CM9330 Datasheet, PDF (8/10 Pages) California Micro Devices Corp – Asymmetrical High Efficiency Three Channel Boost LED Driver

PRELIMINARY
CM9330
Application Information (cont’d)
softstart, the CM9330's internal NMOS drives an exter-
nal inductor and Schottky diode delivers the inductor's
stored energy to the load.
Setting the LED Current
The output current is set by the value of the RSET con-
nected between the ISET pin and GND, according to
the equations:
(a) Interleave
R_SET(kΩ)
=
---------4---5---0-----------
I_LED(mA)
(b) Non-interleave
R_SET(kΩ)
=
---------7---3---0-----------
I_LED(mA)
PWM Brightness Control
The brightness WLEDs level can be continuously con-
trolled for each channel using a PWM signal in 1-50
KHz range (recommended value is 10 kHz). As an
example the PWM signal can be applied directly
through R_SET resistor for negative slope or by using a
switch transistor for positive slope. See Figure 1 for dif-
ferent brightness control methods and results.
PWM
signal
R_SET1
22k
R_SET2
36k
R_SET3
91k
ISET1
ISET2
ISET3
CM9330
(a) Schematic
25
20
ILED1
negative slope
ILED2
15
10
ILED3
5 _{positive slope}
0
0 10 20 30 40 50 60 70 80 90 100
DUTY (%)
(b) Brightness Curves
Figure 1. Brightness Control Using
Different Methods
Inductor Selection
The inductor is used to store energy in a boost con-
verter. The amount of energy stored in the inductor and
transferred to the load is controlled by the PWM. The
inductor is operated in the discontinuous conduction
mode, and to assume proper operation, the inductor
value must be limited to a maximum value.
An inductor with low series resistance (DCR)
decreases power losses and increases efficiency. The
core material should be capable of operating at I MHz
with minimal core losses. An inductance of 4.7-µH is
optimum for most applications, but low DCR inductor
values in 1.5-15uH range are also recommended for
high efficiency applications.
To ensure proper operation of the current regulator
over a wide range of conditions, the inductor should be
selected based on the required load power and the
minimum input voltage. The saturation current rating
should be chosen well above the steady state peak
inductor current. At minimum V_IN and full duty cycle
(worse case), this is approximately:
I_PEAK
≅
-V----^I--^N----⁽--^M----^I--^N----⁾---^×-----t-^O----^N--
L
≅
3----V------×_{----0---.--9---5----}×_{-----1--------M------1-----H---------z--}
4.7μH
≅
0.7A
Diode Selection
The low forward voltage and fast switching time make
Schottky diodes the choice for high efficiency opera-
tion. Make sure the diode has a reverse voltage rating
greater than the maximum output voltage. The diode
conducts only when the power switch is on, so a peak
current rating above 1A should be sufficient for a typi-
cal design.
Capacitor Selection
For proper performance, use surface-mount, low ESR
ceramic capacitors for CIN and COUT. X7R or X5R
ceramic dielectric provides good stability over the oper-
ating temperature and voltage range.
In most LED applications, high frequency output ripple
is not a concern because it will not cause intensity vari-
ations that are visible to the human eye.
© 2006 California Micro Devices Corp. All rights reserved.
8 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
04/26/06