CM9321 Datasheet, PDF (8/11 Pages) California Micro Devices Corp – Asymmetrical High Efficiency Three Channel Boost LED/OLED Driver

PRELIMINARY
CM9321
Application Information (cont’d)
CM9321 Operation
When a voltage that exceeds the undervoltage lockout
threshold (UVLO) is applied to the VIN pin, the
CM9321 initiates a softstart which limits the inrush cur-
rent while the output capacitors are charged. Following
softstart, the CM9321's internal NMOS drives an exter-
nal inductor and Schottky diode to deliver the inductor's
stored energy to the load.
Setting the LED Current
The output currents for channel 1 (up to 40mA) and
channel 3 (up to 30 mA) are set by the value of their
R_SET resistor, located between the ISET(1,3) pin and
GND, according to the equations:
(a) Interleave
R_SET(kΩ) = _I---L---_E---_D^4---5-₍--0m-----A-----₎-
(b) Non-interleave
R_SET(kΩ)
=
---------7---3---0-----------
I_LED(mA)
switch transistor for positive slope. See Figure 1 for dif-
ferent brightness control methods and results.
PWM
signal
R_SET1
22k
R_SET3
91k
ISET1
VSET2
ISET3
CM9321
(a) Schematic
25
ILED1
20
negative slope
15
10
ILED3
positive slope
5
Setting the OLED Voltage
The output voltage for the OLED is the difference
between V_OUT the voltage at I_LED3 pin. The voltage is
programmed using the voltage divider R22 and R21,
according to the equation:
V_OLED = 20 × V_SET2
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
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 4.7-µH 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
selected based on the required load power and the
minimum input voltage. The saturation current rating
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04/26/06