:
Where:
•
•
R01
D = duty cycle
t
V
average LED current.
V
off
LEDstring
in(min)
45
40
35
30
25
20
15
10
5
0
D
F
= Off-time
0
S
=
=
V
------------------------- -
is the minimum DC input voltage.
1 D
------------ -
V
LEDstring
t
–
is the LED string voltage at the desired
off
in min
500
(
)
1000
R
T
vs Off-Time
R
1500
T
(KΩ)
t
off
2000
(μS)
T
resistor
2500
3000
www.clare.com
S
,
The minimum inductor value for a given ripple current
is:
Where:
•
The inductor peak current is given by:
2.9 Gate Output Drive
The CPC9909 uses an internal gate drive circuit to
turn on and off an external power MOSFET. The gate
driver can drive a variety of MOSFETs. For a typical
off-line application, the total MOSFET gate charge will
be less than 25nC.
ΔI
L
I
L
LPeak
= Ripple Current
MIN
=
=
I
V
------------------------- -
LED
LEDstring
ΔI
+
L
0.5ΔI
L
×
t
off
CPC9909
7