EUP2561
Application Information
Soft-Start
The EUP2561 attain soft-start by charging C
COMP
gradually with a current source. When V
COMP
rises above
1.25V, the internal MOSFET begins switching at a
reduced duty cycle. When V
COMP
rises above 2.25V, the
duty cycle is at its maximum. See the Typical Operating
Characteristics for an example of soft-start operation.
Shutdown
The EUP2561 enter shutdown when V
CTRL
is less than
100mV for
more
than
8.2ms. In shutdown,
supply current is reduced to 0.3µA by powering down
the entire IC except for the CTRL voltage-detection
circuitry. C
COMP
is discharged during shutdown, allowing
the device to reinitiate soft-start when it is enabled.
Although the internal N-channel MOSFET does not
switch in shutdown, there is still a DC current path
between the input and the LEDs through the inductor
and Schottky diode. The minimum forward voltage of
the LED array must exceed the maximum input voltage
to ensure that the LEDs remain off in shutdown.
However, with two or more LEDs, the forward voltage is
large enough to keep leakage current low, less than
1µA(typ). Typical shutdown timing characteristics are
shown in the Typical Operating Characteristics.
Overvoltage Protection
Overvoltage lockout (OVLO) occurs when V
OUT
is
above 27V.The protection circuitry stops the internal
MOSFET from switching and causes V
COMP
to decay to
0V. The device comes out of OVLO and into softstart
when V
OUT
falls below 25V.
Adjusting LED Current
Adjusting the EUP2561 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:
VCTRL
I LED
=
10
×
R SENSE
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.
Capacitor Selection
The exact values of input and output capacitors are not
critical. The typical value for the input capacitor is
2.2µF and the typical value for the output capacitor is
0.1µF. Larger value capacitors can be used to reduce
input and output ripple, but at the expense of size and
higher cost. C
COMP
stabilizes the converter and con-
trolls soft-start. Connect a 0.1µF capacitor from
COMP to GND. For stable operation, C
OUT
must not
exceed 10 times C
COMP
.
Inductor Selection
Inductor values range from 10µH to 47µH.
A 22µH inductor optimizes the efficiency for most
applications while maintaining low 15mV
P-P
input
ripple. With input voltages near 5V, a larger value of
inductance can be more efficient. To prevent core
saturation, ensure that the inductor-saturation current
rating exceeds the peak inductor current for the appli-
cation. Calculate the peak inductor current with the fol-
lowing formula:
I PEAK
=
+
VOUT(MAX)
×
I LED(MAX)
0.8
×
VIN(MIN)
VIN(MIN)
×
0.8
µ
s
2
×
L
Schottky Diode Selection
The EUP2561 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 V
OUT
. The RMS diode current can
be calculated from:
I DIODE(RMS)
≅
I OUT
×
I PEAK
The V
CTRL
voltage range for adjusting output current is
0.24V to 1.62V. To set the maximum current, calculate
RSENSE when V
CTRL
is at its maximum as follows:
R SENSE
=
1.62V
10
×
I LED(MAX)
Power dissipation in RSENSE is typically less than 5mW,
making a standard chip resistor sufficient.
DS2561 Ver1.0
Dec. 2006
7
EUTECH [ EUTECH MICROELECTRONICS INC ]
