CM9320 Datasheet, PDF (7/10 Pages) California Micro Devices Corp – Asymmetrical High Efficiency Two Channel Boost LED Driver

PRELIMINARY
CM9320
Pin Descriptions (cont’d)
10
EPad
EN
GND
PIN DESCRIPTIONS
Enable pin. The circuit is ON when V_EN is above 1.0V. The circuit is OFF when V_EN is below
0.2V. Active High (ON) by default.
Ground (backside exposed pad).
Application Information
The CM9320 is a high efficiency, constant frequency
current regulating boost driver ideally suited for driving
white LEDs to backlight LCD color displays and a cam-
era flash in Li-ion powered portable devices. The
CM9320 is the perfect driver for portable applications
such as cellular phones, digital still cameras, PDAs and
any application where small space, compact overall
size and low system cost are critical.
With a maximum 100 mA/19V output capability, the cir-
cuit can drive up to 10 WLEDs (5 series x 2 parallel),
allowing up to 35 mA per channel. It includes a switch
and an internally compensated loop for regulating the
current into the LEDs. The CM9320 delivers a constant
current to series-connected LEDs, ensuring uniform
brightness and color purity regardless of any LED for-
ward voltage variations.
The proprietary design architecture allows asymmetri-
cal loading on each channel and maintains high effi-
ciency (typ 85%) at low V_IN resulting in longer battery
life, and cool, reliable operation when an adapter is
supplying high V_IN. The maximum LED current for
each channel is independently programmed with exter-
nal low power resistors avoiding the need for ballast
resistors.
An 1MHz constant frequency PWM scheme saves
board space with the use of small, low cost external
components, allowing designers to avoid sensitive IF
bands in RF applications. The circuit operates with low
value inductors and low value output ceramic capaci-
tors, keeping voltage and current ripple in the 1%
range.
The output over-voltage protection circuit prevents
damage in case of a high impedance output (e.g. faulty
LED). The controlled current limit circuit limit prevents
large inductor current spikes, even at start-up. To avoid
possible leakage currents, the EN control pin discon-
nects the LEDs from ground during shutdown.
CM9320 Operation
When a voltage that exceeds the undervoltage lockout
threshold (UVLO) is applied to the VIN pin, the
CM9320 initiates a softstart which limits the inrush cur-
rent while the output capacitors are charged. Following
softstart, the CM9320's internal NMOS drives an exter-
nal inductor and Schottky diode that delivers the induc-
tor's stored energy to the load.
Setting the LED Current
The output current is set by the value of the R_SET con-
nected between the I_SET 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
© 2006 California Micro Devices Corp. All rights reserved.
05/08/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
7