CM9320 Datasheet, PDF (8/10 Pages) California Micro Devices Corp – Asymmetrical High Efficiency Two Channel Boost LED Driver

PRELIMINARY
CM9320
Application Information (cont’d)
switch transistor for positive slope. See Figure 1 for dif-
ferent brightness control methods and results.
PWM
signal
25
20
15
R_SET1
22k
R_SET2
91k
ISET1
ISET2
CM9320
(a) Schematic
ILED1
negative slope
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
≅
3V × 0.8 × -------1--------
----------------------------1----M-----H-----z-
4.7μH
≅
0.5A
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.
10
ILED2
positive slope
5
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 minimum core losses. An inductance of 4.7µH is
optimal for most applications, but low DCR inductor val-
ues 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
Capacitor Selection
For proper performance, use surface-mount, low ESR
ceramic capacitors for C_IN and C_OUT. 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.
For such applications, when low ripple is needed, a
22μF input capacitor and/or 2.2 μF output capacitor are
recommended.
REF
DES
C_IN
C_OUT
L₁
D₁
DESCRIPTION
Capacitor, 10μF,
10V, Ceramic, 1206
Capacitor, 1μF, 16V,
Ceramic, 0805
Inductor, 4.7μH, 1A,
Low DCR
Schottky Diode, 1A,
20V, SMD
SOURCE
Murata, GRM319R61A106KE19D
Vishay, VJ1206G106KXQ
Murata, GRM188R61C105KA93D
TDK, C2012X5R1C105K
Coilcraft, LP06013-472ML
TMP Electronics Co., SPC-03802-4R7
CHILISIN, SCD03015-4R7
SUMIDA, CDH3D13/S4R7
IR, MBRS120
CHENMKO, SSM5817S
Input Filter
If CM9320 is more than 4" from main power supply
point, use an input RC filter to avoid high ripple and
input transients to the circuit input pin (see Figure 2).
© 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
05/08/06