EUP2983
Application Information
PWM Dimming Control
CTRL is also used as a digital input allowing LED
brightness control with a logic-level PWM signal applied
directly to CTRL. The frequency range is from 200Hz to
200kHz, while 0% duty cycle corresponds to zero current
and 100% duty cycle corresponds to full current. The error
amplifier and compensation capacitor form a lowpass filter
so PWM dimming results in DC current to the LEDs
without the need for any additional RC filters; see the
Typical Operating Characteristics.
Soft-Start
The EUP2983 attains soft-start by charging CCOMP
gradually with a current source. When VCOMP rises above
1.25V, the internal MOSFET begins switching at a
incremental duty cycle. When VCOMP rises above 2.25V,
the duty cycle is at its maximum. See the Typical
Operating Characteristics for an example of soft-start
operation.
Shutdown
Capacitor Selection
The EUP2983 enters shutdown when VCTRL is less than
100mV for more than 8.2ms. In shutdown, supply current
is reduced to 150µA by powering down the entire IC
except for the CTRL voltage-detection circuitry and
internal regulator. CCOMP is discharged during shutdown,
allowing the device to reinitiate soft-start when it is
enabled. Typical shutdown timing characteristics are
shown in the Typical Operating Characteristics.
The exact values of input and output capacitors are not
critical. The typical value for the input capacitor is 10µF
and the typical value for the output capacitor is 2.2µF.
Larger value capacitors can be used to reduce input and
output ripple, but at the expense of size and higher cost.
CCOMP stabilizes the converter and controls soft-start.
Connect a 0.1µF capacitor from COMP to GND.
Inductor Selection
Overvoltage Protection
Inductor values range from 10µH to 47µH. A 22µH
inductor optimizes the efficiency for most applications
while maintaining low 50mVP-P input ripple. To prevent
core saturation, ensure that the inductor-saturation current
rating exceeds the peak inductor current for the application.
Calculate the peak inductor current with the following
formula:
Overvoltage lockout (OVLO) occurs when VOUT is above
32V.The protection circuitry stops the internal MOSFET
from switching and causes VCOMP to decay to 0V. The
device comes out of OVLO and into soft-start when VOUT
falls below 30V.
In normal operation, EUP2983 VOUT calculates as follows:
V
) × I
V
) × 0.9µs
IN MIN
(
+
V
= V + V + V
IN
× K
OUT
(
MAX
LED
MIN
(
MAX
)
OUT
FB
LED
I
=
PEAK
0.9× V
2× L
IN
(
)
Where
Schottky Diode Selection
K----------number of LEDs in each string
The EUP2983’s high switching frequency demands a
high-speed rectification diode (D1) for optimum efficiency.
A Schottky diode is recommended due to its fast recovery
time and low forward-voltage drop. Ensure that the diode’s
average and peak current rating exceed the average output
current and peak inductor current. In addition, the diode’s
reverse breakdown voltage must exceed VOUT. The RMS
diode current can be calculated from:
VIN--------input voltage
VFB--------feedback voltage
VLED-------a LED forward voltage
Adjusting LED Current
Adjusting the EUP2983’s output current changes the
brightness of the LEDs. An analog input CTRL and the
sense-resistor value set the output current. Output current
is given by:
I
=
I
× I
OUT PEAK
DIODE
PC Board Layout
RMS
( )
V
CTRL
I
=
LED
10× R
SENSE
Due to fast switching waveforms and high-current paths,
careful PC board layout is required. An evaluation kit is
available to speed design. When laying out a board,
minimize trace lengths between the IC and RSENSE, the
inductor, the diode, the input capacitor, and the output
capacitor. Keep traces short, direct, and wide. Keep noisy
traces, such as the LX node trace, away from FB. The IN
bypass capacitor (CIN) should be placed as close to the IC
as possible. PGND and GND should be connected directly
to the exposed paddle underneath the IC. The ground
connections of CIN and COUT should be as close together as
possible. The traces from IN to the inductor and from the
Schottky diode to the LEDs may be longer.
The VCTRL voltage range for adjusting output current is
0.24V to 1.68V. To set the maximum current, calculate
RSENSE when VCTRL is at its maximum as follows:
1.68
R
=
SENSE
10× I
LED MAX
( )
Power dissipation in RSENSE is typically less than 10mW,
making a standard chip resistor sufficient.
DS2983 Ver1.2 Nov. 2008
7
EUTECH [ EUTECH MICROELECTRONICS INC ]
