CPC9909 Datasheet, PDF (8/12 Pages) IXYS Corporation – High Effciency,Off-line, High Brightness LED Driver

INTEGRATED CIRCUITS DIVISION
The switching frequency is determined by:
F_S = ^1---_t--–_o---_f-^D-_f--
Where:
• D = duty cycle
• t_off = Off-time
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_S,
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_T resistor
selection for the desired off-time.
CPC9909
The minimum inductor value for a given ripple current
is:
L_MIN = ^V----^L---^E--_--^D--_I-^s--_Lt--^r--ⁱ--ⁿ---^g  t_off
Where:
• I_L = Ripple Current
The inductor peak current is given by:
I_LPeak = I_LED + 0.5I_L
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.
R_T vs Off-Time
45
40
35
30
25
20
15
t_off (μS)
10
5
0
0
500
1000
1500
2000
2500
3000
R
T
(KΩ)
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:
D = ^V----_V^L---^E-_i-_n--^D-_--_ms---^t-_ir-_n-ⁱ--ⁿ_---^g
Where:
• V_LEDstring is the LED string voltage at the desired
average LED current.
• V_in(min) is the minimum DC input voltage.
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